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VHF Receiver VHF Receiver For Weather Satellite For Weather Satellites Here’s a compact, low-cost 2-channel VHF FM receiver for pulling in weather satellite signals in the 137-138MHz band. It has good sensitivity and adjustable RF gain, combined with the correct bandwidth for weather satellite APT signals. You can also operate it from either a plugpack or a 12V battery, for greater flexibility. By JIM ROWE W HILE YOU CAN use a standard VHF scanner or communications receiver to pick up weather satellite signals, the results are often disappointing. The reason for this is that most scanners and communications receivers only provide a choice of two bandwidth settings for VHF FM reception: “narrow” and “wide”. The narrow setting 18  Silicon Chip gives a bandwidth of ±15kHz or less, which is fine for NBFM reception. However, it is too narrow for undistorted reception of the weather satellite signals which need a bandwidth of at least ±25kHz. By contrast, the wide bandwidth setting usually gives a bandwidth of about ±100kHz, so this is the setting that must be used. Unfortunately, this is really too wide for weather satellite signals and, as a result, the demodulated audio level is relatively low. At the same time, the wider reception bandwidth allows more noise through, so the signal-to-noise ratio can become quite poor. In short, for best results you really need a receiver with an effective bandwidth of ±30kHz, or not much more. This type of specialised VHF receiver is available but they are not very thick on the ground and those that are available are fairly pricey. Hence the motivation for developing the low-cost weather satellite receiver described here. As you can see from the photos, the receiver is built into a very compact plastic instrument box. All of the circuitry is mounted on a double-sided PC board, so it’s quite easy to build. It has switch tuning between two preset frequency channels, for ease of use. There are RF Gain, Audio Muting and Audio Gain controls and the receiver www.siliconchip.com.au Fig.1: block diagram of the Philips SA605D low-power FM mixer and IF system. It contains a local oscillator (LO) transistor, a balanced mixer, a high gain IF amplifier and IF limiter, a received signal strength (RSSI) detector, an FM quadrature detector and an audio muting circuit. can drive a small monitor speaker or headphones, as well as providing a line level signal to feed into your PC for recording and decoding. The sensitivity is quite respectable, at about 0.7µV for 12dB of quieting. At the same time, the effective bandwidth is approximately ±35kHz, which is quite suitable for weather satellite reception. Bear in mind though that for good reception of these signals, you really need to use a masthead preamp as well. The receiver provides 12V DC at the antenna connector, for “phantom powering” such a preamp. We’ll describe a matching preamp in the third of these articles, along with an easy to build turnstile/reflector antenna for 137.5/137.62MHz. Circuit description At the heart of the receiver is an SA605D IC, which is described by Philips as a high-performance low-power FM mixer and IF system. www.siliconchip.com.au As you can see from the block diagram of Fig.1, it contains a local oscillator transistor and balanced mixer, plus a high-gain IF amplifier and IF limiter, a received signal strength (RSSI) detector, an FM quadrature detector and finally an audio muting circuit. The local oscillator transistor can operate at frequencies up to about 500MHz in an LC circuit, or up to 150MHz with a suitable crystal. The mixer can operate up to 500MHz as well, while the IF amplifier and limiter can operate up to about 25MHz with a combined gain of about 90dB. That’s not bad when you consider it’s all packed inside a 20-pin small outline SMD package! Fig.2 shows the complete circuit details. In this receiver, we’re using the SA605D in a fairly conventional single-conversion superhet configuration, with the IF amplifier and limiter working at 5.5MHz. This allows us to take advantage of high selectivity 5.5MHz TV sound IF ceramic filters to provide most of our bandwidth shaping. The two filters in question are CF1 and CF2, which are both Murata SFT5.5MA devices. As shown in Fig.2, CF1 is connected between the mixer output and the IF amplifier input, while CF2 is connected between the IF amplifier output and the limiter input. The resistors connected to the filter inputs and outputs are mainly for impedance matching, while the 10nF capacitors are for DC blocking. The 90° phase shift