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Using Cheap Asian Electronic Modules – by Jim Rowe W A TINY Digital RF Power Meterr Mete The new little Digital RF Power MeThe block diagram of Fig.1 shows ould you like to measure the RF power output from oscil- ter module we’re looking at in this arti- that it’s really quite straightforward in lators or low power transmit- cle is quite similar to our 2008 design, terms of circuitry. The RF input is terminated with close to 50Ω and then fed ters operating at frequencies between at least in terms of its functionality. For example, it again uses an AD8307 straight into the AD8307, which con100kHz and 500MHz? This very small module from Bang- logarithmic amp/detector at the input, verts it into a DC output voltage varygood (China) will let you do just that. and then uses a MCU to process the ing between about 100mV and 2.5V in It has a 16x2 LCD readout and gives readings and display them on a 16x2 proportion to the logarithm of the RF input level. quite accurate readings for signals be- LCD readout. But in this case the MCU is an STC The AD8307’s nominal conversion tween +16.0dBm (40mW) and -75dBm 12C5A60S2 rather than a PIC16F88, characteristic is shown in Fig.2. It’s (32pW). and the whole circuit is on a single very close to a straight line between All for less than $50! Back in the October 2008 issue of PCB measuring only 59 x 57mm. So input levels of -75dBm and +18dBm, SILICON CHIP, we described a Digital RF it’s much more compact than our 2008 with a slope of 25mV per dB (decibel). design, as you can see from the photos. The Analog Devices data sheet for the Level and Power Meter project. AD8307 shows the It used an OFF – ON Logarithmic ConAnalog Devices DC INPUT AMS1117-5.0 formance as ±0.5dB. AD8307AN loga(6–12V) +5V IN OUT This DC output rithmic amplifier/ signal is then fed detector IC at the GND 16 x 2 into one of the ADC input, followed LCD MODULE (analog-to-digital by a digital meterconverter) inputs ing circuit with a RF of the 12C5A60S2 programmed PIC INPUT MCU, where it is first 16F88 microcon- (SMA) IN+ AD8307 digitised with a resotroller (MCU) driv3V3 DC–500MHz ADC IN 47 GND LOGARITHMIC lution of 10 bits. The ing a 16x2 LCD IN– STC 12C5A60S2 Rx AMP/DETECTOR MCU 11.0592MHz firmware in the MCU readout. Tx (ENHANCED 80C51 CPU) L1 then uses this digital The frequency value to calculate range was from bethe equivalent RF low 50kHz to over ENTER SUB ADD SC input level, which it 500MHz, with a 2020 displays on the LCD measuring range Fig.1: the block diagram of the digital RF Power Meter. Ours came from readout. from +20dBm Banggood (China) but it is no doubt also available from other sources. Fig.1 shows that down to -60dBm. siliconchip.com.au Australia’s electronics magazine July 2020  27 Fig.2 (right): the nominal conversion characteristic of the AD8307 logarithmic converter. It’s very close to a straight line between -75dBm and +18dBm, with a slope of 25mV/dB. the module is designed to accept a DC input of between 6V and 12V, and uses an AMS1117-5.0 regulator to provide the rest of the module with a regulated 5.0V supply. Incidentally there’s also a reverse-polarity protection diode across the input of the regulator, to ‘take the fall’ and protect the rest of the components in the event of the power supply being connected with reversed polarity. The current drain of the module is less than 45mA (including the current drawn by the LCD backlighting). Note that the MCU is provided with an 11.0592MHz crystal for its master clock. It also has three small pushbutton switches connected to three of its I/O pins, with the switches labelled ENTER, ADD and SUB. These allow you to move the reference level up or down in increments of 1dB, to correct for any known variations in the meter’s log-law characteristic at a particular frequency. More about this a little later. Checking it out I was interested in putting this little meter through its paces, if only to compare its performance with that of our 2008 project. So I powered up my RF signal generator, connected the input of the RF Power Meter module directly to the generator’s output (to negate cable losses) and began plotting its performance at various frequencies in the claimed range of 100kHz - 600MHz, for 10 different power levels: +13dBm, 0dBm, -10dBm, -20dBm, -30dBm, -40dBm, -50dBm, -60dBm, -70dBm and -75dBm. (I couldn’t test at +16dBm, because the maximum output of my generator is +13dBm.) It took quite a while, but the results were quite impressive, as shown in the curves of Fig.3. 