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By JIM ROWE T RADITIONALLY, RF level/power meters have been quite expensive beasts costing many hundreds of dollars, even secondhand. Small wonder that many of us have simply had to do without them. Such RF level/power meters have always been expensive because of the measurement technique they used: converting the RF energy into heat and then measuring the temperature rise using a sensitive thermocouple system. Luckily for us, advancing semiconductor technology now provides an easier way: the wideband logarithmic amplifier/detector IC. Its DC output is closely proportional to the logarithm of the RF input voltage. We can achieve the desired result by combining one of these chips with an “intelligent” metering circuit, capable of processing this logarithmic DC voltage to indicate both signal level and the corresponding power level. In a nutshell, our circuit uses an Analog Devices AD8307AN logarithmic amplifier/detector to convert RF signals into DC which is processed by a PIC microcontroller. The micro uses some fairly fancy maths routines to work out the signal level and power, which is then displayed on a standard 2-line LCD panel. The whole set-up works from a 9V battery or DC plugpack and draws less than 35mA. The AD8307 log amp/detector Digital RF Level & Power Meter Need to measure small signals at radio frequencies? Here is a low-cost digital level and power meter which will allow you to measure RF signals from below 50kHz to above 500MHz. As well as indicating the signal level in volts and dBV, it also shows the corresponding power level (into 50 ohms) in both milliwatts and dBm. 30  Silicon Chip To help understand logarithmic amplifier/detector ICs, take a look at Fig.1. This gives a simplified view inside the AD8307AN device. The incoming RF signals are passed through six cascaded wideband differential amplifier/limiter stages, each of which has a gain of 14.3dB (about 5.2 times) before it enters limiting (ie, clipping). This gives a total amplifier gain of about 86dB or about 20,000 times. The outputs of each amplifier/limiter stage are fed to a series of nine fullwave detector cells, along with similar outputs from three cascaded passive 14.3dB attenuator cells connected to the input of the first amplifier/limiter. The differential current-mode outputs of all nine detector cells are added together and fed to a “current mirror” output stage, which effectively converts them into a single-sided DC output current. Because of the combination of cascaded gain and limiting in the amplifiers (plus an internal offset compensation loop), the amplitude siliconchip.com.au Specifications SIX 14.3dB GAIN, 900MHz BW AMPLIFIER/LIMITER STAGES +INP INTERC. SET –INP • 3x PASSIVE ATTENUATOR CELLS MIRROR Iout NINE FULL-WAVE DETECTOR CELLS WITH DIFFERENTIAL OUTPUT CURRENTS – ALL SUMMED ENB BAND-GAP REFERENCE AND BIASING OUT 25mV/dB 12.5k INPUT OFFSET COMPENSATION LOOP Fig.1: block diagram of the AD8307AN amplifier/detector IC. The incoming RF signals are passed through six cascaded wideband differential amplifier/limiter stages and these in turn drive full-wave detector cells (see text). 4.7 TO MAIN BOARD (CON1) 100nF 47nF 8 Rin* 7 VPS IN H 6 EN 4 OUT IC1 AD8307AN 5 INT 47nF 1 10 IN L COM 2 OFS INTERCEPT ADJUST SLOPE VR2 ADJUST 50k 1 2 3 4 VR1 50k 100nF 3 100nF 51k SC RF LEVEL & POWER METER 33k HEAD END CIRCUIT Fig.2: the head-end circuit is based on the AD8307AN. The incoming RF signals are fed to pins 8 & 1 via 47nF capacitors, while the detected output appears at pin 4 and is fed to pin 3 of a type A USB socket. of this output current is closely proportional to the logarithm of the RF input voltage, over an input range of 100dB from about -93dBV (22.4mV) up to +7.0dBV (2.24V). In fact this “logarithmic law” relationship is linear to within ±0.3dB over most of the range. The output current Iout increases at a slope of very close to 2mA per dB increase in RF input level. This current passes through an internal 12.5kW resistor, resulting in a DC output voltage which increases at the rate of 25mV/dB. This slope can be fine tuned using an adjustable external resistor in parallel with the 12.5kW internal resistor. So what’s that “intercept set” input for? This allows us to adjust the DC offset in the output current mirror, which adjusts the effective “zero level” point of the chip’s output current and siliconchip.com.au • • • • • Input impedance: 50W (can be changed to 75W or 1.1kW) Measuring frequency range: from below 50kHz to above 500MHz Maximum input signal level: 2.238V RMS (+7.0dBV) Minimum input signal level: 22mV RMS (-93dBV) Maximum input power level: 100mW into 50W (+20dBm) Minimum input power level: 1nW (0.001mW/-60dBm) Measurement linearity: approximately ±0.3dB Measurement accuracy: approximately 0.2% Power requirements: 9V DC at 35mA (no backlight) or 120mA with backlight CON5 USB TYPE 'A' SOCKET * Rin = 100 //220 //220  FOR 50  INPUT 2008 • • COM RF INPUT CON4 • voltage, ie, the “origin” from which the output slope rises. You can think of it as setting the detector’s zero point. Head-end circuit It’s desirable to separate the RF detector section from the rest of the meter circuitry, partly because it is the only section handling RF signals and partly because it has very high gain and