required for IC1’s quadrature FM Main Features • • • • • Two presettable channels in the 137-138MHz band Sensitivity: 0.7µV for 12dB of quieting Bandwidth: ±35kHz (approx.) Plugpack or battery powered Provides 12V DC phantom power to power a masthead amplifier detector is provided by coil L4 and its parallel 390pF capacitor, which are tuned to 5.5MHz. The local oscillator transistor inside IC1 is connected in a Colpitts circuit. This includes coil L3, together with the two 15pF capacitors (which provide the emitter tap) and a 10pF capacitor in series with varicap diode VC3. Varicap diode VC3 is the receiver’s tuning capacitor. Its tuning voltage for each of the two channels is set by 10-turn trimpots VR4 and VR5, with switch S1 selecting between them. We can tune the receiver simply by changing the local oscillator frequency because we only need to tune over a relatively small range (ie, 137.3 - 137.85MHz maximum), which is within the selectivity curve of the “front end” tuned circuits. Moving now to the front end, this uses a BF998 dual-gate MOSFET (Q1) connected in a standard cascode amplifier configuration. The incoming December 2003  19 Parts List 1 PC board, code 06112031, 117 x 102mm (double-sided, not plated through) 1 small instrument case, 140 x 110 x 65mm 2 5.5MHz ceramic filters, Murata SFTRD5M50AF00-B0 2 RCA sockets, 90° PC-mount 1 2.5mm concentric power socket, PC-mount 1 3.5mm stereo headphone jack, PC-mount 1 SPDT miniature toggle switch 3 PC board terminal pins, 1mm diameter 1 TO-220 heatsink, 19 x 18.5 x 9mm 3 small skirted instrument knobs, 15mm diameter 1 coil former, 4.83mm OD with F16 ferrite slug 1 6-pin former base and screening can 1 short length of 0.25mm enamelled copper wire for RFC1 1 F29 ferrite bead (for RFC1) 1 short length of 0.8mm tinned copper wire for L1 1 length of 0.8mm enamelled copper wire for L2, L3 8 4g x 6mm self-tapping screws 1 M3 x 6mm machine screw, M3 nut and lock washer 1 20 x 92mm piece of 0.3mm tinplate for shield Semiconductors 1 SA605D mixer/IF amplifier/FM detector (IC1) 1 TL072 dual op-amp (IC2) 1 LM386 audio amp (IC3) 1 7812 +12V regulator (REG1) 1 78L05 +5V regulator (REG2) 1 BF998 dual-gate MOSFET (Q1) 1 PN100 NPN transistor (Q2) 1 5.1V 400mW zener diode (ZD1) 1 3mm red LED (LED1) 1 3mm green LED (LED2) 1 ZMV833ATA varicap (VC3) VHF signals are fed into a tap (for impedance matching) on antenna coil L1, which is tuned to about 137.55MHz using trimmer capacitor VC1. The signal from the top of this tuned circuit is then fed directly to gate 1 of Q1, while gate 2 is bypassed to ground but also fed with an adjustable DC voltage via VR1 for RF gain control. 20  Silicon Chip 1 1N4004 1A power diode (D1) Capacitors 1 2200µF 16V RB electrolytic 1 470µF 25V RB electrolytic 1 330µF 16V RB electrolytic 3 10µF 16V RB electrolytic 1 10µF 35V TAG tantalum 1 470nF MKT metallised polyester 8 100nF multilayer monolithic ceramic 1 47nF MKT metallised polyester 1 22nF MKT metallised polyester 5 10nF multilayer monolithic ceramic 1 4.7nF MKT metallised polyester 7 2.2nF disc ceramic 2 2.2nF SMD ceramic 1 1nF disc ceramic 1 1nF MKT metallised polyester 1 390pF NPO ceramic 2 15pF NPO ceramic 1 10pF NPO ceramic 2 3-10pF trimcaps (VC1, VC2) Resistors (0.25W 1%) 1 470kΩ 1 1.8kΩ 1 390kΩ 1 1.5kΩ 1 150kΩ 1 1.2kΩ 1 110kΩ 4 1kΩ 1 100kΩ 1 360Ω 1 47kΩ 1 300Ω 1 39kΩ 1 240Ω 3 22kΩ 1 220Ω 2 10kΩ 2 100Ω 1 5.6kΩ 1 47Ω 1 4.7kΩ 1 22Ω 2 2.2kΩ 1 10Ω Potentiometers 1 50kΩ linear pot, 16mm PC board mount (VR1) 1 25kΩ linear pot, 16mm PC board mount (VR2) 1 50kΩ log pot, 16mm PC board mount (VR3) 2 50kΩ 10-turn trimpots, PC board mount (VR4,VR5) The amplified VHF signal on Q1’s drain is fed to pin 1 of IC1 via a 1nF coupling capacitor. Additional RF selectivity is provided by coil L2 and trimmer capacitor VC2, which are again tuned to about 137.55MHz. The 100Ω resistor and 10µH RF choke form an untuned high-impedance load for Q1. Notice that as well as being coupled to the tap on L1 via a 2.2nF capacitor, the antenna input is also connected to the +12V supply line via RFC1 and a series 22Ω resistor. As you may have guessed, these components are there to provide “phantom” DC power for the masthead preamp. At the output end of IC1, we take the demodulated APT signals from the “muted audio” output at pin 8. This allows us to take advantage of the SA605’s built-in muting circuit, which works by using comparator stage IC2b to compare IC1’s RSSI output from pin 7 (proportional to the logarithm of signal strength) with an adjustable DC control voltage from muting pot VR2. When the RSSI voltage rises above the voltage from VR2, IC2b’s