Re: Tuner alignment procedure causing thermal drift


 

I'm not quite sure I understand your inference EJP, though I do not disagree with it. Obviously I was doing something wrong. I just don't follow your "tail chasing" observation.

By applying an RF signal of say 98MHz, modulated 100% at say 1kHz (no pilot), at medium signal level (say 65dBf) at the FM antenna inputs and then observing the IF signal after the last ceramic filter (right before the input pin to the IF detector chip) I am able to see the frequency modulated IF signal on the scope. It appears on the scope as a time-shifted sinewave with two peaks separated by perhaps a division due to the FM modulation. As I shift the tuner's knob from either side of 98MHz I can see either the left or right peak of the sine wave be attenuated as that side of the modulated frequency band is shifted out of the ceramic filter and/or tuner pass band window. I assume it is the three ceramic filters that are  performing this windowing function of the scope signal, but I could be wrong.

Does adjusting the tuner knob so that the two sine wave peaks (98MHz+75kHz and 98MHz-75kHz) are of even height mean that the tuner is centered on the applied RF signal of 98MHz? If not, what am I missing?

Joe mentioned that I  MAY be running into IF limiter action, but I don't think this is the case as the IF limiter is in the IF chip which is AFTER my scope probe. My 100MHz 10X scope probe may be causing a problem due to capacitive loading on the IF signal, but removing the probe does not seem to result in any shift in the tuning meter so I don't think that is the case.
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Mike Miller
PA, USA

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