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Review by Andrew Woodfield R80 Synthesised Aviation Band Receiver Kit This moderately priced receiver kit (about $50) is easy to build, simple to use and ideal for monitoring local airport traffic. It uses digital frequency synthesis for excellent stability and ease of tuning, and has a digital frequency readout. C ommercial aviation uses HF, VHF or even satellite frequencies to serve their communication needs. The majority of voice calls use the 118136MHz VHF aviation band. This band extends to 137MHz in many countries, including Australia. Conversations between pilots and airport towers, air traffic controllers, ground services and local aero club aircraft traffic are all routinely heard on this band. It has long been a very popular band for those interested in monitoring local radio services. Amplitude modulation (AM) is used on this VHF band, rather than either frequency modulation (FM) or one of the new digital modes, which are usually encountered with commercial VHF and UHF mobile radio or amateur (‘ham’) radio services. While some perceive AM as outdated, it improves communications safety and has proven to be very reliable over many decades. Even today, AM is also surprisingly spectrally efficient. This R80 aviation band receiver is a recent entry targeting this band. Offered primarily as a DIY kit, it features a digital phase-locked loop (PLL) tuning system and digital display on a compact 120 x 85mm PCB. This kit offers several improvements over older aviation receiver kits, which typically used traditional analog tuning and lacked any form of frequency display. 40 Silicon Chip Kit delivery While it is available from various internet suppliers (including on eBay and Amazon), I bought mine from a seller on AliExpress. It was well-packed with all of the parts and PCBs in plastic bags. A couple of layers of bubble wrap had been wrapped around the kit before placing it inside a cardboard box. The parts supplied are of good quality, with the seven ICs shipped in pin-protecting foam. It is not antistatic foam, but that’s still a lot better than getting a bag full of loose ICs (and that is, sadly, all too common when you order from places like AliExpress these days). Most of the chips supplied are not static-sensitive, only the PIC microcontroller. Assembly instructions must be obtained by email from the kit supplier. These were in Chinese, but most details were fairly obvious. The schematic, also partly annotated in Chinese, was included in these instructions. A detailed English translation is available can be downloaded from siliconchip.com.au/Shop/6/5950 Three PCBs are supplied in the kit: the main receiver PCB, a smaller display PCB and a PLL PCB. SMD parts are pre-fitted on these PCBs, saving builders from any anxiety on that issue. One minor point: the 7-segment LED display driver SMD IC had its part Australia’s electronics magazine number sanded off. If it fails, finding a replacement could be a problem. Checking against the parts list in the instructions revealed that two parts were missing: a 100μF capacitor and a 10-way right-angle pin-strip connector for the display PCB. Three extra ceramic capacitors were supplied. To avoid delay, I purchased replacements from a local retail supplier and set the extra parts aside. How the receiver works Fig.1 shows a block diagram of the receiver. It’s a double-conversion superhet with a first intermediate frequency (IF) of 10.7MHz and second IF of 455kHz. The incoming signal passes first through a bandpass filter (BPF) and the NE5204 10dB gain RF amplifier, then into the first mixer, an NE602. The oscillator for this mixer uses the popular Si5351a digital PLL chip. Its 25MHz reference crystal delivers both excellent stability and tuning accuracy. One of the three square-wave outputs of this chip is filtered via a five-pole low-pass filter to give the desired sinewave signal for the mixer. A Motorola MC3361 FM mixer/ demodulator chip contains the second mixer. This converts the 10.7MHz first IF signal down to the second IF of 455kHz using a 10.245MHz crystal oscillator. The receiver’s selectivity is mainly provided by 15kHz bandwidth 455kHz ceramic IF filter. siliconchip.com.au Fig.1: the block diagram of this doubleconversion superhet AM receiver. It features a four-digit LED display coupled with a stable phase-locked loop and digital volume control. Since the MC3361’s FM IF chain cannot detect AM signals, the receiver’s IF amplifier chain and AM detection is handled by a TA7640. This also supports a ‘signal level’ red LED indicator. Its brightness depends on the strength of incoming signals. The detected receiver audio is then passed to both the LM386 audio amplifier and the receiver squelch circuit via a two-channel software-controlled FM62429 audio attenuator chip. The receiver audio output can be muted until the squelch circuit detects a signal. The MC3361 supports this squelch functionality. A PIC18F1320 microcontroller monitors the rotation of the tuning encoder, drives the four-digit 7-segment LED display, controls the audio levels and the PLL. The rotary encoder also includes a switch to select the 100kHz or 10kHz tuning step size. In addition, a small pushbutton on the front panel gives access to volume, squelch and PLL reference settings. solder-tack this onto the main PCB, and a tidy design solution for this sub-assembly. A further small display PCB requires adding a pair of capacitors and the LED display before being fitted to the main PCB, one of the final steps of