output switches high and this is fed to pin 5 of IC1 via a 2.2kΩ series resistor to unmute the audio. ZD1, a 5.1V zener diode, limits the swing on pin 5 of IC1 to less than 6V. Transistor Q2 and LED1 form a simple signal strength indicator. This also uses the RSSI output from IC1. In operation, the voltage across the 390kΩ resistor and 100nF capacitor rises from about +0.26V under no-signal conditions to about +5V with a very strong input signal. So with Q2 connected as an emitter follower and LED1 in its emitter load, the LED current and brightness are made to vary quite usefully with signal strength. Low-pass filter The demodulated APT signal from pin 8 of IC1 is first fed through op amp IC2a, which is configured as an active low-pass filter. This has a turnover frequency of 5kHz and is used for final de-emphasis and noise reduction. From there, the signal is fed to audio gain control VR3 and then to audio amplifier stage IC3. This is a standard LM386 audio amplifier IC, configured for a gain of about 40 times. Its output is fed to both the monitor speaker socket and to a line output socket for connection to your PC’s sound card. Power supply Most of the receiver’s circuitry operates from +12V, with the exception of IC1 which needs +6V. As a result, the power supply circuitry includes REG1 to provide a regulated and smoothed +12V supply from an external supply such as a 14.5-18V plugpack. This is followed by 5V regulator REG2 which www.siliconchip.com.au www.siliconchip.com.au December 2003  21 Fig.2: the complete circuit diagram for the VHF Weather Satellite Receiver. Dual-gate MOSFET Q1 functions as an RF amplifier stage with adjustable gain. Its output is fed into IC1 and the demodulated output from IC1 fed to low-pass filter stage IC2 and then to audio output stage IC3. The local oscillator (LO) inside IC1 is tuned using VR4, VR5 and varicap diode VC3. Fig.3: install the parts on the top of the PC board as shown here. The red dots indicate where component leads and “pin-throughs” have to be soldered on both sides of the PC board. Note that S1 is not directly soldered to the board but is instead connected to three PC stakes using flying leads. has its output “jacked up” using 300Ω and 47Ω resistors to provide close to +6V for the SA605D (IC1). Note that if you want to run the receiver from a 12V battery, this can be quite easily done by replacing REG1 with a wire link. In addition, the 2200µF capacitor should be replaced with a 16V zener diode (ZD2) for over-voltage protection. Construction Construction is straightforward with virtually all of the parts mounted on a small PC board coded 06112031 (117 x 102mm). The board is double-sided but the top copper pattern is used mainly as a groundplane. This means that the board doesn’t need to have plated-through holes but there are quite a few component leads which do have to be soldered on both sides of the board. 22  Silicon Chip There are also a number of “pinthrough” wires which have to be soldered to both sides of the PC board around IC1. These connect the earth patterns on both sides of the board and ensure that this high-gain chip operates in a stable manner. Fig.3 shows the assembly details. As shown, the various input and output connectors are mounted along the rear edge of the board, while the controls and indicator LEDs mount along the front edge. The only component not actually mounted on the board is S1, the channel select toggle switch. This mounts on the front panel, with its three connection lugs wired to PC board terminal pins directly underneath using very short lengths of insulated hookup wire. Start the assembly by fitting these three terminal pins first (they are the only pins used in the receiver), then Fig.4: here’s how to install the three surface-mount parts (Q1, IC1 & VC3) on the underside of the board. Q1 and VC3 can be held in position using epoxy resin to make soldering easier – see text. fit the project’s only wire link, which goes on the righthand side of the board just to the left of IC2. Note that this the link must be insulated, as it carries +12V and passes over ground­plane copper. Next, fit the four connectors CON1CON4 along the rear edge, followed by the resistors. Table 1 shows the resistor colour codes but it’s also a good idea to check each value using a digital multimeter before soldering it in position. All resistors are fitted to the top of the PC board but note that some of them have one lead soldered to the top copper as well as the bottom copper. This is indicated by the red dots on the overlay diagram. Once the resistors are in, you can fit