Daily Archives: April 5, 2020

Software Defined Radio

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[Yesterday (Saturday): 5 miles (to the Casey’s store)]

I’ve been trying to get a better grip on software defined radio, which is also digital signal processing.

You still have an antenna.

After that things can get interesting.

In the old days you would have maybe an amplifier or a filter or some combination of those. Those steps were to make the voltage coming off of the antenna bigger and to filter it by frequency. Then you might have oscillators and mixers, more filters, more amplifiers, more oscillators/mixers and then an audio amplifier and a speaker/headphone.

This digital thing means that you interrupt that string of events and put in a AtoD converter. Basically it takes snapshots of the current signal level. And it can be constrained by how fast it can snap and how many bits are used to represent the signal level. A lot of bits means it can measure really small signals. And a faster snap means it can go to higher frequencies.

You can then treat the signal like numbers and do math stuff on it. Eventually you turn might turn it back into an analog signal if you are driving a speaker/headphone. But if you wanted to draw a picture on a display it might be digital the rest of the way.

Over time the AtoD converters have become more snappy, so for some radio frequencies you can hook the antenna straight into the AtoD converter and do the rest in software.

That’s what this WWV receiver thing does. It has an antenna which is “tuned” in a rough way, so there is some filtering. The signal is only 60,000 Hz. Many computer sound cards can go that fast; CD-ROM data is 40k plus a little.

To get a good picture of the 60,000 Hz signal you need a AtoD conversion that will go at least twice that fast, 120,000 snaps per second. The Teensy audio board can be coaxed into 192,000 snaps. So the WWV receiver has the somewhat-tuned antenna connected straight into the audio board microphone input.

I had that working. I’m having some trouble getting it going again since I took the project back up. But I think I’ll eventually get that worked out.

The thing I want to do next is decode the phase shift that has a 2nd layer of information on it. This is supposed to be able to get through when the amplitude-varying signal is too weak. To do that I think I need to be able to do 4 times the 60,000 Hz rate. I’m not sure. I’m slowly making my way through a general DSP book and I also am posting some questions in the hope that someone will take interest and mentor me a little bit.

Filed under Ham Radio
Apr 5, 2020
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