FM Synchrodyne

@Jan/17, 2016

Homodyne and Synchrodyne receiver , I rarely hear of it. might because it's so close to a regen receiver, if one play regen first and became familiar with oscillation mode regen, which receiving the CW and SSB, even the AM signal. then, running into a schematic which very similar to the Regen and claim it's  a kind of synchronized receiver, i don't believe it at first time. 

I found Jon.'s web site, it's a great site, there is a article introduce Synchrodyne, http://www.thevalvepage.com/radtech/synchro/section5/section5.htm . 

1947's Synchrodyne, from Jon.'s web site

Synchrodyne some time equal to Direct Convert receiver, especially for SSB and CW, the carrier beating signal is generated by a local Oscillator. i don't want to discuss receiver like that.

there is a phenomena, while a oscillator been injected a signal,  the oscillator likely tend to synchronized with this external injected signal somehow. some research point out the oscillator's phase will related to external signal's phase. deploy such a oscillator, utilized the DC principle, form a Synchrodyne receiver, is the focus topic here.

actually the Synchrodyne have a long history almost old as superhet(1918) receiver, synchrodyne almost same as a improved version of homodyne, and homodyne is trace back to 1924's Colebrook, all of these explain clearly by Jon.'s web site. the name Synchrodyne officially named in 1947.

synchrodyne as a form of homodyne, imply a same principle to detect signal, a very weak oscillator, and injecting an external signal. while a clear sound appear, the oscillator actually synchronized with external signal. 

homodyne from Jon.'s web site

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,

in the same time some China magazine published a similar version:

Some built note: 

• original schematic use 1.5V power, but now is 9V, so remember change transistor bias.

• bias of BJT need adjust to provide weak oscillation, which improve the receiver sensitivity and lock range.

• signal inject to BJT base might need  connect to top of the L1 hot end, which improved the lock range greatly, because strong signal inject.  i believe this just because my input tank actually not tune to the correct range of the FM signal. i don't spend  enough time to pay with it.

Result: very loud beautiful and clear sound.

Working for this little receiver.

Simulation result: 

• 64 ohm load get voltage Gain 22 @1.8khz,   8 ohm load gain  3

• input impedance about 600 ohm (difference on 8 ohm load and 64 ohm load is negligible)

but after box it, it's behavior  get strange cuase of bad layout.