the “pin-throughs” using some of the resistor lead offcuts. The location of these “pin-throughs” are again indicated by the red dots on the layout www.siliconchip.com.au This is the view inside the assembled receiver. Note that a tinplate shield is fitted around L2, VC2 and most of the components in Q1’s drain circuit (see text). Note also that the metal bodies of the potentiometers are connected together using tinned copper wire and then connected to the groundplane copper on the PC board. diagram. Each “pin-through” is fitted by simply passing a wire through the hole in the board, then soldering it on both sides and trimming off the excess lead lengths. The small ceramic capacitors can now all be installed on the lefthand side of the board. Note that some of these also have their “cold” leads soldered on both sides of the board, as indicated by the red dots. Once they’re in, install the MKT capacitors and the electrolytics, making sure that the latter are all correctly orientated. Now for trimmer capacitors VC1 and VC2. These should be fitted so that their adjustment rotors are connected to earth (this makes it much easier to www.siliconchip.com.au align the receiver later). It’s simply a matter of orientating them on the board as shown in Fig.3. A small number of receivers have needed a small earthed shield plate over the top of the IF chip, to keep it from taking off. Similarly, one or two have needed 47nF SMD bypass capacitors from the source of the RF amplifier to ground, to keep it stable. RF chokes RFC2 and RFC3 are both supplied pre-wound (10µH and 68µH respectively) but RFC1 needs to be wound on an F29 ferrite bead. It’s very easy to wind though, because it requires only two turns of 0.25mm enamelled copper wire. Winding the coils At this stage, it’s a good idea to wind and fit the remaining coils. Table 3 gives the winding details. As shown, L1-L3 are air-cored types, each consisting of five turns of 0.8mm dia­ meter wire wound on a 5mm mandrel. Note, however, that L1 is wound using tinned copper wire, while L2 and L3 are both wound using enamelled copper wire. Don’t forget to scrape off the enamel at each end, so they can be soldered to the board pads. L1-L3 should all be mounted so that their turns are about 2mm above the board. After you’ve fitted L1, don’t forget to fit its “tap” connection lead as well. This can be made from a resistor lead off-cut, since it’s very short. It connects to a point 1/3 of a turn up from the “cold” (earthy) end of the coil – ie, just above half-way up the side of the first turn. December 2003  23 Table 3: Coil Winding Details to scrape away the passivation from the pot bodies in order to get good solder connections. Mounting the semiconductors The final coil to wind is quad detector coil L4. Unlike the others, this is wound on a 4.83mm OD former with a base and a copper shield can. It’s wound from 20 turns of 0.25mm enamelled copper wire and tuned with an F16 ferrite slug. Once L4 is wound, fitted to the board and covered with its shield can, you can fit the two ceramic filters CF1 and CF2. These devices can be fitted either way around but make sure that their pins are pushed through the board holes as far as they’ll go before you solder them underneath. The next step is to fashion and fit the small tinplate shield at the location shown in the overlay diagram – ie, around L2, VC2 and most of the components in the drain circuit of Q1. This shield is U-shaped and measures 20mm high, with the front and back “arms” 36mm long and the side section 20mm long. The bottom edges of all three sides are soldered to the board’s groundplane in a number of places, to hold it firmly in position and to ensure it stays at earth potential. to connect their metal shield cans together and then run a lead to the board’s top copper to earth them. This is done using a length of tinned copper wire, with a short length of insulated hookup wire then connecting them to the board copper at front right (see photos). Note that it will be necessary Fitting the pots The two 10-turn trimpots (VR4 and VR5) can now be soldered in position at the front-centre of the board. They can then be followed by the three main control pots, which are all 16mm dia­ meter types. Trim each pot’s spindle length to about 9mm before fitting it and make sure you fit each one in its correct position as they are all different. In particular, note that VR1 and VR3 both have a value of 50kΩ but VR1 is a linear pot while VR3 is a log type. After fitting the pots, it’s a good idea 24  Silicon Chip This view of the underside of the PC board shows the