Sunday 20 September 2015

Acorn II

Eagle-eyed readers in the UK with their fingers on the pulse will have noticed that one of the receivers discussed in prototype on the bench a few weeks back, the "Acorn II", is to be the subject of the buildathon at next month's RSGB Convention.

Those wishing to learn more about the Acorn II should see Dan, m0tgn's build video. Those wishing to get hold of one can do so at (or after) Kanga's launch at next week's National Hamfest.

One of the key features of the Acorn II is the provision of the external oscillator input. Having provided the input, I thought I better show the world how to make best use of it - so I have written some code to drive an Si5351 to work as a local oscillator, with rather more flexibility than the temporary, jury-rigged arrangement I had set up before...

Here's the Acorn II set up on the bench, along with an adjustable L.O. system (so much better than a fixed crystal HI HI)...

The oscillator consists of a Kanga/m0xpd Si5351 shield (of course - I'm biased) sitting on an Arduino UNO, with a user interface formed by a rotary encoder and an alphanumeric display.

The encoder allows the user to tune the radio across all of the 40m band (and the code includes all the support to switch to other amateur bands - but these require the two push buttons of the standard m0xpd interface used on all my rigs and VFOs, which I hadn't hooked up - no problem as the Acorn II only suppots 40m on the board, other bands needing external filters).

The display shows the effective LO frequency, but the oscillator produces the signal at four times this frequency, as is required to drive the standard quadrature detector on the SDR.

Here's the display (which is notoriously difficult to photograph), set to be appropriate to listen at the CW end of the 40m band...

While writing this post, I changed the oscillator up to 7.13 and listened to gb2eaa - the Essex Air Ambulance Special Event Station - on 7.12 MHz LSB. Just the sort of thing that is infinitely easy to do with this flexible oscillator - and (potentially) impossible to do with fixed crystals.

The code is not ready for public release yet - but I shall make a copy available eventually.

As the bands are a bit of a movable feast - particularly in a buildathon context, I have also knocked up a system I've called the "Buildathon Beacon", which just pumps out "QRP" in CW on 7.03 MHz directly from the output of a DDS module. There's enough radiation from a four inch length of wire for a receiver in the same room to pick it up easily.

Here's the "Buildathon Beacon", with its massive antenna looming towards the camera...

As you see, it is made from an early prototype of the m0xpd DDS shield atop a MEGA - these happened to be lying around.

Here's a screenshot of the output of the Acorn II running into HDSDR, with two foot of wire for an antenna, receiving the Buildathon Beacon...

I had done nothing either to match the two audio channels of the receiver (there's a ten-turn pot for gain adjustment) or to up the software to correct for gain imbalance - I just turned it on and "ran". Accordingly, you can see an attenuated image of the beacon at 7.05.

The Buildathon Beacon was actually developed to assist with the second of my two receivers - but that is a piece of news which hasn't broken yet.

Watch this space!

...-.- de m0xpd

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