Plug-In BITX Modules

Anyone who reads these notes will already know that I like plug in modules for solder-less breadboards. Here's a couple more to add to my fleet...

I've been playing around with some ideas for cheap 'n dirty SSB generation and I needed a broadband amp - so I thought I'd make up one using the bidirectional design from the BITX. I enjoyed making it so much I made a matching mixer with integral driver too...

The amp is one of the standard building blocks from the BITX...

I spent some time figuring out a layout on a scrap of Veroboard - trying to fit it onto a piece 12 holes wide (as that's the best width to work with on my breadboards).

Here's the layout I came up with (seen from the "component" side - the red "x's" indicate breaks in the copper tracks)...

and here's the finished article...

Flushed with the success of that module (and needing a mixer) I made up the second unit, implementing this section of the original BITX design...

Again, I wanted it on a piece of perforated board 12 holes wide, so I kept to the same overall size - here's the layout...

and here's the mixer in vivo...

Needless to say, the little modules work fine (after I'd remembered to make the last break in the copper track on the mixer circuit - my forgetfulness was causing an obscene amount of current to flow and making a 100 Ohm resistor get very warm).

No doubt the big boys will tell you that Veroboard and (in particular) solder-less breadboards aren't the thing for RF work. I don't care - they're really useful for trying out quick ideas and kludging systems together to see if things work.

With the added flexibility that these little ready-made modules bring, playing with RF is - literally - play.

...-.- de m0xpd

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