is therefore susceptible to electromagnetic interference. The AD8307AN and its accompanying components are therefore mounted on a small “head-end” board which in turn is mounted inside a small diecast aluminium box, for shielding. The circuit of this head-end section is shown in Fig.2 and involves little else apart from the all-important AD8307AN (IC1). The incoming RF signals are coupled into the inputs of IC1 via two 47nF capacitors, with Rin providing the desired 50W input termination. (Rin is a combination of paralleled surface-mount chip resistors, to give a value of 52.4W with very low parasitic inductance. As the input impedance of the AD8307AN is itself very close to 1.1kW and this is in parallel with Rin, the resulting total input resistance is very close to 50W). Trimpot VR1 and its 33kW series resistor are connected between the output (pin 4) of IC1 and ground, so they are effectively in parallel with the 12.5kW resistor inside the chip itself. This allows the output slope of the detector to be fine tuned to a value of 20mV/dB, when the meter is calibrated. Trimpot VR2 is used to adjust the DC voltage fed to pin 5 of IC1. This is the “intercept set” input, so VR2 effectively becomes the detector’s zero set adjustment. The head-end section connects to the main meter unit via a standard USB cable. This cable carries the detector’s output voltage to the main board via pin 3 of CON5 and also supplies IC1 with +5V power via pin 1. Main circuit The processing part of the circuit is shown in Fig.3. Here is where the real “work” is done, by the firmware October 2008  31 Volts, dBV, Milliwatts & dBm The RF Level and Power Meter described in this article gives four indications for every measurement: the RF input voltage (in volts or millivolts), the corresponding value in dBV, the corresponding power level in the meter’s 50W input load (in milliwatts or microwatts) and the corresponding value in dBm. The voltage and power levels probably need no explanation but I should perhaps explain the significance of the two decibel readings. For many years, engineers working in the communications and RF fields have found it convenient to describe signal amplitude and power levels in decibels, because of the very wide ranges involved – from microvolts (mV) to kilovolts (kV), and from nanowatts (nW) to kilowatts (kW). Because decibel scales are logarithmic, they make it easier to work with signals varying over such wide ranges. To describe the voltage gain of an RF amplifier in terms of decibels, for example, we simply take the base-10 logarithm of the voltage gain (Vout/Vin) and multiply this figure by 20. So a voltage gain of 10 corresponds to +20dB, a gain of 100 corresponds to +40dB, a gain of 1000 corresponds to +60dB and so on. Similarly an attenuator which reduces the voltage level by a factor of 10:1 can be described as having a “gain” of -20dB. Get the idea? When we’re describing power levels rather than voltage, the power gain of an RF amplifier can be found by again taking the base-10 logarithm of the power gain (Pout/Pin) but this time multiplying the figure by 10. So a power gain of 10 times is +10dB, while a power gain of 100 times is +20dB and so on. (If you’re a bit puzzled by the difference between voltage and power when calculating the decibels, it’s merely because power increases with the square of the voltage. That’s why we multiply the log of voltage ratios by 20 but we only multiply the log of power ratios by 10). dbV and dBm So what’s the difference between “dBV” and “dBm” figures? Well, these are both decibel scales but in this case they are used to compare one specific voltage or power level with a known reference value, rather than to compare two specific values. So the contractions dBV and dBm indicate that the figures they accompany are absolute, rather than relative. “dBV” is a voltage level expressed in decibels with reference to 1.000 volts. So +6dBV (2V) is 6dB greater than 1V, while -20dBV (100mV) is 20dB smaller than 1V. So expressing a voltage in dBV merely indicates that it is measured on a decibel scale which refers to 1.00V as its 0dB point. Similarly, “dBm” is a power level which is expressed in decibels with reference to a specific reference power level of 1mW (milliwatt); in other words, on a decibel scale where 1mW corresponds to 0dB. So +10dBm corresponds to 10mW, +20dBm to 100mW and -30dBm to 1mW (microwatt). There is another “absolute” decibel scale used for expressing voltage levels, the dBm scale. This refers to a level of 1mV (microvolt) as its 0dB point. So +120dBm is the same as 0dBV, while 0dBu is the same as -120dBV. One last point: since the dBV and dBm scales are “absolute”, surely they can be related to each other? Yes they can but to work this out you need to know the impedance level – because that is what relates voltage and power in any circuit. In most RF work, the impedance level is 50W. At this level, a voltage of 1V corresponds to a power level of 20mW (12/50), so 0dBV equals +13dBm. On the other hand -30dBm (= 1mW) corresponds to 7.07mV, or -43dBV. In other words, there’s a fixed 13dB difference between the two scales. This difference changes with impedance level, though. For example when the impedance level is 600W, 0dBm or 1mW corresponds to 0.7746V or -2.218dBV, so there’s a fixed 2.2dB difference between dBm and dBV. Older RF level and power meters often indicated in just dBm or perhaps in dBV as well. If the user wanted to know the actual voltage and power level, they had to either refer to a chart or grab a calculator and work them out. This could be pretty tedious, and that’s why we’ve given this new RF Level and Power Meter the ability to calculate and display not just dBm and dBV but the equivalent volts and milliwatts as well, for every measurement. 