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Old 21st May 2022, 1:37 pm   #40
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Many many apologies that I have not responded to your messages. For some unknown reason, I did not get any email notifications about replies of this thread since my last post in March. I have not logged onto to the forum since mid-March and I have seen the ten posts for the first time today. Maybe I switched off the email nortification accidentally.

The reason I have been away from the forum was because I have been busy at work and I was lost in my little world of making different solid-state VHF tuned amplifiers and bandpass filters. I got side-tracked completely spending lots of time designing and experimenting both capacitive and inductive coupled bandpass filters. In April I bought a £1500 Siglent SVA1015X dual VNA and spectrum analyzer but it developed faults and I returned it for a refund.

This week I have returned to this project after a long period of "procrastination" because I have taken three days off from work. To my astonishment, the 6-gang valve tuner actually works very well the first time i powered it today. Murphy Law states that anything can go wrong, it will go wrong but this project seems escape it. This is very pleasing and encouraging! This is after doing some very crude alignments using the NanoVNA and NWT200. It receives all stations and track perfectly from top and bottom FM band.

Last night I aligned it with the NanonVNA plus 4 using 50 ohms matching capacitors in the input and output terminations using "cold" tuning. Now this is not really correct way of aligning the bandpass filters because the center frequency and bandwidth depend on the input and output terminations. I did calculations for the Butterworth filters using excel and software with both cases of 50 ohm terminations and high Z source and load .If I powered up the tuner, the NanoVNa indicates that the center frequency and bandwidth completely shift completely and there is a large Z mismatch. This is because the input and output impedance of the valves are really unknowns. I only have ballpark figures from the datasheet.

To do correct alignment of the 6-gang tuner, I will need some sort of Z probe with variable input and output potentiometers with very little stray capacitance and inducatance. I was working on some variable Z input and output FET probe last year for my NTW200. It did not work work well at VHF and now I am going to build it again.

For FM tuner, I would opt for slight under-coupling instead of the critical coupling with optimally flat bandpass response commonly used in fixed tuned HF filters. In the case of slight under-coupling, it introduces higher insertion loss but higher filter Q. A more round top response curve is desirable for FM tuner in adjacent channel selectivity. The Butterworth filter tuning is very sharp. A small out of alignment introduces massive insertion losses. This makes alignment very challenging.

The last screenshot shows the alignment and measurement of unloaded Q of the single tuned front end using the NWT200.


I have started working on the solid-state dual gate FET version but I am toying this idea of building a third version using SMD GAL-51+ and Gilbert cell mixer with 50 ohms terminations. It is much easier to align 50 ohms filters!

Quote:
ollowing that RCA paper was a whole sequence on FM front ends in which the bipolar signal path devices were progressively replaced, firstly by single-gate mosfets, and then by dual-gate mosfets.
Thanks for the papers. i will have a look.
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Last edited by regenfreak; 21st May 2022 at 1:45 pm.
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