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

Saturday, 19 September 2015

Bridging the Gap

The best things in life are either free or simple. Here's an example of the latter...

I'm a big fan of Return Loss Bridges, enjoying the fact that they give you useful information pretty much for free. They are simple instruments, which teach you while you're using them - rather like dip oscillators and slide rules. 

I've been using my old, home-brewed return loss bridge for a long time...

Whilst it hasn't had a write-up in these pages, it did make a brief appearance back here in the description of my experiments with mag loops.

I've been intending to return to return loss bridges (!) again and could have used the old warrior above. But the form-factor isn't quite right - I wanted something a little easier to incorporate into a different kind of experimental context. So...

First try was a little plug-in module, designed to fit into my beloved solderless breadboards...

As you see, input is applied at the left side (in the orientation of the photograph) and the load is connected at the right side. The output from the detector is output from the male header pins on the right side of the board. 

This 0v1 build worked fine - but did not achieve the sort of Directivity I was aiming for and certainly did not match the Directivity of my trusty old RLB. 

For readers who don't know about these sort of things, Directivity (in this context) is a single figure-of-merit for return loss bridges, describing the ratio of the detector output when the bridge is operated into an open circuit to the output when the bridge is terminated by a reference impedance equal to the characteristic impedance the system is designed for - in my case, 50 Ohms. To be practically useful, the bridge needs a Directivity of greater than 40dB.

Not satisfied with 0v1, I made 0v2, which features a physically smaller transformer and bridge resistors...

These improvements gave the required increase in directivity - to comfortably over 50dB at HF frequencies.

However, it doesn't take good eyesight to spot the bigger difference between the two versions - the later bridge also features a relay, by which the output can automatically be switched from the load to open-circuit conditions, through the application of an active-high logic input. 

For readers who don't know about these sort of things, the return loss bridge can be used to derive Voltage Standing Wave Ratios from return loss measurements in i) loaded and ii) open-circuit conditions. Having the ability automatically to switch between these two loading conditions will obviously allow a machine to make such measurements without the intervention of human hands.

The 0v2 hardware also incorporates an on-board reference impedance (a swell name for another parallel pair of 100 Ohm resistors), which can be used as a load just by putting a jumper between the two header pins on the board. I was tired of fishing around for a pair of external resistors.

Here's the new bridge in its native environment...

Notice that the Bridge PCB is dimensioned such that the rows of input and output pins make best possible use of the "blocks" of contacts on the breadboard. 

Off to the left of the new bridge you can see a little QRPppp linear, providing the power to drive the load. 

It all works well - watch this space (and some other UK radio events in the forthcoming weeks) for developments.

...-.- de m0xpd