8 Band Superhet AM Receiver

Amplitude Modulation holds a fascination for me. It dates back to those hours spent as a teenager listening to the big broadcast-like amateur AM stations in the 1970s and 80s, on 160, 80 and 40 meters. Signals that seemed as wide and loud as medium wave commercial broadcast stations, bearing sonorous, paced voices that projected a wealth of wisdom and experience, and in many cases, a grandfatherly manner. Back then, AM was known as ‘The Gentleman’s Mode’.

Compared to single side band it is a woefully inefficient and power-hungry means of transmission- SSB draws a quarter of the power budget. Compared to digital modes, AM is from another epoch, a true dinosaur, unjustifiable on most any communications-theoretical basis. Some might think AM is pursued only by eccentrics, romantics, Luddites all, and a handful of purveyors of broadcast transmission purity.

Digital consumer technologies and the internet have eaten AM broadcasting just as they have nibbled away at many other industries and technologies. The AM broadcast bands, once packed with every nation’s voices and propaganda are now relatively silent. In my country, Radio Australia, onetime bastion of Pacific and Oceanic shortwave broadcasting, turned off its transmitters in January 2017. Rumor has it that the former 500 acre site was recently sold, no doubt to speculators or developers for what will surely be another soulless housing estate.

Despite all this, AM persists in the amateur radio community, perhaps for largely nostalgic and aesthetic reasons, or because each generation holds on to its childhood memories. Just as auto restorers keep cars from every decade running, a small coterie of radio enthusiasts ensure the best of these boat anchor transmitters remain on-air. In the United Kingdom the Vintage Military Amateur Radio Society (VMARS) Net on 3615kHz runs 6 days a week as a showcase of mostly wartime equipment. In the USA, the AM scene is alive and well on 75m where the surplus of AM broadcast transmitters are kept alive alongside the new breed of Class D, E and PWM designs. AM will be alive for a good while to come.

An AM Receiver

To complement two 160m AM transmitter projects I built in 2019 (one finished, one on the workbench) I wanted to build an AM-only receiver. No BFO, no product detector, just a ‘proper’ AM detector and a fixed 6kHz IF bandwidth, to serve as a dedicated analogue receiver to go with the homebrew transmitters.

Since AM transmissions on the amateur bands are infrequent, I wanted to make it multi-band, so that at any time I could tune in an AM signal of some sort. So I resolved to cover the essential amateur bands where AM nets can be heard in Melbourne, 160 and 40m. I would include 80m as occasional AM activity occurs there, and it is a secondary band for some WIA and regular special interest group AM broadcasts. And I wanted to include the medium wave band, because there are a number of programs that I enjoy on the ABC AM stations (Radio National), and because MW would provide a reference band for receiver testing.

Having only these bands would still leave big gaps in the listening week. I wanted to be able to turn on the receiver and hear a short wave or long wave AM signal any time of the day or night. So I resolved to include 5MHz and 10MHz for WWV/WWVH, and a long wave band for the aircraft beacons below the 630 meter amateur allocation. Seven bands in total. My usual demux/IO expander IC (PCF8571) supports 8 outputs, so 8 bands it would be.

Design

For LW, MW and lower HF AM coverage, an IF of 455kHz is suitable, and readily available IFTs speed up construction. Inspiration was drawn from the WN5Y’s Electroluminescent Receiver for the IF stages, and this design for the detector. My design would use a bank of switched band pass filters, a switchable RF preamp (2N3904), an SA612 mixer, two dual gate MOSFET IF stages (BF961), an infinite impedance FET AM detector (MPF102), an audio preamp (2N3904) and power amp (TDA2003). The VFO would be a single si5351 clock, under control of an Arduino Nano, running my script. Band pass filter selection would use the familiar PCF8574, necessitating no code changes.

The IF filter is perhaps the critical component in any receiver. I picked up a few of these 455kHz ceramic filters from Minikits. They are around 2k ohm input and output impedance with 6dB points at +-6kHz. A number of bandwidths are available and all are very reasonably priced. For AM at the frequencies of interest, these should suffice.

My choice of design and components was motivated by availability, familiarity, and a desire to build this receiver in a matter of weeks, not months. A higher performance receiver might use a switching mixer or a diode ring mixer, more IF gain, and convenience features such as a switchable attenuator, audio filtering, and speed-selectable AGC. These extras all add time and effort. I wanted to finish this project before Christmas and move on. Modular construction and a comfortably open chassis and layout would make later substitution or improvements easy, if the homebrew energies dictated down the track.

Circuit

The full kicad circuit is here.

Receiver RF modules.
Controller, display, PLL (VFO).

VFO, controller

I used my usual VFO/Controller module, an Arduino Nano, LCD, si5351 breakout, pushbuttons, rotary encoder and a 7805 regulator on a heatsink, built to the Raduino pattern. This one was made up on Veroboard. My script is here.