locations of the three surface-mount devices (SMDs). Refer to the text for the mounting details. Now you should be ready to fit the semiconductor devices – or at least those that go on the top of the board. Begin by installing diode D1, 5.1V zener diode ZD1 and transistor Q2. That done, install regulator REG1 (if you’re using it) and its associated heatsink, as shown in Fig.3. These parts are secured to the board using a 6mm-long M3 screw, nut and lockwasher. Note that REG1’s centre pin should be soldered on both sides of the board but take care not to touch either of the two large adjacent electrolytic capacitors with the barrel of your soldering iron. Next, fit regulator REG2, followed by IC2 (TL072) and IC3 (LM386). Note that pin 4 on both these devices should be soldered to the copper on the top of the board as well as the bottom. The two LEDs (LED1 & LED2) are both mounted horizontally, so that they later protrude through matching 3mm holes in the front panel. Note that they are both fitted with their cathode leads towards the left. Bend their leads down through 90° about 5mm from the LED bodies, then solder them in position so that the axis of each LED is 5mm above the board. The final components to fit are the surface-mount parts, which all fit underneath the board – see Fig.4. We’re talking here of varicap diode VC3 (ZMV833ATA), the BF998 dual-gate MOSFET (Q1), and the SA605D IC (IC1). The first two in particular are in very tiny packages and need very careful handling. In fact, these very small devices are not easy to hold in position while you solder them but there is a way around this. The trick is to mix up a small amount of 5-minute epoxy resin cement (Araldite or similar) and then apply an extremely small “dot” of epoxy to the underside of the board at each component position (if you use the end of a resistor or diode lead offcut as the cement applicator, this should apply about the right amount). It’s then just a matter of using tweezers to carefully place each component in its correct position over the epoxy “dots”, with the correct orientation. When you’re satisfied that they’re all located accurately, carefully put the www.siliconchip.com.au Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o o o o o o o o o No.   1   1   1   1   1   1   1   3   2   1   1   2   1   1   1   4   1   1   1   1   1   1   1   1 Value 470kΩ 390kΩ 150kΩ 110kΩ 100kΩ 47kΩ 39kΩ 22kΩ 10kΩ 5.6kΩ 4.7kΩ 2.2kΩ 1.8kΩ 1.5kΩ 1.2kΩ 1kΩ 360Ω 300Ω 240Ω 220Ω 100Ω 47Ω 22Ω 10Ω board aside for 10 minutes or so to let the adhesive cure. After this, you can solder their leads to the PC pads without having to worry about them moving. IC1 can be mounted in the same way if you like but it’s not nearly as small as the other two parts and so isn’t as difficult. The main thing to watch out for here is that you don’t create solder bridges when you’re soldering its leads, as they’re spaced at just 1.25mm. Make sure you use a clean fine-tipped soldering iron for this job and work quickly so that you don’t overheat either the IC or the copper pads on the board. After soldering all three SMD devices in place, it’s a good idea to inspect them very carefully using a magnifying glass. Check that all joints have been made correctly and that there are no solder bridges. Final assembly The completed PC board is housed in a low-profile plastic instrument case. If you purchase a kit, this will probably come with all holes predrilled. If not, you will have to drill the front and rear panels yourself using Figs.4 & 5 as drilling templates. www.siliconchip.com.au 4-Band Code (1%) yellow violet yellow brown orange white yellow brown brown green yellow brown brown brown yellow brown brown black yellow brown yellow violet orange brown orange white orange brown red red orange brown brown black orange brown green blue red brown yellow violet red brown red red red brown brown grey red brown brown green red brown brown red red brown brown black red brown orange blue brown brown orange black brown brown red yellow brown brown red red brown brown brown black brown brown yellow violet black brown red red black brown brown black black brown By the way, it’s always best to drill a small pilot hole at each location and then carefully enlarge it to size using a tapered reamer. As well as the holes shown, you might also want to drill small “blind” holes in the rear of the front panel to mate with the locating spigots on VR1, VR2 and VR3, and the spigot on the backing washer for toggle switch S1. Once the panels have been prepared, you can mount switch S1 on the front panel and connect three 30mm