Sunday, 21 December 2014

Arduino SDR IV

Time to try once again to squeeze a quart into a pint pot...

One of the failings of my Arduino SDR, with its limited (12-bit) input stage, is the tendency to be swamped by big guns. The resulting distortion is of the particularly ugly "digital" variety that takes no prisoners.

At present, I'm dealing with this by knocking my ATU way off tune whenever a strong station comes on - but this is hardly an elegant solution.

Sure - I could add a simple potentiometer on the RF input - but that would never be addressable by anything other than my hand - barely an improvement over the ATU approach. I thought about adding a digital potentiometer at the RF input - but a quick look at the data sheets reveals that this approach is a non-starter.

Accordingly, I decided to play with a digital potentiometer in the "IF" of my software-defined radio which (of course) is actually in the high audio frequency range.

I happened to have an AD5242 in the "junk box" - as used in the experiments with the Digital Tube Screamer. This is a two-channel device (I needed a two-channel device to scale both the In-phase and Quadrature components of my system). The particular AD5242 in my collection was the 1MOhm variant.

The device features an I2C digital interface  - which was a bit of a concern, as I'd never had much joy with I2C and the Arduino DUE.

Fortunately, Rob Tillaart's library seemed to compile fine for the DUE and worked pretty much first time (once I had figured which of the two available I2C interfaces on the DUE was being used, with the help of my Ripoff logic analyser). I soon had the DUE talking to the AD5242 and implementing various resistances, tracking d.c. voltages read from a potentiometer...

Having confirmed I could control resistor values at will from the DUE, I thought about building a twin-channel, variable gain amplifier. It uses another Analog Devices chip from the "junk box", this time a quad OpAmp, which will run on a 3v3 supply - the AD8608...

Now, the physical potentiometer generates a voltage which is read into the DUE and then generates a control code to set the GAIN of the amplifiers. This gain varies between 6.5 dB and -41.5 dB. The (partial) schematic below gives you an idea of what I'm up to...

So - now the variable gain amplifier is added into the SDR receiver's input path...

It is, as you can see, very much a temporary "lash-up" at this stage. But it works fine. I have the ability to take shelter when the big guns open up. I also have a little more gain available to increase overall sensitivity. Plus, I've retained the variable receive bandwidth control from before.

But what - you might ask - is the advantage over putting the potentiometer on the RF input, just as I mentioned at the top of this post? Well...

One day soon, I will be able to allow the DUE to take over control of the AD5242 - to give automatic level control.

For the moment, I'll satisfy myself with manual control and focus on those other distractions the next few days have to offer.

...-.- de m0xpd

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