Electronics > RF, Microwave, Ham Radio

Would DC current variation detune an IF transformer?

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Circlotron:
Just been messing around with a late 40s AM broadcast band receiver, in particular experimenting with different detectors, biasing a 1N4148 etc. One setup has about 20 uA diode bias current going through the final IFT secondary and if I double this figure the 455kHz signal on the secondary drops down by about 60%. Then I got to thinking, DC current through an inductor will shift the inductance somewhat, so what about when the AGC line alters the bias of the mixer and IF amp tubes? Their current would vary quite a bit, so would this have the effect of detuning the IF trannies somewhat?

CopperCone:
I don't know much about what you are talking about, but I did notice that putting a permanent magnet near a inductor connected to a LCR meter also varied its inductance reading somewhat. Maybe its a cleaner way to adjust things then putting a DC current on it.

My work with fluxgate magnetometers says that this is true, I went to great lengths to keep any kind of DC out of the device (did not want to couple capacitvely), ended up using complicated hybrid chopper amplifier. They use a servo loop to maintain linearity and balance the currents.. might be a bit relevant to what you are saying.

It's probably possible to actively null that current, variation in sensitive areas, but this is another feedback loop, so another source of distortion, as any feedback loop is going to end up oscillating somewhat.

I bet that the designers of what you are studying just did not care or thought it was negligible. My guess is that if you implement it, it will be a much nicer piece of equipment. Each version has their merits though.

I believe typically it's described that you can alter the Q of a circuit (to make it wider) by adding a magnetic field on the tuning inductor. A hack if you will.

4CX35000:

--- Quote from: Circlotron on August 22, 2017, 11:19:03 am ---Just been messing around with a late 40s AM broadcast band receiver, in particular experimenting with different detectors, biasing a 1N4148 etc. One setup has about 20 uA diode bias current going through the final IFT secondary and if I double this figure the 455kHz signal on the secondary drops down by about 60%. Then I got to thinking, DC current through an inductor will shift the inductance somewhat, so what about when the AGC line alters the bias of the mixer and IF amp tubes? Their current would vary quite a bit, so would this have the effect of detuning the IF trannies somewhat?

--- End quote ---


Been a while since I did the theory, but I suspect the saturation on the transformer is being changed, rather than inductance. DC current generates a magnetic field which would change the dynamics of the transformer and possibly causing it to reach saturation when passing the 455 KHz signal through it as well.

CopperCone:
you can add a quick DC winding to a simple transformer and check with a LCR meter.

What you say makes sense, but I defiantly saw a inductor (ferrite, cylindrical, part of a ~6Amp SMPSU) change when it was exposed to a hard drive magnet by about 5% connected to a LCR meter. Did not look at the other things it measured though, like Q.. only the henries reading, I want to say at 1KHz. Happened a few years ago, don't remember the details.

I don't think its causing hard saturation with just a small magnet.

is it fooling the meter some how? (genrad)

Codebird:
The physics is clear enough.

A constant DC current, on an air-wound inductor, does nothing. It may be ignored. It has no effect. Not does any stationary magnetic field. However, once you have a core material in play it can get more complicated as it might push the material into saturation.

I can't see that happening on 40uA though, so whatever is going on for you is something else.

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