Band Pass filter board

I made up a back-board for the 8 BPFs, each filter on a small plug-in daughter board, sitting over a telco-style 12v DPDT relay. For more demanding receiver applications a relay would be used at each end of each filter, but for these low frequencies, I judged that any stray contact capacitance could be tolerated. Unused BPFs are earthed at both ends. Each relay has its own 2N7000 FET switch, driven by one of the PCF8574 control lines. A parallel LED and dropping resistor light up the active filter, the LED visible through a hole drilled in the daughter-board. The PCF8574 (an eBay piece for a few $) saves that bit of extra work to make up the board for the bare IC and includes address jumpers and headers.

The band pass filters are a variety. For 160, 80, 40 and 30 (10MHz) I used the W8DIZ kitsandparts filters, which I’ve used before with good results. For 60 (5MHz) I extrapolated the values, half way between 80 and 40m values, which is not generally recommended, but it worked. These filters are narrow, about 200kHz on 160 to 30m, so the receiver gain drops off sharply when you move above 40m into the 39 meter shortwave band, but I was prepared to live with this, as I am mainly interested in amateur AM reception.

For the medium wave broadcast band, I installed a coupling capacitor (no filter). This works, but does nothing to suppress the image, and needs to be replaced with proper filtering. Image rejection needs some thought at MW because the image falls within the band when using 455kHz as the intermediate frequency. When tuned to a station on 1500kHz (with a high-side VFO) the high side image frequency will be 455kHz above the VFO (=1500+ 2×455 or 2410kHz). With a band pass filter installed, anything on 2.4MHz should be well outside the filter pass band. However, at the other end of the band, say 621kHz, the image is at 1531kHz — inside the band. An unfiltered receiver will hear a signal on both 621 and 1531kHz equally. This problem is ignored for the time being.

On Long Wave, I wanted to be able to monitor a couple of the airport beacons, particularly the nearest one, Moorabbin Airport (callsign MB), about 25kM south of my QTH. I used two back-to-back 455kHz IF transformers coupled with 18pF. This filter works but is sharp and has too much attenuation. I might replace it with one with hand-wound toroid inductors down the track. Another homebrew LF receiver I’ve seen by Dale VK1DSH uses a low pass filter with a cutoff at around 500kHz — this might be a better option.

Receiver board

The long, narrow board to the right of the chassis houses the main receiver stages — a switchable RF Preamp (2N3904), mixer (SA612), two IF stages (BF961), AM detector (MPF102), audio preamp (2N3904) and audio power amp (TDA2003). These are all of standard design and a minute studying the circuit diagram should make it all clear.

The only unusual thing is the use of SA630D RF switches instead of relays to switch the RF Preamp in and out. I bought a strip of these devices a few years back and made up a prototype board with this arrangement. At small signal levels they are an excellent RF switch, even if overkill in this application.

The IF stages have their gain fixed with a trimpot. This could easily be bought out to a manual front panel control or given over to a dynamic AGC circuit. I was prepared to fix the IF gain and just ride the audio volume for simplicity.

Chassis

The receiver is mounted on an aluminium base panel measuring 247mm x 165mm with front and rear panels made from 40x1mm angle. This makes for a simple and cheap but effective chassis. Sides (of angle) and a top will be added down the track.

Evaluation

The video captures every AM signal I heard over one week. Broadcast stations and beacons include ABC Radio National 621kHz (Late Night Live’s Phillip Adams interviewing the Late Clive James), WWV/WWVH on 5 and 10MHz after sunset, Asian shortwave broadcast stations in the 39 meter band (Radio China?), and the Moorabbin Airport navigation beacon (MB) on 398kHz.

Amateur band transmissions include members of the daily 160m AM ‘Coffee Break’ 11am net, some 40m AM stations who are mostly active late afternoon Saturday and Sunday mornings, VK3ASE’s weekly crossband re-transmission on 160m NBFM (slope detected) and 80m AM, in which Dave runs a robot reader to enunciate the text of old wireless and electronics magazines.

Antennas used are a 160m inverted-L, 80m and 40m dipoles. I live on a typical suburban block in Eltham, about 20 km north-east of Melbourne’s city center. My noise level averages S5 to 7 across the HF bands, dropping to S3 between midnight and 6am. It is a reasonable but not spectacular receiving location.

The WebSDR in the video used for remote monitoring is courtesy of Paul VK3KHZ and is at Croydon North, about 24km to my south-east. Signal strength off the ionosphere is typically similar but has been seen to differ by up to two S-points between our stations. This was most noticeable on WWV/H 10MHz.

Post-build Reflection

This relatively conservative design gave me no headaches and performed beyond my expectations. A fun project! The receiver sits on the bench, crackling away, an acoustic window on the bands, and able to fill the shack with an AM signal from somewhere in the world, any time. Having a dedicated WWV monitor is something I’ve wanted for a long time and it is useful, a steady WWV beat is a sure indicator of 40m activity.

Acknowledgments

Thanks to Paul VK3KHZ for the SDR, an invaluable remote monitoring tool for the experimenter.

Thanks to David WN5Y the designer of the Electroluminescent and to the creator of Mid-Century Radios for taking the trouble to publish, and for continuing to keep their pages alive.

Thanks to Mark at Minikits for stocking the Murata 455kHz ceramic filters, without which I wouldn’t have attempted this project.

More?

If you got this far, and you are interested in simple AM receivers and detectors, including the infinite impedance FET detector, check out this excellent article by Felix VK4FUQ.

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