Tune Out Before Tuning In, Part 2

In part 1 (Tune Out Before Tuning In, Part 1), we looked at the method for tuning and calibration of multiple stages of frequency sensitive circuitry. We saw that properly setting up the stages is not as easy as it might seem to be. A proper method is counter-intuitive.

In the initial phases of using an antenna tuner, I observe that perhaps 70 percent of the time (an accident related to the normal distribution?) I'm on the right track very quickly with this “away and forth” technique, and the variables all nicely converge towards their sweet spot. It seems to me if you're good at circuit design and modeling, you might see the method in this madness by way of a sensitivity analysis of each control element. But whatever works for you.

I'm sure the experienced engineer is thinking, “Of course.” But I doubt the young tech or engineer thinks of tuning away even now. Indeed, the truth is that students largely weren't thinking that way 45 years ago (and I suspect it was the same for working technicians) or else they would have been able to align that FM discriminator — at least the particular module I was looking at. No, time and time again I saw each student initially push a component directly towards a desired meter null or peak, and then further try to maximize (or minimize) the meter reading by adjusting the next component. But what did that achieve?

More often than not, it led them to a sort of dead spot, which is why students ended up starting from scratch again. They missed the plot: Until you reach sweet spot status, the more pots, inductors, and capacitors you have to tune, the more critical it is initially to move each variable element slightly in the opposite (wrong) direction for a while in order to give the other components some play, some role in the circuit.

A different kind of forgotten solutions made it easy to make accurate (in terms of percentage) measurements of various variables using equipment with poor accuracy, even equipment that was all but broken. Again, my memory isn't perfect, but I remember the approach was that you recognized that your cheap meter was a lot more useful as a differential tool — i.e., one for measuring small differences in circuit voltage rather than measuring absolute values.

Then there was also a one-control type of tuning away test to determine if the signal being received on your traditional superhet radio was on the actual frequency indicated on the dial, or an image. While improved front ends have largely eliminated discussion of image responses, would you know how to determine easily if one existed?

It goes like this: Tune in the signal of interest — ideally a carrier or continuous wave (CW) signal. Couple in an external frequency source at about the same frequency to the receiver's input (e.g., close coupling near the receiver's antenna jack is one way). Turn on the receiver's AM detector (we assume your receiver comes with one) and adjust the external frequency source for a 1 kHz beat note with the signal you're testing. Maintain minimum signal levels to avoid overloading any stage.

Now tune the receiver's main tuning dial away from the desired signal/beat note. If the beat note stays constant, the sampled signal is on the frequency initially indicated on the main tuning dial. If the pitch of the beat note changes, the sampled signal is an image.

Why is that? I leave it to you to figure out. If you're familiar with the old-school tune away technique for aligning the aforementioned FM discriminator, bring it forth!

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