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Saturday, 16 July 2011

How Noisy is this?

Well, there have been a few developments here of recent; firstly the FT-920 has gone and been replaced by an ICOM IC-756 PRO II. The radio is currently sat permanently WSPRing on 6m.

But getting back to Homebrew, I have been trying to make a noise source for HF; the idea being that I can use this for testing filter bandwidth. I stumbled upon the following schematic which is from the RSGB publication Hombrew Cookbook - strangely my wife looked at the receipt and wondered why I wanted what she thought was a recipe book for making wine!

And this is my dead bug style build of the circuit:
The observant amongst you will spot that I have a reverse biased zener diode in there in place of the transistor that has been substituted for same in the schematic. I plan to try a few different diodes and transistor junctions to see what difference it makes. I'm certainly not seeing quite what I expected from this circuit. This is what the spectrum analyser makes of the circuit output (the large spike in the middle is 0 MHz):

 and if I move the 0MHz point left and increase the scan width to 2MHz per division:
So I wasn't expecting to see such a dramatic decrease in amplitude as the frequency increased. I cetainly wasn't expecting the spectrum analyser to show the inverse image from 0MHz and below!

So, I'm planning to try a few different zener diodes to see what the difference is; the theory is that there should be a good amount of white noise generated when a zener is reverse biased to the point where it reaches its reverse breakdown voltage.

Weird results, egh?

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