32  Silicon Chip running inside the PIC16F88-I/P micro (IC3). The PIC16F88-I/P device is well-suited to this application, as it includes an analog-to-digital converter (ADC) module with 10-bit measuring resolution. The ADC is also flexible in terms of its operating mode, with a choice of positive and negative reference voltages and also a 7-channel input multiplexer. We take advantage of these features by using a positive reference voltage of 3.50V which is fed into pin 2 of IC3 and by using three of the ADC input channels to allow firmware selection of the measuring range via pin 1 (AN2), pin 18 (AN1) and pin 17 (AN0). Why do we need three ranges? Because it allows us to get higher measuring resolution when the RF input signals (and hence the output voltage from IC1) are quite small. For these signals, we are able to amplify the DC output voltage from IC1, in order to use a larger proportion of the ADC’s measurement range of 0-3.5V and hence increase the measurement resolution. We provide the three ranges in the following way. The incoming DC voltage from IC1 enters via pin 3 of CON1, and is then passed through a simple input protection circuit using diodes D1 & D2, the 100nF capacitor and the 100W and 1MW resistors. It is then fed to the paralleled inputs of op amps IC2b, IC2c & IC2d. Each of these provides a different amount of gain, to change the effective slope of the log-law input signal. The gain for the normal default measuring range is 1.75, provided by IC2b with its 1.5kW and 2.0kW feedback resistors. This gives the incoming DC signal an effective slope of 1. x 20 or 35mV/dB, translating to a total span of 100dB for the ADC’s 3.5V measuring range. For signals of less than 223.9mV (-13dBV), we select the output from IC2d, configured for a gain of 2.19. This gives the incoming DC signal an effective slope of 43.74mV/dB, translating to a total ADC measuring span of 80dB. Then for signals of less than 22.39mV (-33dBV) we select the output of IC2c, with a gain of 2.916. This gives the incoming DC signal a slope of 58.32mV/dB, which translates to a total span of 60dB. Using this approach we obtain much better measuring resolution for these much smaller signals. The siliconchip.com.au siliconchip.com.au October 2008  33 2 3 4 1 100nF D2 100 A K A K D1 1M +5.00V 100nF 6 5 13 12 9 IC2c 4  LED1 330 7 1.5k 2.4k 14 3.9k TP1 2.0k 2.0k 220k 2.0k 220k TPG 1 18 17 2 2.2k RB4 AN2 AN1 Vss 5 10 11 3 CLKo 15 9 RB3 8 RB2 7 RB1 6 RB0 IC3 PIC16F88-I/P AN0 RB5 RA4 4 14 Vdd MCLR Vref+ 12 RB6 13 RB7 16 RA7  LED3 +3.50V K A MAIN BOARD 11 IC2b IC2d 8 200 6.8k  LED2 3.0k K A IC2: LM324N 10 K A RF LEVEL & POWER METER USB TYPE 'B' SOCKET CON1 100nF +5.00V 6 4 Vdd 2 SET 5.00V 330 TPG D3 K A K D1,D2: 1N4148 A TP2 (2.0MHz) IN K A 5 R/W IC2a LEDS 2 3 A OUT ADJ 1 3 LM317T IN – + CON3 9–15V DC INPUT OUT CON2 CAL/MOD OUTPUT LCD VR4 10k CONTRAST 68k 1.5k 9V BATTERY S2 POWER CONTRAST 470 F 16V K D3 1N4004 16 x 2 LCD MODULE VR3 100 220 F ADJ OUT D7 D6 D5 D4 D3 D2 D1 D0 GND 1 14 13 12 11 10 9 8 7 EN RS S1 SELECT RANGE 10k 100nF 120 REG1 LM317T Fig.3: the main-board circuit is based on an LM324 quad op amp (IC1) and a PIC16F88 microcontroller (IC3). The incoming signal is fed to paralleled op amp stages IC2b-IC2d, each operating with a different gain to provide three ranges. Their outputs in turn drive the ADC inputs of IC3 which processes the signals and drives a 16 x 2 LCD module. SC 2008 FROM HEAD END (CON5) TPG TP3 ALTRONICS 16X2 LCD MODULE Z-7000A OR Z-7011 (B/L) A 18090240 8002 C K RE W OP/LEVEL FR LATI GID )DRA O B NIA M( RETE M 14 330 SET 5.00V TP3 5.00V REG1 LM317T 1 2.0k 4148 LED2 –20dBV LED3 –40dBV 470 F 2 3 CON1 POWER S2 4004 0dBV 1M 4148 100nF D2 LED1 4 100nF D3 S1 2.4k 220k 1 100 330 3.9k 220k RANGE SELECT D1 1 IC2 LM324N 2.0k 2.0k 100nF 220 F 1.5k CAL OUT CON3 9–15V DC IN TPG CON2 INPUT FROM HEAD END 68k LCD CONTRAST 1.5k TP2 (2MHz) 200 6.8k 2.2k 10k 3.0k 100 VR3 120 TPG TP1 (3.50V) IC3 PIC16F88 -I/P 100nF 10k RBL* VR4 TPG 18  0.5W 1 + – BATTERY * SEE TEXT Fig.4: follow this parts layout diagram and the accompanying photograph to build the main board. Both IC2 and the PIC16F88 microcontroller (IC3) should be installed in sockets. outputs from op amps IC2c, IC2d & IC2b are fed directly to the AN0, AN1 & AN2 (ADC) inputs of the PIC and its firmware selects the appropriate ADC input channel by stepping from one range to the next each time you press the range select button (S1). To indicate which range is currently selected, the firmware switches on LED1, LED2 or LED3 and automatically changes the scaling factor, so that the displayed values are correct. After performing the calculations for each measurement, the firmware then displays the results via the LCD module. Power supply The complete circuit runs from 5V DC, which is derived from either a 9V 34  Silicon Chip battery or a plugpack