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Old 25th Oct 2020, 8:20 am   #17
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: Hallicrafters SX-43 FM drift.

Quote:
Originally Posted by falcon123 View Post
radio wrangler, can you post or provide a link to your design?
It's in the ARRL handbook, starting in 1995 and repeated in subsequent years. It's in the Oscillators and Synthesisers chapter, sometimes titled 'AC/RF Sources'.

I've seen chapters from the book appear on the internet for a while before they're taken down for breach of copyright, so I can't point to any at the moment. If you know any radio amateurs it's quite likely that some of them have paper copies of the book.

It consists of a stable voltage supply (resistor into a temperature compensated zener diode) I used an 8.2v diode from the 1N823A family.

This feeds the slider of a pot which is added into a bridge of four resistors. The top resistor on one side and the bottom resistor on the other are thermistors (both the same type)

A second pot is connected across the horizontal axis of the bridge and its slider goes to bias a varactor diode in your oscillator.

At room temperature you use the top pot to balance the bridge. Wind the centre pot to-and-fro between end stops. The oscillator frequency will shift. Adjust the top pot until the frequency shift is zero. The bridge is now balanced at this temperature, and there is no voltage difference across the second pot.

Now, note the oscillator frequency. Don't touch the tuning of the oscillator, but change the temperature to the highest you want to compensate to. Give it time to settle. Now, adjust the second pot to return the frequency to the value you noted.

You now have temperature drift nulled at two spot temperatures, and it will give a good approximation between these temperatures and for some temperature change beyond them.

The bridge and two pots trick allows you to adjust it without lots of trial and error.

I didn't give values of components, they can be chosen to suit whatever thermistors you can get, but thermistors around 10k Ohms at room temperature would be suitable, 10k fixed resistors, a 5k pot at the top of the bridge, and a 100k pot in the second position.

I could draw it in a moment with paper and pencil, but I don't have a schematic drawing package on this computer.

David
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