28 Silicon Chip 3.0 RF INPUT AT 10MHz 2.5 RF INPUT AT 100MHz AD8307 OUTPUT VOLTAGE (Left): how big is it? This shot of the power meter is same size – 57 x 59mm. It can be powered from 6 to 12V DC. 2.0 RF INPUT AT 500MHz 1.5 RF INPUT AT 300MHz 1.0 0.5 DASHED LINE HAS A SLOPE OF 25mV/dB SC  0 –80 –70 2020 For the upper seven power levels, the indicated power levels are within the ±0.5dB tolerance bands (shown in yellow), for frequencies between 500kHz and 300MHz – and in many cases between 200kHz and 300MHz. The indications do fall off above 300MHz, though, and are often about 4dB low at 500MHz and about 7-8dB low at 600MHz. They also fall off below 200kHz (this could probably be remedied by increasing the value of the AD8307’s input coupling capacitors). At the three lowest power levels (-60dBm, -70dBm and -75dBm) the low-end performance falls away earlier. But overall, the new RF Power Meter’s performance over the frequency range 500kHz – 300MHz compared very well with that of our 2008 project. And if you want to use it to make measurements at frequencies above 300MHz, you could do so by making use of those ENTER/ADD/SUB buttons to correct the readings. For example if you want to make measurements at 450MHz, you could use the buttons to add 3dB to the readings. You may have noticed in the pictures that the Meter’s LCD display Australia’s electronics magazine –60 –50 –40 –30 –20 –10 0 10 20 RF INPUT LEVEL (dBm) has an indication at the RH end of the second line, showing any correction figure that may be active for current readings. For example if you don’t enter any correction figure, it will display ‘AT:00’ after the power reading. This is the default figure, by the way. But if you use the buttons to add say 20dB to the readings to allow for a 20dB attenuator you have connected to the input, it will display ‘AT:20’. So the bottom line is that the performance of this tiny little RF Power Meter compares quite well with that of our 2008 project. Radio amateurs, hobbyists and service technicians should therefore find it a handy addition to their test instruments – especially considering its low price. Before closing I should note that you will find this RF Power Meter module on the Banggood website, (www. banggood.com). ID no. is 1221705. At the exchange rate in early June 2020, it was priced at about AU$40.50 plus $3.50 for shipping via Air Parcel. What next? I suppose my only real reservation about this tiny RF Power meter is that siliconchip.com.au +20 +13 +13dBm INPUT 1.0V +10 710mV 0.0dBm INPUT 0 RF LEVEL in dBm (and volts RMS into 50 ) 224mV –10dBm INPUT –10 71mV –20dBm INPUT –20 22.4mV –30dBm INPUT –30 7.1mV –40dBm INPUT –40 2.24mV –50dBm INPUT –50 710 V –60 –60dBm INPUT –70 –70dBm INPUT 224 V 71 V –75dBm INPUT –75 39.8 V ZERO INPUT READING = –78.3dBm –80 22.4 V 100kHz 200 500 1MHz 2 5 10MHz FREQUENCY 20 50 100MHz 200 500 1GHz (YELLOW BANDS INDICATE ±0.5dB DEVIATION) Fig.3: the measured performance of the RF Power Meter we reviewed, at 10 different input power levels and at frequencies between 100kHz and 700MHz. At most power levels the performance is very good between about 500kHz and 300MHz. even though it’s listed as “600MHz”, realistically its maximum frequency is more like 450-500MHz. It would nice if the people who make this module came up with anoth- siliconchip.com.au er version using one of the AD8307’s more agile sister chips, like the AD8317 or the AD8318. Either of these should lift the maximum frequency to at least 8GHz – a Australia’s electronics magazine very useful extension. You may recall that I reviewed an RF Detector module using the AD8318 in the March 2018 issue of SILICON CHIP, and found it an impressive performer. This chip could be used to produce an RF Power Meter like the one we’ve looked at in this article, but with a much wider frequency range. It would need some changes to the MCU’s firmware, since the output voltage of both the AD8317 and the AD8318 has a negative slope, compared with the positive slope with the AD8307 (as shown in Fig.2). SC July 2020  29