DigiTIA receiver

The guys at the Solder Smoke podcast have inspired me to do a bit more solder melting and this is the result – a CW/SSB HF receiver for 40m band. It is a bit “al fresco”, with no case, built on a piece of copper clad with modules stuck down using a double sided tape.

DigiTIA receiver

Detail of the main board

The rig is “half-DigiTIA” – there is no transmitter. I am not planning to add one because I live in a rental apartment and don’t have neither space for a proper antenna nor a local HAM license (too lazy to do the paperwork).

The receiver is very simple, using the 3 termination independent amplifiers (the purple modules in the photo) as described by Wes Hayward W7ZOI. I have only built them using surface mounted parts to make them smaller and they are not bidirectional because I don’t need that for a receiver.

Termination Insensitive Amplifier

TIA, as designed by Wes Hayward, W7ZOI

The mixers are ADE-1 from MiniCircuits – also SMD parts, but they fit perfectly on a small off-cut of stripboard as a carrier.

The VFO and BFO are using Si5351 clock generator module from Adafruit, driven by an ATMega328 micro and a small OLED display module. I have adapted the code used by Pete Juliano N6QW to use the more recent U8G2 library by Oliver Kraus because the Adafruit one didn’t work correctly with my module. My display is slightly different and the text was not displaying right.

Si5351 VFO/BFO

The receiver uses 11.997450 IF – that is the frequency most of my 12MHz crystals matched on, so I have used that frequency for the IF – digital VFO/BFO make it trivial to move the IF as required. The crystal filter could use some tweaking but it works well enough.

The audio was a bit low, so I have used an NE5532 opamp as a low noise preamplifier and I am feeding it into a classic LM386 audio amplifier. The LM386 has separate power supply for now because it likes to break into loud oscillations otherwise – the power supply decoupling and grounding are not ideal.

The input has a bandpass filter for 40m, I have used the same design as used in the BitX transciever, by Ashhar Farhan VU2ESE.

BitX40 bandpass filter

And how does this contraption sound? Well … judge for yourselves – there was a CQ WW WPX Contest – SSB contest going on, so the band was pretty busy. This from northern France, using a magnetic loop hanging on the wall in an apartment:

 

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