Saturday 1 August 2015

Two Receivers

Two receivers are being prototyped on the bench here at m0xpd. Interestingly, they sit close to opposite ends of the spectrum. Not the frequency spectrum - but what might be called the spectrum of sophistication...

At one end is a re-work of a Software-Defined Radio...

The original Acorn is a simple SDR, designed by g0nqe and available from Kanga UK. It uses only through-hole components, which differentiates it from many other SDR systems and makes it attractive to novice builders who shy away from surface mount components - for perfectly good reasons.

I've re-worked the Acorn for Kanga, to provide the "Acorn II" prototype seen above. It retains the "though-hole" philosophy and improves the connectors on the board. Support for an on-board crystal is retained but the main upgrade is the provision of an external clock input. You can see both the (plug in) crystal and the external clock inputs on this picture.

The external input liberates the Acorn II to tune anywhere, given an appropriate source - such as that conveniently provided by my Si5351 board...

In the picture above, the Acorn II is powering an Arduino UNO (through the red and black wires), which is hosting a Kanga/m0xpd Si5351 shield.

The shield is generating a 28.6 MHz clock (applied through the green and black wires), to place the SDR in the middle of the 40m voice band - I was listening to David, g3sqa, chatting on 7,160 after calling "CQ WAB" at the time the photo was taken...

Notice that I hadn't changed the LO setting on WinRad to reflect this tuning!

At the moment I have the two "amplified" output channels of the Si5351 shield set to two frequencies appropriate for 40m use - one at 28.6 MHz (as above) for voice and the other at 28.08 MHz for CW. Given a few more moments spare time, I'll add a rotary encoder to allow changes of the LO setting - but you all know how life is too short, etc etc.

The Acorn II also inherits from the original Acorn an on-board input band-pass filter. The Acorn II will be delivered with components to build this filter for 40m. The external clock input allows the radio to tune other bands - but the radio should use an input filter appropriate to these bands. Such filters have to be constructed off-board and there are routing jumpers to steer the signal to external filters as required.

The Acorn II will soon be available from Kanga UK, along with code to set up the Kanga/m0xpd Si5351 shield to generate LO signals.

I mentioned at the top of the post that there were TWO receivers on the bench at present...

At the lower-tech end of the spectrum is a Theremin Regenerative Receiver I'm putting together for a little event later in the year (watch this space)...

Here's a rear view, which shows more of the giblets...

The Theremin gag is a reference to the "charming" consequences of hand capacitance that you get with these old-time toys. I'd nearly forgotten about all that stuff since playing with sensible, isochronous oscillators, like the AD9850 and the Si5351.

It is nice to have both ends of the "sophistication" spectrum on the go at the same time. Nice - but it does rather squeeze available time for other projects.

...-.- de m0xpd


  1. I'd like to mod my existing ACORN SDR and would like to add an external IO for an oscillator that I'm building. Eventually I will buy your new design but would like to see the schematic for it so I can change what I have now. Thanks.

  2. The Acorn II design is proprietary; all rights rest with Kanga UK.

  3. Do you happen to have the schematic for 20 and 30 meter BPFs. Your Acorn 11 works great and I am using a tuned LC circuit to cover 20, 30 and 40 Meters but I would like to use BPFs rather than the 365 pf variable cap that I use now.

    1. Jack
      My 'go to' BPFs are these...
      but I've not even thought about how/if they might work with the Acorn II.