Drift & Sensitivity

[pwdrive]

Thoughts on drift and sensitivity in a regenerative detector:

If I use a low LC ratio in the grid tuned circuit of the detector then the drift caused by thermal movement of the inductor, capacitor and any stray capacitance is much reduced but would this reduce the sensitivity to received signals when the feedback is adjusted just to the point of oscillation?

Lawrence.

[Skywave]

Tricky one that Q. Lawrence.

My first thought in connection with the temp. variable components that you list, it that it will be the coil that is likely to be the main contributor to changing the characteristics of that LC cct. as the temperature changes. So to reduce any such changes, the coil needs to be so constructed to minimize such effects - i.e. standard practice one would adopt for constructing a high stability VFO.

My second thought is that the effect of temperature changes will depend on the type of reaction arrangement that is being used. Where a tickler coil is used to couple the anode back into the grid, then the effect of temperature on that coil needs to be considered as well. How is the degree of reaction to be controlled? By varying the degree of mutual inductance between those coils? By changing the voltage on the screen grid of a pentode? By a variable capacitor: manual adjustment of the amount of reactive current in the feedback coil? Different arrangements will probably have different temperature dependencies.

Overall, for moderate temp. changes, I wouldn't expect the change in sensitivity to be a concern, especially considering that when the main tuning is manually changed, even by a small amount, it is not unusual to then find that the reaction control (or whatever type) needs an adjustment also.

I suppose the only way that you're going to get a definite answer is to follow classic R & D practice: build such a cct., get out a hair dryer - and see what happens!

Al. / Aug. 17, 2013 //

[pwdrive]

Hi Al, it's not so much about the effect temperature change has on sensitivity per se, the question was more a about taking the classic stability (including temperature) measures for the tuned circuit ie: low LC ratio which you probably know can reduce the degree of drift caused by changes in capacitance (including stray) due to temperature change and also reduces the degree of drift caused by the inductor due to temperature change because of the fact that it's inductance is lower (less copper etc) because of the low LC ratio.
As I understand it the Q of a low LC ratio tuned circuit is lower than that of a tuned circuit where the LC ratio is higher, if that is the case then one would assume that the overall magnification of that circuit would be lower, on the other hand does that matter in the context of a regen as the detector is taken to the point of oscillation (for SSB/CW) which means that the stage has a very high level of gain anyways due to the feedback/oscillatory nature of it?

"""How is the degree of reaction to be controlled? By varying the degree of mutual inductance between those coils? By changing the voltage on the screen grid of a pentode? By a variable capacitor"""

That is by screen grid voltage control, feedback is cathode tapped Hartley, I have that bit working nice and smooth, no plops or ear shattering birdies.

Lawrence.