I had the idea of building a 6-gang FM tuner a couple of months ago after making a 3-gang FM valve tuner. I acquired three sets of 6-gang silver-plated, air variable capacitors that are spanking new in original boxes. The seller said they were ex-MOD items. I guess they were used in 2 meter band military communication equipment?
I am going to build two versions: (1) valve version and (2) dual gated mosfet version (see attached schematics). The designs are inspired by the top-end FM tuners in the 1970s that had 4 up to 9 gangs. My schematics do not copy the designs of any particular tuner but are just a cocktail of ideas from many designs. I just hope they will work somehow.
6-gang valve FM tuners did not exist and it was rare to have more than 4 gangs. The topology here is:
single tuning + double tuning + doubled tuning
This is really a learning journey for me. I still consider myself a newbie, only taking up the hobby in 2018. I don't have a real spectrum analyzer and VHF sweep generator but I can make it work using NanoVNA and cheapo Chinese sweeper.
The first step was to design the 2nd order Butterworth filter. I did hand calculations based on the method by Zverev and validated the results using software. They give identical results. I measured the unloaded and loaded Q of the single and double tuned filters using the test rigs attached. The sweep results matched the simulation very closely.
Thanks to Synchrodyne in this forum, I was made aware of a 1965 IEEE paper about the performance of 3 coils versus 4 coils FM tuner. This is the only paper technical paper written on the comparative study on the effect of double-tuned gang vs single-gang front end.
The key considerations of high-performance FM tuners are (list below is taken from Pioneer TX8500 service manual):
1. noise figure,
2. spurious response,
3. image rejection,
4. IF rejection
5. sensitivity,
6. 50db quieting sensitivity
7. image rejection IMD interference
8. cross-modulation interference
I still struggle to understand concepts like quieting sensitivity and limiting sensitivity.
There will be lots of shieldings around the coils. Lets the battle begins!