supply of similar voltage, using regulator REG1, an LM317T adjustable device. We use this rather than a fixed regulator because this allows us to set the supply rail accurately to 5.00V. We need to do this because the 3.50V reference voltage for the PIC’s ADC is derived directly from the 5V rail, via a voltage divider using 3.0kW, 6.8kW and 200W resistors. This reference voltage for the ADC is fed into pin 2 of the PIC, which is configured as the Vref+ input. Notice that there are a number of test points provided in the main board circuit, to allow more convenient setup and calibration. TP1 allows you to measure the ADC reference voltage, so you can adjust trimpot VR3 to achieve exactly 3.50V at pin 2 of the PIC. TP3 also allows you to measure the 5.00V rail directly, if you wish, while TP2 allows you to check the PIC’s internal clock oscillator. This runs at 8MHz, which means that the signal available at TP2 should be very close to 2MHz (Fc/4). So if the PIC is running correctly, you will find a 2MHz square-wave of 5V peak-topeak at TP2. Finally, the fourth op amp, IC2a, is provided purely as a voltage follower/ buffer from the output of IC2b (the default ADC driver). Its output is made available via CON2, to allow you to monitor the amplified output voltage from the AD8307AN head-end extersiliconchip.com.au nally, with a DMM or oscilloscope. This could be convenient for calibration and also for looking at any amplitude modulation of the RF signals being measured. Note that any observed modulation envelope is likely to be distorted because of the logarithmic response of the head-end amplifier. 51k 4 3 2 10 100nF 100nF 1 VR1 50k 33k TOP VIEW OF HEAD END BOARD SIDE OF BOX CON4 CON5 1 100 220 47nF 220 BNC INPUT SKT TO MAIN BOARD CON5 2 3 4 47nF Fig.5: these two diagrams & the above photo show the parts layout on the head-end board. Use a fine-tipped soldering iron to solder the SMDs to the copper side of the PC board and take care to ensure that IC1 is correctly orientated. Do not use a socket for ICs – it must be soldered directly to the PC board. C 2008 04208082 siliconchip.com.au 50k 100nF IC1 AD8307 Construction As noted earlier, the project is comprised of two parts: the AD8307AN head-end fitted into a small metal box for shielding and the main meter circuitry which is fitted into a UB1-size plastic jiffy box (158 x 95 x 53mm). The two are connected together using a standard USB interconnect cable. The meter’s main circuitry is all fitted on a PC board coded 04210081 and measuring 146 x 84mm, and with a recess in each corner so that it fits neatly behind the lid of the UB1 box. The head-end circuitry is installed on a second PC board coded 04210082 and measuring 43 x 44mm. There is actually a third PC board for this project, coded 04210083 and measuring 95 x 38mm. This is for an optional 20dB/50W attenuator, to allow measurements of higher-level signals. The location and orientation of all parts mounted on the main board are shown clearly in the board overlay diagram of Fig.4. Note that connectors CON1, CON2 and CON3 are all mounted directly on the board, along the righthand side. Power switch S2 also mounts directly on the board, with its connection lugs passing through the board and soldered to pads underneath. Range select switch S1 can be mounted in the same way or mounted on the box lid with its leads extended through the board using short lengths of tinned copper wire. The three range indicator LEDs are again mounted directly on the board, with the underside of their bodies spaced up by about 14mm so that the LEDs just protrude through the matching holes in the front panel (ie, the lid) when the board is mounted behind it. Use sockets for IC2 & IC3, rather than soldering them directly to the board. There are four wire links on the board and it’s a good idea to fit these before any of the components so that they’re not forgotten. The test point terminal pins can also be fitted at this VR2 4.7 8002 C 28080240 COPPER SIDE OF HEAD END BOARD The head-end board is attached to a panel-mount BNC socket and mounted upside down inside a diecast metal case. A type A to type B USB cable connects the unit to the main PC board. stage, along with the two further pins used for the optional battery connections. By the way, these last two pins are mounted from the rear, to make the battery connections easier. Mounting the LCD module The LCD module used for this project is the Altronics Z-7000A or Z-7011, with the second type number signifying the version with backlighting. Regardless of which version you use, the module is mounted above the main board using four M3 x 15mm machine screws, with M3 x 6mm tapped Nylon spacers used as standoffs. Then nuts are fitted under the board to hold everything together – but with one Nylon flat washer under the nut at lower left, to ensure that it doesn’t short-circuit October 2008  35 What The Firmware Does As we explain in the main text, the AD8307 chip in the RF Meter’s “head end” detects the incoming RF signals and converts them into a DC voltage according to a logarithmic conversion scale. A PIC micro then measures and converts this into the equivalent RF voltage and power readings, under the control of a firmware program. To do this, it makes use of a suite of maths routines made available to PIC programmers