I always have some project on the back burner and this is one.
This should have part of my "syncrodyne" tuner experiments but I never got to it.

I wanted to use a phase discriminator transformer to control the local oscillator frequency and "auto tune" an AM receiver.
No particular reason why, just experimenting.

In this first part I revive my prototyping chassis and wind up a discriminator coil that provides a control signal.
As usual I have very little idea of what I am doing but just go ahead and do it anyway.
That is what prototyping is, put something together and make it work and then try and make it look like you know really do know what you are doing!


Fred.

Auto Frequency Control

The project turned into a theory exercise as there was not much practical use for a AFC control on an AM superhet.
Interesting exercise only.

I'll send a complete part 1 and 2 to Brad and get on with something else!

Fred.

Document uploaded.

Did post to this & it seems to have gotten lost.

I would note that in the library here there is a book that does have some data on AFC & pictures

Marc

Hi Marc, what was the name of the book?

Fred.

Hi Fred

Your circuit resembles that of the RCA Synchrolock hor. osc. as used in HMV TVs E and F series. No varicap diode though!

Do you have access to it or will I send you a copy?

Interesting idea but as you say, limited use. I have one radio that drifts rather alarmingly, so much that 2CH can disappear entirely after a while. An AFC could help there. Also for shortwave where heat can easily shift you to an adjacent broadcast.

I most likely would use the modern route though, just a standard varicap diode like was used in a CB radio's clarifier circuit. Using a valve may fit in with the historical time period, but is such a lot more work to implement.

Yeah Rob, using valves was the point of the exercise.
I could step up to parts per million using a PPL chip, but thats not a valve!
I needed more loop gain and less varicap, but that is for another time.

Hi Ian, aha, the E and F series are in my JR manual and yes, a 6BX6 as a "reactance valve" with a 6AL5 and a discriminator coil.
Wow, they went to a lot of trouble in those models to lock the horizontal up tight. Look at the parts count! They neatened it up by the P series and were into diodes and 6EJ/H7's.

I have reconfigured my chassis and looking at the Syncrodyne tuning again with carrier cancellation in a valve mixer.

All Fascinating.

Fred.

Re the synchrodyne, a thought I had last time to turn it into a usable receiver would be to have a normal diode detector for tuning and AGC and have a Lock Detector output on the PLL. Unlock would select the diode as the audio output, Lock would select the synchrodyne.

Oh, and a Lamb noise blanker. And an active notch filter for the 9kHz beat.

Trouble is, there is really nothing worth listening to on the MW band so what's the point?

However some modern designers must think it's worth doing. The radio in my IS250 uses DSP to process the AM. It does a very good job for sound quality on strong signals and has very effective noise processing on weak ones or where there is interference.

Hi Ian, I had very good results the last time using a mixer valve as the means of cancelling the carrier.

I'll go back to that and use a 6BE6 and direct trigger the outboard oscillator through a sync seperator like an old B/W TV!
No PPL, no discriminator.
What could go wrong?
See how far I get.

Feed.

Book is called "Radio Engineering" No circuit where it is mentioned but its explaining using a discriminator and actually how its applied.

No date on it but one of the circuits features what would be possibly 6J8 so perhaps late 30's to 40's?

Came of of England & I somehow have two copies both from different printers which would suggest popular.

May be more in other books here? They're reference books, so one does tend not to read them like novel.

Marc

There are several pages on AFC in Radiotron Designer's Handbook 4th Ed including ideal and commercial circuits, mathematics etc. Can scan if it would be useful.

Thanks STC I have the Designers handbook and yes, love the mathematics!
Fred.

" love the mathematics!"
Out of practice with that - not sure if I know my tan from my arctan these days. Wouldn't attempt it with a slide rule or a calculator, but should set up OK in a spread sheet if needed.

Have never designed and built anything from scratch - my job didn't require it so I don't have that knowledge in my bones, especially solid state stuff. Can build and trouble shoot a kit or fix a radio usually but am in admiration of your "cobbles" from "that's a good idea, let's give it a try".

Its fun building things from scratch, albeit I have not built anything radio for a while. Its an Engineering cycle here at the moment. Mate broke his photo tripod so I have turned a new bit for it & added a bush as I did not consider a 5 mm rod in a 6.5mm hole, desirable, in the situation its in. He borrowed one of mine & has managed to break it now?

Hopefully tomorrow I will finally get at the tractor 8Km away & re-tighten the cylinder head. Of course bits have to be removed to accomplish that.