Would DC current variation detune an IF transformer?

[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]

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?

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.

[glarsson]

Once upon a time before triacs and thyristors we used a component called a transductor. A transductor was like a transformer but dc current is passed through one winding to control the level of saturation in the core. However, I don't know if 40uA dc current is enough to saturate the core of a small IF transformer.

[CopperCone]

I'm pretty sure there is no lower limit to saturation. I think it creeps up there slowly and then hits a region where its a strong effect.

You might need a high resolution meter though.

You can't really ignore the fact too that it changes the material dimensions due to stress and heat.

[chris_leyson]

To use DC to "tune" an inductor you really need a closed magnetic path, it makes life a lot easier. Imagine you have lets say a small ferrite toroid, wind a few turns on to get the required inductance and then you need to add a control winding with perhaps a few hundred turns. The idea is, as you increase the DC current in the control winding then the flux density increases and you move the magnetic operating point along the B/H loop as the flux density B increases. With enough control current you eventually reach a point where the ferrite starts to go into soft saturation i.e. the operating point is on the corner of the B/H loop. This is the point where the permiability starts to drop and so does the inductance in the smaller winding. By setting the magnetic operating point on the "corner" of the B/H loop gives you a DC controlled inductance, if you know the material properties for the ferrite and the dimensions of the core you can calculate the required number of turns for the control winding. I have seen a few amateur designs for current controlled VFOs but can't find any links. Earlier on I said that a closed magnetic path made life easier because if you put a gap in the magnetic path then most of the flux is across the gap and not through core making it difficult to increase the flux density. (might have edit that last sentence)

Magnetic amplifiers use the same principle but they use materials with a square B/H loop and they use the "control winding" to saturate the core at the right time and effectively use it as a switch in switched mode supplies. But that really is old school.

[4CX35000]

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.

i suppose the slug could only be partly into one coil and fully inside the other ?. Difficult to answer further without knowing more about the transformer concerned.

[T3sl4co1l]

Are you certain it is the current and not the voltage?

The grid input capacitance varies with bias, because, in effect, the space charge has mass -- it's made of electrons!

The capacitance listed in the datasheet is cold capacitance.  Typical high performance tubes have more than double that capacitance at full bias.  (The capacitance is also lossy, but this isn't significant at 455kHz.  A 6J6 is around 10kohms per grid at 100MHz.)

Miller effect (if applicable) also varies with bias, because it's proportional (almost) to gain.  This gives rise to the reactance modulator circuit for generating FM and PM.

Tim

[Circlotron]

^^ Tim, is there =anything= you don't know???

[babysitter]

I had some boat anchor which contained sweepy oscillators that got a mains-derived voltage exactly to permanently detune this oscillator around a center frequency - the voltage is applied to a additional winding on a core (the UHF section was a cavity with a voice coil actuator humming at mains frequency too.)

Rohde and Schwarz SWOB 2, it was. In a new nice home now, had to go due to required childrens room and aquisition of a rigol sa.

Years later I made a demonstrator for https://en.wikipedia.org/wiki/Respiratory_inductance_plethysmography the same way at work. Thanks ancient engineers.