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Old 30th May 2022, 6:16 pm   #60
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Quote:
The flatnees of group delay characteristics of a filter has in general little to nothing in common with the flatness of its S-parameters characteristics.
Of the three celebrated generic filter types, the Chebyshev offers the worse flatness of group delay, as Radio Wrangler already pointed out.
Anyway, you should perhaps consider to open another thread for discussion on your filter synthesis as mixing it together with the 6-gang FM project introduces some confusion and unnecessary distraction.

The group delay of basic parallel LC filter (i.e. with lumped L and C elements) is not related to the real time delay encountered (which is null), but to its bandwidth. It is merely a calculated value, used for analysis in most modern modulation schemes, where phase linearity is of merit.
Group delay (precisely its flatness) is relevant to FM systems, independent of filter types used. If you need details, I could put some, but it doesn't go without a bit of basic maths.
(The linked radio-labs page https://www.radio-labs.com/DesignFile/DN004.pdf has highly entertaining qualities, especially the attempt to explain the term "group delay" exploiting the the idea of a filter "waiting" before making decision upon what to pass and what not. And then they found a "negative group delay" through a parallel LC tank and tell, you can't violate the causality principle, yet they didn't mind to try it. However, they would fail though, because their "negative group delay" is a mirage. So, you've had some fun, and the world is saved )
Thanks. I was scratching my head after reading the bits in the article https://www.radio-labs.com/DesignFile/DN004.pdf.
Presto! Negative group delay means time travel.

Quote:
urning back to the roots ...
It would be interesting to see few sweeps of your current front end' response (from ANT in to IF out), preferably ~1-2(max)MHz wide, in three points of band, say 94MHz, 98MHz and 102MHz, with input level ~100uV (for gain & selectivity check), and few sweeps across the whole band, 88-108MHz, with unit tuned to points as above, with input level ~200mV (for distortions check).
To preserve the selectivity add 25 Ohm in series on the generator hot side to match its 50 Ohm output to ANT input, and for the same reason, another series resistor to the IF out hot side, as required (value depending on IF transformer ratio; ~50-150 Ohm should do).
Can you manage it?
I am not interested in making sweeps from antenna input to IF out because my FM IF strips use complicated staggered tuning (in contract to synchronous tuning in mono FM or AM IF strips) to max out the stereo fidelity. The IF responses will mask out the misalignment issues in the individual single and double tuned RF front end stages. It is easy to do frequency sweep of IF stages but it is hard to do visual sweep on the FM front ends.

I would not use 50 input injection from signal gen or sweep generator. I am in the middle of winding my own 1:6 balun (50 unbalanced to 300 balance antenna input).

The most challenging part of the project is to align the double-tuned filters in the cascode stages (powered-on) using 50 ohm instrument. It does not matter if I have a £50 or £3000 spectrum analyzer, the challenges remains the same. In Zverev's method, there are two solutions for the same 2nd order Butterworth filter: 1) one with I/O coupling caps matched to 50 ohms source and load 3) one with high impedance I/O terminations. I have 50 matching ports soldered to the I/O of the bandpass filters of the FM tuners. ( it is also possible to match 50 ohms by tapping the I/O to the coils near the ground ends) I have managed to sweep the bandpass filter S11, S21 and SWVR response using the NanoVNA V2 Plus 4 with both power on and off. However, they may not represent the actual response curve due to the fact that the I/O of the cascode stages do not necessarily have the same I/O impedance of LC bandpass. I have tried high Z probes and variable Z probes, they have not been working well. I am still searching for the solutions. If i am able to crack this, I will be on my way to Nirvana. The other thing is that the 6-gang tuner is already working very well, I am reluctant to touch it further. Sometimes something is working well and you mess it up by changing things around, you can't get back where you were easily (Murphy's Law).

At the moment, I am messing around with dual gate FETs and Gali 51+. I can see the beauty of having bandpass filters with 50 ohms I/O impedance that I can characterise them easily with VNA.

Last edited by regenfreak; 30th May 2022 at 6:30 pm.
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