Sunday, 17 August 2014

Breadboard BITX

Having first built a BITX bidirectional amp as a plug-in module then developed a mild obsession for plug-ins associated with the BITX, I found the temptation to build a complete plug-in BITX radio too much to resist...


It started with some batch production


of the remaining two bidirectional amplifiers...


Then I turned my attention to the second mixer. I used the circuit I published in my article in SPRAT 158 (see page 7), which includes a driver stage appropriate to the use of a digital oscillator.

I was embarrassed to spot an error in the way I’d drawn the secondaries of the transformer in the schematic in SPRAT – corrected below...


Here’s the layout I used on the little piece of Veroboard, which is of the now standard “12 holes” width for this project...


and here’s the little mixer in the flesh...



The only thing missing from a viable receiver was an AF amplifier – so I made up a module based directly on Farhan’s original ‘386 design (seen here with corrected pin-out)...


My vero layout looks pretty relaxed...


but the large junk-box capacitors make it all a bit crowded and ungainly...


Here’s how the whole shooting match fits together – starting with (at top left) one of the g3rjv / G-QRP band-pass filters (another of my standard plug-ins) and then the set of recently described modules associated with the BITX. My legend "BA" signifies Bidirectional Amplifier. I am running a 12 MHz IF; the VFO is derived from a DDS and the BFO is implemented on the VXO seen in the photo...


The system on the bench shows the VFO running the new Kanga VFO 3 code in a new, sloping front desk-top enclosure I’m in the process of building. I haven’t any nice pushbuttons (which will occupy enlarged versions of the pilot holes either side of the tuning knob) in the junk box – hence the temporary pushbuttons on the outside of the enclosure in best Heath Robinson style.


You can see (given my near 12 MHz BFO) that I was listening to the 40m voice band – in fact, I was listening to the Lighthouses On The Air weekend.

I now have a hugely flexible experimental platform, on which I can mess about with some new ideas. I haven't made a driver or PA stage (because I'm not interested in transmitting with this set-up - it has just been made to investigate some receive concepts). But all the functionality is there - this is a complete BI-DIRECTIONAL system, missing only the mic pre-amp, driver and PA. I have those - of course - in my main h/b rig.

An interesting experiment but – most important - it has been fun to build.

...-.- de m0xpd

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