the build process. Here’s where the missing right-angle pin strip was required, although component leads off-cuts can be used if necessary. An experienced constructor could build this receiver in around four hours. Those with less experience, or constructors wanting to enjoy the kitbuild process a little more, will probably take eight to ten hours. While not a difficult kit, the absence of detailed step-by-step instructions means it does require some attention. It’s not suitable as a first kit for beginners but, if help is obtained from an experienced constructor, it could be successfully completed by those with only a few builds under their belt. Just watch out for the four tiny semi-transparent display spacers! They are hard to spot if they fall out of the plastic bag onto the floor. I’m just saying... Performance Following construction, the receiver worked first time. That’s important for a kit of this relative complexity. The alignment was delightfully simple. I first adjusted the IF transformer for maximum noise output with no input signal. I then adjusted the two input bandpass filter inductors for Kit construction Assembly follows the usual approach: fit the lowest-profile components to the PCB first, then move on to the taller components. The component locations are all clearly marked (in English), and the boards are all logically laid out. Fit the wafer-sized PLL PCB subassembly once all of the smaller parts are mounted. It’s a quick job to siliconchip.com.au The R80 aviation band receiver kit looks complicated, but two PCB subassemblies and a welldesigned main PCB make it straightforward to build. Australia’s electronics magazine November 2021  41 with the pushbutton and display menu system. The PLL required no additional adjustment for accurate operation, although the reference frequency can be precisely adjusted via the front panel controls if desired. Squelch problem & resolution Fig.2: the front-end filter response at 3dB/division, showing the 118-136MHz band (red bar) is within the -6dB response. The receiver has an adjustable squelch. It’s set with the front panel pushbutton and LED display. The receiver’s squelch circuit aims to silence (‘mute’) the receiver noise when no signals are present, and weak signals too, if desired. Unfortunately, this part of the R80 circuit did not appear to work properly. If a signal unmuted the receiver, the squelch would then promptly mute the receiver again, an effect known as ‘talk-off’. This occurs when the squelch circuit initially (correctly) detects a reduction in receiver noise when a valid signal arrives, but then (incorrectly) detects the desired received audio as noise and promptly mutes the receiver! This occurred repeatedly until the signal disappeared. The result was a series of brief bursts of chopped-up speech whenever a signal was received. This effect was also clearly audible in one of the early online video reviews of this receiver kit. I modified the original squelch circuit into a more conventional AM noise detector. Combined with a signal level squelch for stronger signals, that solved the problem. Details of this modification are provided in the separate panel. Putting it in a case Fig.3: the front-end filter response at 10dB/division, showing the valuable attenuation of the 88-108MHz FM broadcast band (blue). maximum signal strength using a signal generator. Because the receiver is very sensitive and the receiver’s frequency display is accurate, it’s likely that this bandpass filter alignment could also be completed using local airport signals. The front-end filter’s tiny green Kuibiaochi MD505-series adjustable inductors are a little unusual. These use brass tuning slugs rather than the typical ferrite core. The coil’s inductance therefore decreases (!) as the slug enters the coil. Fig.2 shows that this simple peak tuning provided a good receiver input 42 Silicon Chip filter response over the desired 118– 136MHz band (shown in red) with low insertion loss, important with such a wide tuning range. Fig.3 demonstrates helpful rejection of the nearby FM broadcast band (shown in blue). Tuning to a specific frequency is easy – just rotate the single front panel control – and precise, thanks to the digital display. The received audio was clear and distortion-free. Weaker signals were naturally a little noisier, but that’s typical for a 15kHz bandwidth AM receiver. Volume adjustment was practically a ‘do it once’ function, easily achieved Australia’s electronics magazine The kit did not come with any sort of case, nor was there an option to purchase one with it. As shown, I housed it in a trimmeddown box I bought from Altronics a couple of years ago, which is unfortunately no longer stocked. The box originally measured 130 x 90 x 48mm (lwh); I trimmed it down to reduce the height to 38mm. No adjustments were required to the length, as the box fitted the receiver board like a glove. After some research, I discovered that similar cases (with apparently identical dimensions) can be purchased from vendors on AliExpress, by searching for “BDH20002” (unventilated) or “BDH20006” (ventilated). Like me, you would have to cut them siliconchip.com.au Squelch modification to fix the ‘talk-off’ problem This modification converts the original high-gain filter/limiter squelch circuit into a conventional noise squelch with an 8kHz active bandpass filter and half-wave noise detector, combined with a large AM signal squelch gate. It is not a difficult modification, and there are no tracks to cut. The circuit along with my changes is shown in Fig.5, while the extra parts needed are listed below. The steps are: 1. Remove CP4 (10μF) and replace it with a 4.7nF Mylar or MKT capacitor. 