by Microchip Technology Inc, the manufacturers of the PIC family of micros. These routines are used to perform 24-bit and 32-bit floating-point addition, subtraction, multiplication and division, base-10 exponentiation, fixed-point multiplication and division, and floating-point to ASCII conversion. Without going into much detail, the PIC firmware program works through the following sequence in making each measurement: First it directs the PIC’s 10-bit analogto-digital converter module to take a measurement of the DC output voltage from the AD8307 chip. It then converts that into 24-bit floating-point form, after which it is multiplied with a pre-calculated scaling factor (24-bit also) for the currently chosen measurement range. The resulting product is then divided by the ADC’s full-scale value of 3FF (in 24-bit FP form), to give the measurement value in what I call the “raw dB” form. This is essentially a 24-bit number varying between 0 and 100. This raw dB value is then used to calculate the equivalent dBV value, by subtracting decimal 93 (in 24-bit FP form) and also the equivalent dBm value (for 50W impedance level) by subtracting decimal 80 (again in 24-bit form). These values are then saved for display but also used to calculate the actual voltage and power levels. The dBV value is used to calculate the equivalent voltage by first dividing it by decimal 20 (in 24-bit FP form) and then raising decimal 10 to that power using EXP1024, the Microchip 24-bit floatingpoint base-10 exponentiation routine. This is equivalent to calculating the antilog­ arithm, so we end up with the equivalent voltage value in 24-bit FP form. After saving this for display, the program then does the equivalent calculation for power, taking the dBm value and first dividing it by decimal 10 and then again raising decimal 10 to that power using EXP1024. This gives the equivalent power in milliwatts, which is again saved for display. Once all four parameters have been calculated, the final steps of the measurement sequence involve taking each 24-bit parameter and processing it for display on the LCD module. For the dBV and dBm figures, this means working out the correct polarity indication (+ or -) and then using a Microchip routine called Float_ASCII to convert the numbers themselves into ASCII digits for display. Things are a little more complicated for the voltage and power values, because these must first have their 24-bit binary exponents analysed to work out their scaling, the position of their decimal point and the most convenient multiplier to give them (eg, milli or micro). After this is done, they are again converted into the equivalent ASCII digits using Float_ASCII. As you can see, there’s quite a bit of mathematical jiggery-pokery involved but luckily most of this is performed by Microchip’s fancy maths routines. The full source code for the firmware will be available on the SILICON CHIP website, along with the source code for the floating point maths routines it uses (in a file called FPRF24.TXT) and, of course, the assembled hex code of the complete firmware ready to burn into a PIC. a PC board track close by. The 14 main connections to these modules are all in a horizontal row at lower left. To make these connections reliably but in a manner which allows easy removal and replacement of the module if this is ever needed, I elected to use a custom-made 14-way plug and socket system. The socket was made from one side of a 28-pin IC socket, cut away neatly and then mounted on the top of the main board. To mate with this socket, I made a plug from a 14-pin length of SIL pin strip, the pins of which were soldered to the pads on the underside of the module. This must be done carefully, so that there is enough clean length of each pin extending down to mate with the socket clips (this is easier to do than to describe). Backlit LCD module This larger-than-life-size view shows how the LCD module is connected to the main PC board. A 14-pin header is soldered to the LCD module and this plugs into a matching 14-pin socket strip cut from a 28-pin IC socket. 36  Silicon Chip If you use the backlit LCD module (Z-7011A) you will have to connect its “A” & “K” terminals (for the backlight siliconchip.com.au A A 19 61 16 63 x 16mm LCD WINDOW 26 63 HOLES A: 3.5mm DIA., COUNTERSUNK 7 24.5 D B HOLES B: 3.5mm DIA. CL HOLES C: 6.5mm DIA. (RIGHT-HAND SIDE OF BOX) 24.75 8.25 B 12 HOLE E: 9mm DIA. 17.75 12 (BOX LID) HOLE D: 11mm DIA. B 7.5 B 15.25 7.5 E B A A 21 3 C 9.5 11.5 C 9 30.5 CL ALL DIMENSIONS IN MILLIMETRES Fig.6: this full-size diagram shows the drilling details for the plastic case that’s used to house the main PC board. The large cutouts can be made by drilling a series of holes around the inside perimeter, then knocking out the centre piece and filing the job to a smooth finish. LEDs) to the main PC board. This can be done using short lengths of tinned copper wire. Similarly, resistor RBL (18W 0.5W) is installed only if you are using this module. It gives a nominal LED current of about 80mA. Once all of the components are mounted on the main board, it can be placed to one side while you assemble the head-end board. Head-end board assembly The board overlay diagrams for the siliconchip.com.au head-end board are shown in Fig.5. The USB type A socket CON5 mounts