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Simple FT8 and JS8 direct conversion receivers

Digital modes are easy to build receivers for. Much of the needed gain and selectivity is provided by your computer and software. All that's really needed is a frequency converter to shift the incoming signal to the audio input your computer needs. This comprises of a local oscillator, mixer stage, some tuned circuits for selectivity and possibly some radio or audio frequency amplification. Grab a few transistors, wire them up to form a direct conversion receiver and away you go.

Of course there are compromises. The most notable is a problem inherent with simple direct conversion receivers. That is reception of what's called the 'audio image'. Because the receiver doesn't have a crystal filter or phasing out circuit it receives signals both above and below the local oscillator frequency at equal strength. That can be a problem if there are signals about 3 kHz away from the desired signal on the other side of the local oscillator signal. Still that's a small price to pay for something that's very simple. And as you'll see from the videos you'll still get good reception of many stations.

Decoding FT8 with a 3 transistor receiver

This is one of my earlier tests. It was done with a receiver that I used in conjunction with SDR software for SSB and CW reception. In this case though I'm receiving FT8 with the WSJT-X software. The converter uses a ceramic resonator to adjust the frequency. This is less stable than a crystal but it still works.


Three transistor JS8 receiver for 3.5 MHz

This uses a similar scheme to the above. Howver it conveniently uses some parallel 3.58 MHz crystals (which are cheaply available) to pull the local oscillator down to 3.578 MHz. This is the frequency used for JS8. JS8 is less popular than FT8 but is more interesting to listen to (or is that 'watch'?) as it's a conversational mode rather than set-piece exchanges like FT8.


HF digital mode crystals from W6OUT (and a simple 7 MHz FT8/JS8/WSPR receiver)

This came about when I was sent some custom-made WSPR, JS8 and FT8 crystals by Matthew W6OUT. These greatly simplified making a simple receiver for these modes on popular bands. The video describes and contains the circuit of this three transistor receiver.


Portable FT8 receiving with direct conversion digital modes receiver

Once happy with the above receiver I used several crystals and added a rotary switch so I can switch for various bands and modes. Video shows the DX decodes possible while I'm outside in an RF-quiet location.



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