2. Remove D1 (1N4148) and replace it with a 6.8kW resistor. 3. Remove CP6 (10μF) and then re-fit it with a 4.7kW resistor in series, while maintaining the original polarity. 4. Add a 150kW resistor on the copper side of the PCB, between pin 4 of U5 (MC3361) and the common connection between C16 and D2. 5. Add the two transistors shown in Fig.4 on the copper side of the main PCB, adjacent to the display PCB connections. Squelch adjustment can now follow the method described in the assembly instructions, ie, select the “Set100kW 100k W tings” mode using the front panel pushbutton, then select Mode 3 (Squelch Adjustment). Without any signal present, rotate the tuning control to adjust the squelch setting until the (noise-only) receiver audio is muted. Then, return 10kW 10k W to the standard tuning mode. To check that the squelch operates correctly, tune the receiver until you can hear a suitable speech signal. The 4.7kW 4.7k W signal audio should be audible when a signal is received, and the audio should be quickly muted when that signal disappears or when no signal is present. Readjust the squelch setting to achieve this, if necessary. Parts required for Squelch modification 1 BC548 NPN small signal transistor 1 BC558 PNP small signal transistor 1 4.7nF MKT or greencap capacitor 1 150kW 1/4W 5% resistor 1 100kW 1/4W 5% resistor 1 10kW 1/4W 5% resistor Fig.4: this shows where the extra components need to 1 6.8kW 1/4W 5% resistor be fitted on the underside of the PCB to fix the squelch 2 4.7kW 1/4W 5% resistor problem. Other changes need to be made to some parts on 1 short length of 1.5mm diameter heatshrink tubing the top side of the board, as described in the panel. Fig.5: the original squelch circuit, as designed by the kit makers, with my changes shown in red. They make a huge difference in how well the squelch function works. The additions are an 8kHz bandpass noise filter, biased diode noise detector and a high-signal level mute using the two extra transistors. siliconchip.com.au Australia’s electronics magazine November 2021  43 Fig.6: the front and rear panel cut-outs required to house the receiver in a suitable case. Even though your case will likely have differently-sized end panels, as long as you keep the clearance requirements in mind, you can transfer these to panels of any shape and size. down, though, taking 5mm off the top and bottom halves, using a rotary cutting tool or similar. Also, the box from Altronics used clips to hold the covers together while these versions use pillars and screws. They would need to be removed to allow the PCB to fit flat. These could be replaced by internal side gussets and screws. Fig.6 show the panel cut-out dimensions. These might seem useless given that the box is no longer available, but that is not so. All the dimensions in those drawings are all referenced to a 44 Silicon Chip pair of vertical and horizontal datum lines, shown in blue, so they can be transferred to any surface. The clearances noted from PCB edges should allow the minimum box size to be determined/verified. Most builders should be able to locate a suitable enclosure, or design/print one themselves. What’s missing? Once I made the modification, the receiver worked very well. But there are a few features I’d like to have seen in the receiver. Australia’s electronics magazine One is full 118–137 MHz coverage. To balance this, the vast majority of VHF aviation communication falls within the current tuning range of the receiver. I would also like support for 8.33kHz bandwidth channels. These are gradually becoming more widely used, particular across Europe. The current receiver software supports 10kHz and 100kHz tuning step sizes only. These neither match legacy 25kHz channel assignments, nor the new 5kHz-based steps required for the mix of 25kHz and 8.33kHz channels in use. The R80 receiver’s current wide bandwidth allows both types to be received, although, on occasions, you may hear traffic from several adjacent channels simultaneously! It would only require a minor software change to provide 100kHz, 25kHz and 5kHz tuning steps rather than the current 100kHz and 10kHz tuning steps, along with the use of a narrower 6kHz wide 455kHz ceramic filter. This filter (around $5) would also reduce receiver noise slightly and further improve performance. I’d also like to have channel memories for a few frequently-used channels. This would reduce the tuning required to move between widely spaced channels. However, that would require a more significant software upgrade. The lack of these features does not limit the widespread use and enjoyment of the current receiver kit. In time, these features may well be developed by the user community, given the performance and functionality of the existing R80 receiver and the ease with which such upgrades can be made. Conclusions Despite the (now resolved) squelch fault, my overall impression of the kit and the receiver is very positive. The kit is well-priced at $50, enjoyable to assemble, easy to align and simple to use. It’s a sensitive little aviation band receiver, completely free of the instability frequently encountered with earlier analog aviation kit receivers. The very stable PLL allows rapid tuning to a precise frequency. That makes it ideal for a variety of monitoring applications, and it’s now in regular use at the writer’s home. In short, I recommend it. SC siliconchip.com.au