lengths of insulated hookup wire to the three pins on the PC board via. That done, the front panel can be mated with the PC board by positioning it on the three pot ferrules and doing up the nuts. The three leads from the PC board pins can then be soldered to the switch lugs. The rear panel is not attached to the board assembly. Instead, it simply slips over CON1 and CON2 and is then slid into the rear slot when the assembly is fitted into the bottom half of the case. Finally, the completed assembly is fastened in place using eight 6mmlong 4-gauge self-tapping screws. Does it work? Now for the final checkout, to make 5-Band Code (1%) yellow violet black orange brown orange white black orange brown brown green black orange brown brown brown black orange brown brown black black orange brown yellow violet black red brown orange white black red brown red red black red brown brown black black red brown green blue black brown brown yellow violet black brown brown red red black brown brown brown grey black brown brown brown green black brown brown brown red black brown brown brown black black brown brown orange blue black black brown orange black black black brown red yellow black black brown red red black black brown brown black black black brown yellow violet black gold brown red red black gold brown brown black black gold brown Table 2: Capacitor Codes Value 470nF 100nF 47nF 22nF 10nF 4.7nF 2.2nF 1nF 390pF 15pF 10pF μF Code EIA Code 0.47µF   474 0.1µF   104 0.047µF   473 0.022µF   223 0.01µF   103 0.0047µF  472 0.0022µF  222 0.001µF   102    –   391    –    15    –    10 IEC Code 470n 100n   47n   22n   10n   4n7   2n2    1n 390p   15p   10p sure it’s working properly. First, turn all three front-panel pots fully anticlockwise, then apply power from a suitable 14-18V DC plugpack (or a 12V battery). Check that the green power LED immediately begins glowing. If it does, check the voltage on REG1’s output lead (ie, the righthand lead) with your DMM – it should be very close to +12V with respect to ground. Similarly, the voltage at REG2’s (righthand) output pin should December 2003  25 The receiver is easy to drive, with just four front-panel controls. These are (from left to right): RF Gain, Channel Select, Muting Level and Audio Gain. In addition, there are two holes in the front panel to provide screwdriver access to the 10-turn pots (VR4 & VR5) during alignment. measure very close to +6V. If you now plug an 8Ω speaker into CON3 and then turn up audio gain control VR3, you should hear a small amount of hiss and noise. When you turn up the RF gain control VR1 as well, this noise should increase a little further but LED1 shouldn’t begin glowing except only very faintly when VR1 is turned fully clockwise. Now turn VR1 fully anticlockwise again and use your DMM to measure the DC voltage at the top of the 390kΩ resistor located just behind transistor Q2 (ie, to the right of 5.1V zener diode ZD1). The voltage across this resistor should be less than 0.30V and preferably about 0.26V. If it’s any higher than 0.30V, the IF amplifier in IC1 may be unstable. Assuming that your receiver has passed all these tests, it should be working correctly and is now ready for alignment. Receiver alignment 26  Silicon Chip A final “touch up” alignment of the receiver is best done with a satellite signal. However, you need to give it a basic alignment first so that you can at least find the signal from a satellite when it’s within range. For the basic alignment, you’ll need access to a frequency counter capable of measuring up to 150MHz and an RF signal generator which can be set to give an output at 137.50MHz and at 137.62MHz. It should be able to provide either unmodulated (CW) output or frequency modulation, with a modulating frequency of 2.4kHz and a deviation of ±25kHz or thereabouts. If the generator can’t be accurately set to the above frequencies, you’ll need to use the frequency counter to help set its frequency. You’ll also need your DMM during the “tuning-up” process, to monitor received signal level. The first step is to set the local oscillator frequencies for the two reception channels. This is done by adjusting trimpots VR4 and VR5 respectively, while measuring the oscillator’s frequency with the frequency counter. The oscillator signal is coupled to the counter via a “sniffer” coil which is connected to the end of a coaxial cable. The other end of this cable is then connected to the counter’s input. Note that there is no direct physical connection between the oscillator coil and the counter’s sniffer coil. Instead, the sniffer coil is placed about 9mm in