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Old 18th Jun 2022, 5:04 pm   #72
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Going back to the idea of having 50-ohm I/O bandpass filters, I have acquired some MMIC Gali-51+, MAR-1, Mar-2 Mar-4, 6 and 11. Theses have Darlington configuration internally matched to 50ohms. The advantages of 50ohm I/O impedance is easy to align with VNA or sweep generator. I thought I could explore the use of passive ring diode mixer like SBL-1+ or active double-balanced Gilbert cell mixer/oscillator NE602.


In order to test the feasibility of the ideas, I built the fixed tuned MMIC cascade amp with Gali-51+ with double-tune capacitive coupled bandpass and inductive coupled bandpass. It is working ok.

Furthermore I made a variable tuned MMIC amp using a 7-gang air gang variable capacitor. It is too lossy and difficult to tune to my liking. After all, it seems not a good idea. When it comes to variable tuned MMIC cascade amplifier, it is impossible to obtain the end coupling caps matching 50ohm I/O impedance across the whole FM band. The Hi-Z to 50-ohm transformation is frequency dependent. To add to the complication, the nodal frequencies are much higher than the filter center frequency in 50 ohms matched version, this makes it impossible to obtain an exact analytical solution that simultaneously satisfies all the constraints: acceptable bandwidth, impedance matching and tracking correctly across the FM band. So I am going back to the high Z double dual gate FET project.

Attachment 3-4 is the testing of SBL-1+ with my new poorman's 100MhZ FY6900 signal gen.

Attachment 6 is my 300 ohms to 50 ohm balun. I tested with 300ohm resistor, I got SWR of 1.4 at FM band. I am working on a new version that will give SWR close to 1 at VHF.

I am hoping to get a combiner and will measure the IP3 two tune measurement of an amplifier.
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