FM Synchrodyne II

@2/11, 2022

Six years past since last time (2016) buit Homodyne/Synchrodyne FM receiver. Last one did works, but there is no evidence the phase lock happens. while discuss with hobbyist, there is lack of evidence and data to convence myself it's really works as it state.

long story short, there are 3 mysteres about FM synchrodyne:

why the amplfier have a huge gain
how the reciver detect the FM
does really the oscillator phase locked by FM station?

There is neccessary to build new one, perferm testing and observation to confirm what happens in a persuasive way.

Simple Design

Russian Radio magzine @1985

and a improved version @1987:

There are 2 LC tank and I had 2 identical variable capacitor, range from 9pF to 55 pF, So i needs 2 coils with proper inductance. One resonant from 88Mhz to 108Mhz, another one 44Mhz to 54Mhz.

Direct parrllen the capacitor with inductor will cover a frequncy range about 2.5:1, 108Mhz:88Mhz is a 1.22:1 range, this gonna lost most of capacitor tune range. So It needs pad and sereies capacitor to get a correct tuning range.

I had 2 builtfull air variable capacitor， capacitance vary from 8pf-50pf. To achivement a nice band of FM, parrallen with 4.7pf and then series with a 30 pf， we got a proper band cover while the capacitaor use it full rotation angle.

Coil Make And Test

Then we needs a 210nH indoctor. Evaluate by Helical calulator provide by https://hamwaves.com, we get a nice helical inductor meet our requiments. and it's insensible for wire diameter(0.5mm - 1mm don't change much, could be compasated by squeeze a little bit)

~210 nH coil, only 4.5 Turns means the coil form too big

The coil form is very nice. My LC meter show it's a 500 nH coil. That meter is obviously out of calibration and did not given meanning full value any more. I decice use resonant method evaluation the coil, the capacitance meter is nice and reliable provide a refrence standard. The Air vaiable capacitor capacitance range from ~9pF to 55pF. Its possible to use 55pF to evaluate the coil inductance, and caculate the high frequncy value then it's low end capacitance ~9pF to verified the result.

signal source use a 1 pf ( ~2k to 100Mhz) couple to testing LC tank (via a 50cm Coaxial Cables and 10cm lengh fly wire)
SA use sing turn detector coil hook up to ceramic coil form.

Evaluation Result:

55pF end got 33Mhz resonants frequncy, indicate a 400nH coil (way to much compare to coil caculator)
~9pF end got 60Mhz resonants frequncy, way too much low than expect......
from 33-50Mhz, the resonants response seems prety ok, but at high frequncy ends, it almost a all-pass element, the 66Mhz resonants response is very shallow, and mess.

Coil evaluation mysteries

So, why evaluation result located so far from mathmatic calulation? resonant at 33Mhz with 55pF, indicate it's 400 nH coil, but it's should arond 210 nH. the extra 1 pF couple capacitor did not contribute so much. 210 nH means the resonant frequnce should be around 45Mhz. At high end, reulst departure so much far, there is no any explanation for that.

Carefully check the set up, the obviously suspicious culpit is the coaxil cable from signal source and plus 2 10cm wires connect to LC tank.

I know, there fly wire is the killer for so high frequncy, but in deep mind, I kind of hope the wire won't affect the resonant it self, it's signal souce anyway.

I suspect the capcitor couple introduce some extra impact than I know. I feel simillar stuff in many other diffrents ciruits. For example, the AM receiver, couple from attenna coil, one or 2 turns coil couple seems more like works better than a small capacitors couple to coil's tap . In a RF oscillor, feedback couple from inductance likely works better than capacitor couple.

I suspect every time tring connect the capcitor to something, there must wonder phenomenon arise way much exceed expection. I planed give a try for single turn coil couple, like the stuff showed below.

Red coil couple the signal into LC tank from 5cm distance or so. Every other thing did not change.

Result is exciting! eveything seems in order. At 55pF end, got a peak at 42Mhz, indicate a 250nH inductor, prediction a 10pf smallest capacitance will peak at 100Mhz, 11pf smallest capacitance will peak at 96Mhz. The setup given a peak at 86Mhz, mean there might have 2 or 3 extra pF parasitic capacitance, that's very reasonable result.

It is good news, everything nice. Why?

What's wrong with capacitor couple, the couple capacitor itslef is not the reason for all the mess. I perform simulations by ltspice, even include the impact of the 10 cm fly wire, result still prety reasonable.

Then, anther big impact become clear to me. The Coaxial Cable is not termitor properly, becasue at tank side it's see a big value resistor even in resonant frequncy. This means the cable act like a big inductor combine a big capacitance! the whole test bed gonna look like a compex LC monster.

This is not all the mess!

Anohter big why? simulation dont' alow the LC tank floating, connect their gnd togherther: run frequncy sweep indicat the parasitic effect been pushed to around 200Mhz. I don't belive that's everything cause it function properly.

A single turn coupler for signal inject did several things nicely:

Isolation the signal source, especillay the ground return coaxial cable inductance and capacitance(cable not terminated by 50R impedance ). Confirm this effect by still use the 1pF couple to tank but use windband RF transformer isolating the signal source.
Single turn couple easly remove extra 10 cm wire attached to cable, which connect to capacitor and grounds.
While use capacitor couple the signal, all system's ground connected electrically, they all sit far from each other.

Synchrodyne principle

what do you think while first time you see this circuits? Advanced your Regen control, until you hear Oscillate begin, in this way you regen receiver capable receiving the SSB, CW, this this a typical application mode for a regen receiver. Wow, in this way, i actually use the homodyne, synchrodyne for a long time but don't know it. John's web site note this: homodyne reception was almost certainly used far more often by accident than by design!

FM Synchrodyne

oh, yeah, i experienced homodyne in AM, SSB, CW, but what about FM? FM synchrodyne is kind of mystery schematic, and being popular in many radio forum. and lot's of discussion, and lot's of name: PLL detector, single transistor superhet etc.

Here is a very popular FM synchrodyne in Russian Radio magzine @1985 : http://zpostbox.ru/fm_receivers_with_pll.html,

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