on the top of the board, along with the two trimpots, three 0.25W resistors and three 100nF monolithic capacitors. IC1 should be soldered directly into the board, to ensure an absolute minimum of input lead inductance. The remaining surface-mount components all mount on the copper side of this board, ie, the two 47nF input coupling capacitors and the three resistors used for the RF input termination. Solder these components carefully using a fine-tipped iron, using the “tack first to hold it in position” technique to avoid damaging either the parts or the board pads. When you have finished wiring up this board, place it aside also while you prepare the meter’s two boxes by drilling and cutting the various holes in them. These are all shown in the drilling diagrams (Figs.6 & 8), so the job should be quite straightforward. To complete assembly of the headend unit, first mount the BNC input connector CON4 in the hole at the October 2008  37 Table 3: Resistor Colour Codes No.   2   1   1   1   1   1   1   1   1   1   3   2   2   1   1   1   1   1   1 o o o o o o o o o o o o o o o o o o o o Value 220kW 68kW 51kW 33kW 10kW 6.8kW 3.9kW 3.0kW 2.4kW 2.2kW 2.0kW 1.5kW 330W 200W 120W 100W 18W 10W 4.7W Fig.7: when the unit is first turned on, it displays “Silicon Chip RF Level/Pwr Meter” as shown at top. The display immediately above shows typical level (top line) and power readings. 4-Band Code (1%) red red yellow brown blue grey orange brown green brown orange brown orange orange orange brown brown black orange brown blue grey red brown orange white red brown orange black red brown red yellow red brown red red red brown red black red brown brown green red brown orange orange brown brown red black brown brown brown red brown brown brown black brown brown brown grey black brown brown black black brown yellow violet gold brown 5-Band Code (1%) red red black orange brown blue grey black red brown green brown black red brown orange orange black red brown brown black black red brown blue grey black brown brown orange white black brown brown orange black black brown brown red yellow black brown brown red red black brown brown red black black brown brown brown green black brown brown orange orange black black brown red black black black brown brown red black black brown brown black black black brown brown grey black gold brown brown black black gold brown yellow violet black silver brown end of the metal box, with the lug of its earthing washer orientated at “3 o’clock” so that once the mounting nut is fully tightened, it can be bent around at 90° ready to be soldered to the PC board copper (along from the socket’s centre spigot). Then mount the head-end PC board upside down inside the upper part of the box, ie, with the trimpots underneath and facing the matching adjustment holes in the top of the box. The board is mounted using two A M3 x 10mm tapped Nylon spacers as standoffs, with M3 x 6mm countersink-head screws holding the spacers inside the box and pan-head M3 x 6mm screws attaching the board assembly to them. Once the board assembly is mounted in position, you can solder the centre spigot and earthing lug to their respective pads on the board to complete the input connections. The USB cable’s type-A plug can then be mated with socket CON5 at the other end of the B C 39 (CENTRE LINE) 5 25.5 12.75 A B A 13.5 31.5 HOLES A: 3.5mm DIAMETER, COUNTERSUNK HOLES B: 3.5mm DIAMETER HOLE C: 9.5mm DIAMETER 7.5 46 5 (UNDERSIDE OF BOX) (ALL DIMENSIONS IN MILLIMETRES) Fig.8: here are the drilling details for the metal case that’s used to house the head-end board assembly. 38  Silicon Chip siliconchip.com.au RF INPUT (Zo = 50) Pmax = 500mW RF LEVEL & POWER METER SILICON CHIP SLOPE ADJUST LCD CONTR SET 5.00V CAL OUT SILICON CHIP RANGE RF LEVEL & POWER METER SENSOR HEAD RF INPUT Zo = 50 Pmax = 4W (+36dBm) INPUT FROM SENSOR SELECT INTERCEPT ADJUST POWER 0dBV SILICON CHIP 20dB (10:1) RF ATTENUATOR (0 – 500MHz) –20dBV –40dBV 9–15V DC INPUT OUTPUT Zo = 50 Fig.9: this full-size artwork can used to make the front panels of the various units, including the Sensor Head case and the optional RF Attenuator (see text). The artwork can also be downloaded from the SILICON CHIP website. The main PC board is attached to the lid of its case via four M3 x 15mm tapped spacers. Four M3 x 6mm countersinkhead screws secure the lid to the spacers, while four M3 x 6mm pan head screws are used to secure the PC board. siliconchip.com.au October 2008  39 Optional 20dB (10:1) RF Attenuator board, after which the cable can be fitted with its P-type clamp, which is then fastened into the box using an M3 x 10mm countersink-head machine screw with a nut and lockwasher. The cable is then looped around and fed out of the box via a rounded slot cut in the end and the box lid screwed on to complete the assembly. Initial checkout At this stage you should be ready to give your RF Level & Power Meter a preliminary functional checkout, because this is easiest done before the main board is attached to the lid/front panel of the main box. Don’t worry 40  Silicon Chip 27k INPUT 2.7k SC 2008 OUTPUT 4 x 1k 16 x 1k 5 x 330 820 P You will have noticed from the specification panel that the maximum input level of the basic power meter is essentially +7.0dBV, corresponding to 2.238V, 100mW into 50W and +20dBm. As