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Old 31st May 2022, 5:49 am   #67
regenfreak
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Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by nemo_07 View Post
Quote:
Originally Posted by regenfreak View Post

I managed to do RF sweep of RF frond end of a 4 gang FM double gate FET tuners with high Z probe but have not been successful with valve FM tuners. Great caution must be taken with valve tuners.

I am starting to build the dual gate FET version of the 6-gang tuner. It should be easier to test and align without worrying about the high voltage or frying the measurement equipment. But it will be still tricky to align those double tuned amp stages because we still have high impedance in the I/O of the filters.

Quote:
the IF transformer bandwidth should be made considerably wider than the expected front bandwidth
The opposite is true. The IF stages provides lots of the selectivity. Typical FM IF bandwidth is about 250KHz for wide IF high fidelity stereo. The RF front end has quite wide bandwidth (see the RCA paper example in my previous post number 20 and 25 of this thread. With double-single tuned stages, it is about 700-800kHz). This is because it is very difficult to align multiple tuned gangs (over 3 gangs) because of the tracking errors and imperfect filter response shapes. The bandpass shape and bandwidth of the double tune filter changes with the frequencies across the band. 2nd order Butterworth bandpass filter has very sharp tuning; a small misalignment would cause massive insertion losses. On the other hand, a single tuned LC gang has a relatively low loaded Q, small misalignment wont hurt much.
Again, you will need a narrow bandwidth preselection to visualize and locate spurs.
But to visualize the bandwidth of the front end you will need a "panorama" bandwidth at the detector (or to kill the IF transformer selectivity). A simplest way would be to replace the IF transformer at the mixers plate by ~1k Ohm resistor and to use a cheapest version of Hi-Z 1:1000 probe consisting of series connected 1pF/500V mica and 1000pF (plus a Si diode for additional DC leak protection, if you insist). This, of course, would require few orders higher generator levels-
Interesting idea. I can give it a try. i have tried all kind of tricks on the book you can imagine. Unfortunately, i dont have a VHF signal generator at the moment. The TinySA may have
not enough power level in its RF generation output.

Last edited by regenfreak; 31st May 2022 at 6:09 am.
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