front of oscillator coil (L3) and roughly on-axis (ie, just in front of the 10pF capacitor). The sniffer coil can be made by winding four turns of 0.8mm enamelled copper wire on a 5mm drill shank. Its ends can then soldered ends to a BNC socket which is then connected to the end of the counter input cable (see photo). This arrangement picks up enough oscillator energy to give reliable counter readings, without needing to be any closer to L3 (to avoid “pulling” the frequency). Assuming you want to receive the NOAA satellites, set the oscillator frequency for channel A to 132.0MHz (using VR4), and the frequency for www.siliconchip.com.au You can make a sniffer coil for your frequency counter by winding four turns of 0.8mm enamelled copper wire on a 5mm drill shank. Its ends can then be soldered to a BNC socket which is then connected to a plug on the end of a coaxial cable. The other end of the cable is then connected to the frequency counter. channel B to 132.12MHz (using VR5). These correspond to reception frequencies of 137.5MHz for NOAAs 12 & 15 and 137.62MHz for NOAA 17. If you want to try for other satellites, you’ll need to find out their APT frequency and set the oscillator frequency to 5.5MHz below that figure instead. Peaking the RF stage Once the oscillator frequencies have been set, the next step is to peak up the RF stage tuned circuits. This is done by setting your RF signal generator to produce an unmodulated (CW) signal at 137.5MHz, initially with a level of about 30µV. That done, connect the generator’s output to the antenna input of the receiver, using a series DC blocking capacitor if the generator doesn’t have one (so that the generator doesn’t short out the +12V phantom power for the masthead amplifier). Next, connect your DMM (set to the 5V DC range) across the 390kΩ resistor just behind Q2 and make sure switch S1 is set to the channel A position. Now turn up RF gain control VR1 to about midway and use an alignment tool or a very small jeweller’s screwdriver to adjust trimcap VC2 until you find a peak in the voltage reading www.siliconchip.com.au Fig.5: here are the full-size (top and bottom) etching patterns for the PC board. on the DMM. If you can’t find a peak, you may have to pull the turns on coil L2 slightly further apart to reduce its inductance. Once the peak is found, adjust VC2 carefully to maximise the DMM reading (the DMM is reading the RSSI voltage from IC1, so it’s essentially showing the received signal strength). When you’re happy that the L2/VC2 circuit is tuned to 137.5MHz, check the actual voltage reading of the DMM. If it’s more than 2.5V, reduce the output level from the RF generator until the December 2003  27 Fig.6: these two full-size artworks can be photocopied and used as drilling templates for the front and rear panels. DMM reading drops to about 2.0V. You’re now ready to peak the receiver’s input tuned circuit – ie, L1 and VC1. This is done in exactly the same way as for L2 & VC2. Just adjust VC1 slowly until the DMM indicates a peak and then carefully set VC1 for the maximum peak reading. If you can’t find another peak, you may need to pull the turns of L1 slightly further apart as before. Peaking the quadrature coil The final alignment step is to set the slug in quadrature detector coil L4 to the correct position for optimum FM demodulation of the 5.5MHz IF signals. This is done by first switching the signal generator so that it’s still producing a 137.5MHz signal but this should now be frequency modulated – preferably with a 2.4kHz tone and a deviation of about ±25kHz. That done, connect an 8Ω speaker to the receiver’s speaker socket (CON3) and turn up the audio gain control (VR3) to about the 10-o’clock position. You may not be able to hear the 2.4kHz modulating signal at this stage but in any case, slowly and carefully adjust the slug in L4 using a non-magnetic alignment tool. Sooner or later you’ll start to hear the 2.4kHz tone and you should also be able to tune the coil for maximum audio level and minimum distortion and noise. Once this has been done, the basic alignment of your weather satellite receiver is finished and it’s ready for final alignment using the signals from a weather satellite. But before you’ll be able to do this, you’ll need to build a suitable antenna and masthead preamp. They’ll be described in another article next month. SC This view shows the rear panel layout. There are two RCA sockets (one at each end) for the antenna and audio output signals, a 2.5mm DC power socket and a 3.5mm stereo jack socket for the loudspeaker. 28  Silicon Chip www.siliconchip.com.au