this may be a little low for some applications, we have designed a compact 20dB (at 50W) wideband attenuator which may be used to extend the meter’s range up to 22.38V (+27dBV) and +40dBm (10W) – although it may not be able to cope with 10W of input power for more than a few seconds if you have to use 0805-type SMD resistors. SMD resistors are used low parasitic inductance and capacitance but they do have a fairly low power dissipation (especially the 0805 size). So try to use the larger 1206 size resistors if you can get them, especially in the input leg. Otherwise the continuous input power rating will be limited to about 4W. Despite this limitation, this attenuator can be built quite cheaply and would make a handy optional extra for the meter for those who want to be able to measure higher RF levels. Please note, however, that when the attenuator is connected ahead of the meter’s head-end, the meter itself won’t be able to allow for the extra 20dB of attenuation. This means that you’ll need to add 20dB to the readings yourself, although this shouldn’t be too much of a chore. 10:1 (20 B) RF ATTENUATOR (50, 5W MAX INPUT) Fig.10: the circuit for the optional 20dB RF attenuator uses a standard pisection configuration. The resistors are all surface mount types. All you need to do is add 20dB to the dBV and dBm readings. You will have to multiply the voltage reading by 10 and multiply the power reading by 100. short pieces of tinned copper wire (leaded resistor lead offcuts) are used to make the connections from the earthing lug of each socket to the earthy side of the board copper. Construction details Shield plate The circuit for the attenuator is shown in Fig.10 and it is a standard pi-section type. Everything fits on a small PC board measuring 95 x 38mm and coded 04210083, which fits in a second diecast aluminium box identical to that used for the head-end. Fig.11 and the photos show the parts layout on the PC board. Note that the board assembly is supported behind the box lid simply by soldering the input and output pads to the “active” spigots of the BNC connectors. Multiple As you can see from the internal photos, the prototype attenuator has a small shield plate which was mounted vertically across the centre of the attenuator, to reduce the possibility of RF energy radiating past the attenuator pad at the highest frequencies. This is probably gilding the lily but you may want to add such a shield to your attenuator also. It can be cut from a small rectangle of blank PC board and is supported by soldering it to four PC board terminal pins fitted to the earth copper at the centre of the main board. if S1 (the range select button) hasn’t been mounted on the main board at this stage – it’s not really necessary for this operation. To begin, make sure that IC2 & IC3 have both been plugged into their sockets the correct way around and then set trimpots VR3 and VR4 to the centre of their ranges. After this, connect the main board to a suitable source of 9V DC, either via a battery connected to the pins at the bottom of the board or a plugpack lead plugged into CON3. There’s no need to plug in the lead from the head-end as yet. When you apply power via switch S2, LED1 should light and you should be greeted by a reassuring glow from LED1 and “Silicon Chip RF Level/Pwr Meter” on the LCD, although you may have to adjust trimpot VR4 before this message is displayed clearly and with good sharpness. Note that this greeting message only lasts for a few seconds, after which it is replaced by the meter’s normal display of readings. If all is well so far, you can now set the Vref+ voltage at pin 2 of IC3 to 3.50V. This is done with one adjustment. Connect your DMM to TP1 and its nearby TPG pin and then adjust trimpot VR3 until you get a reading as close as possible to 3.50V. This should also set REG1’s output to close to 5V. siliconchip.com.au oo 330 330 330 820 330 1k 1k 27k 1k 1k 330 2.7k 1k 1k 1k 1k 1k 1k 1k 1k (INPUT) 1k 1k 1k 1k 1k 1k 1k 1k o SILICON CHIP 04209083 (OUTPUT) 20dB RF ATTENUATOR Fig.11: follow this diagram to build the RF Attenuator board. The copper side of the board carries the SMDs plus four PC stakes to support the central shield plate (see photos below). The BNC input and output sockets are mounted on the other side of the board. Above: because RF signals are involved, the RF Attenuator must also be housed in a metal diecast case. Left: the RF Attenuator board is secured to the lid of the case via the BNC input and output sockets. Note how the central shield plate (consisting of blank PC board material) is supported by soldering it to four PC pins in the centre of the attenuator’s PC board. Use your most accurate DMM when making this adjustment because to a large extent, the accuracy of this setting will determine the accuracy of your RF Level Meter. That completes the initial set-up, although if you have access to a scope or a frequency counter you may want to check the PIC’s clock signal at TP2 and its TPG pin. You should find a 5V peak-to-peak square wave with a frequency very close to 2MHz. Main box assembly You are now ready to mount the main board assembly behind the lid of the main box (the lid becomes siliconchip.com.au the front panel). It attaches to the lid via four M3 x 15mm tapped spacers which are fastened using M3 x 6mm countersink-head screws. The board is then attached to the spacers using four pan-head M3 x 6mm screws. You will need to remove the upper mounting nut from switch S2 so that the threaded ferrule of S2 can pass up through its matching hole in the lid during this assembly. You also need to make sure that LEDs 1-3 are positioned so they pass up through their corresponding holes in the lid. If you have elected to mount S1 on the lid before this assembly, you’ll also need to ensure that its connection lugs or their extension wires pass down through their corresponding holes in the board. When this part of the assembly is complete, the top nut for S2 can be carefully refitted to the top of the switch ferrule and the lower nut and its lockwasher underneath carefully wound up to support the lid. Your meter’s main board assembly should now be complete and can be lowered into the box. This needs to be done with the righthand side angled downwards, so that the outer sleeve of RCA connector CON2 slips into its hole in the side of the box, allowing the lid assembly to be swung down as October 2008  41 Parts List 1 PC board, code 04210081 (146 x 84mm) 1 PC board, code 04210082 (43 x 44mm) 1 Jiffy box, UB1 size (158 x 95 x 53mm) 1 diecast aluminium box, 111 x 60 x 30mm 1 16x2 LCD module, Altronics type Z-7000A or Z-7011A (with backlight illumination) 4 M3 x 6mm tapped Nylon spacers 4 M3 x 15mm machine screws 1 SPST pushbutton switch, momentary (S1) 1 SPDT mini toggle switch (S2) 1 USB type B socket, PC-mounting (CON1) 1 RCA socket, PC-mounting (CON2) 1 2.5mm concentric DC socket, PC-mounting (CON3) 1 14-way SIL socket (half of 28pin IC socket) 1 14-way length of SIL terminal strip 1 18-pin IC socket 1 14-pin IC socket 4 M3 x 15mm tapped metal spacers 4 M3 x 6mm countersunk machine screws 5 M3 x 6mm pan head machine screws 5 M3 nuts, with star lockwashers 1 M3 Nylon washer 8 1mm-diameter PC board pins 1 PC-mount type A USB socket, PC-mounting (CON5) 1 panel-mount BNC socket 2 10mm long M3 tapped Nylon spacers 2 6mm long M3 machine screws with lockwashers 2 6mm long M3 countersunk machine screws 1 USB cable, standard type A to type B 1 P-type 5mm plastic cable clamp 1 10mm long M3 countersunk machine screw 1 M3 nut, with flat and star lockwashers well. The self-tapping screws supplied can then be used to fasten the lid assembly inside the box. the head-end into CON1 on the main board, then fit a 50W termination load plug to the RF input of the head-end so that it has a nominal RF input of “zero”. Now turn on the meter’s power switch (S2) and check the LCD readout after the greeting message has been replaced by the normal readings. Pay particular attention to the dBV reading, because initially you’ll probably find that it shows a figure rather higher than it should. After leaving it for a few minutes for the circuit to stabilise, try adjusting the “Intercept Adjust” trimpot (VR2) on the head-end carefully with a small screwdriver or alignment tool, to reduce the reading down to the lowest figure you can – ideally below -80dBV. Final adjustment Now we come to adjustment and calibration. To do this, you’ll need an RF signal generator which is able to supply an RF signal (preferably unmodulated) of known level. If you don’t have access to such a calibrated generator, an alternative is to use an uncalibrated RF oscillator with another RF measuring instrument of some kind to let you adjust its output to a convenient level – such as 1.0V RMS. The calibration process is quite simple. First, plug the cable from 42  Silicon Chip Semiconductors 1 AD8307AN log detector/amplifier (IC1) 1 LM324N quad op amp (IC2) 1 PIC16F88-I/P microcontroller (IC3) programmed with 0421008A firmware 1 LM317T adjustable regulator (REG1) 1 3mm green LED (LED1) 1 3mm orange/yellow LED (LED2) 1 3mm red LED (LED3) 2 1N4148 diodes (D1,D2) 1 1N4004 diode (D3) Capacitors 1 470mF 16V electrolytic 1 220mF 10V electrolytic 7 100nF monolithic 2 47nF ceramic, 1206 SMD chip Resistors (0.25W 1%) 1 1MW 3 2.0kW 2 220kW 2 1.5kW 1 68kW 2 330W 1 51kW 2 220W (0805 SMD) 1 33kW 1 200W 1 10kW 1 120W 1 6.8kW 1 100W 1 3.9kW 1 100W (0805 SMD) 1 3.0kW 1 18W 0.5W 1 2.4kW 1 10W 1 2.2kW 1 4.7W Trimpots 2 50kW mini horizontal trimpot (VR1,VR2) 1 100W mini horizontal trimpot (VR3) 1 10kW mini horizontal trimpot (VR4) Optional 20dB attenuator 1 PC board, code 04210083, 95 x 39mm 1 diecast aluminium box, 111 x 60 x 30mm 2 BNC sockets, panel-mounting 1 27kW resistor, 1206 or 0805 SMD chip 1 2.7kW resistor, 1206 or 0805 SMD chip 20 1kW resistor, 1206 or 0805 SMD chip 1 820W resistor, 1206 or 0805 SMD chip 5 330W resistor, 1206 or 0805 SMD chip 4 1mm-diameter PC pins The next step is to remove the 50W termination plug from CON4 and instead connect a cable from the output of your RF generator. Set the generator to some convenient frequency (say 100MHz) and of course with a known RF level – say 1V (0dBV). It’s then a matter of adjusting the ‘Slope Adjust’ trimpot (VR1) on the head-end unit – again with a small screwdriver – until you get a reading of +00.0dBV on the LCD. Once that’s done, your RF Level and Power Meter is finished, set-up and ready for use. Finally, note that you will have to power this device from a plugpack if you use the backlit LCD, as the current SC is too high for battery power. siliconchip.com.au