Regen Receiver, Heterodyne Oscillator.

[G6Tanuki]

Originally Posted by AlanC

It occurred to me that in a direct conversion receiver with a traditional variable VFO, it ought to be possible to phase lock it to the carrier by DC amplifying the output of the ring mixer and feeding it to a varactor in the VFO.

I have a Sony ICF2001D which has a synchronous AM mode, there is a chip in it that in effect locks a VCO to the IF (there is a panel LED that tells you when it's locked) and the audio is demodulated in a balanced mixer. It eliminates the distortion caused when the carrier is reduced relative to the sideband(s) by selective fading, resulting in what looks to a diode detector like an overmodulated signal. It actually works spectacularly well.

Yes, PLLing the VFO to the carrier is a well-established technique and works just fine so long as you don't get the 'selective fading' that dips the carrier out to the point where your loop unlocks.

Another AM technique is what's often called "Exalted Carrier" detection - you split the signal and take one part through a very narrow [10Hz or so] filter to remove the sidebands, then amplify the result and put it through a clipper/squarer.

You then use this as the carrier to drive a classic double-balanced-mixer stage (push-pull FET, diode-quad): essentially the locally-reconstituted carrier is of such an amplitude that it overrides the carrier component of the signal you're trying to listen to, which is then demodulated as if it was a DSB signal. If you've got enough gain/Schmitt-trigger-waveform-cleanup effect in the carrier-processing circuit it'll continue producing a good carrier even when the signal's fading badly. Schottky 7400-series TTL is fast enough to do the processing for medium-wave signals.

It's a nice approach, but generally falls down because of the phase-shift and delay inherent in the carrier filtering/processing stage meaning that when you reinsert this carrier into the original DSB AM stream it's not 'quite' right.

It's also interesting that you can use a local carrier to improve operation of crystal-sets: http://www.crystalradio.net/crystals...FO_Article.doc

I wonder if this effect could explain how sometimes people have reported much-better-than-predicted "DX" reception with a crystal-set? Could a nearby broadcast-station 'tuning up' with an unmodulated carrier have biased the detector so it was much more sensitive to a more-distant station on the same (or nearby) feequency?

Whatever, it's always interesting to look at the different ways to detect a signal. I have somewhere a diagram for a VHF valve superregenerative receiver where - unlike most designs where the quench is obtained by making a regenerative detector 'squeg' - this uses a regenerative detector quenched by an externally supplied supersonic signal with a waveform that looks like that fed to the vertical deflection plates of a TV.

[AlanC]

Hi G6Tanuki, very interesting! Mike Tuggle's paper makes very interesting reading; I can see this becoming a project for a rainy day!

Super-regens are a big interest of mine, it was constructing one in the mid 70's to an FG Rayer design, but covering PMR low band, that led to my interest in that field and ultimately to being employed by Pye Telecom!

The WS19 B set uses a separate quench oscillator as I'm sure you know. I've tried this concept out using solid state devices without too much success; whereas the self-quenched circuits always worked fine for me. I've even made them work at 432 MHz using a BFY90 and a stripline inductor!

If you're interested in unusual methods of detection, you might be interested in the locked-oscillator detector FM broadcast tuner whose circuit diagram I have attached below, it's a photo of a page in an old notebook I kept when I worked at Pye's in the 1980's. I cannot remember where I got this circuit from, and I've never seen it anywhere since.

The heart of it is the extremely unusual oscillator built around the two BFR38's TR2 and 3. They form a VERY low power oscillator which becomes locked to the incoming signal amplified by TR1. As the input signal deviates, the voltage at the oscillator emitter common point varies in sympathy, representing the recovered audio signal which is buffered by TR4 and amplified by TR5 and 6.

In use, the receiver is surprisingly sensitive and gives good quality audio. I still have it somewhere- must try and dig it out!

[G6Tanuki]

While happy to discuss further, I fear the lurking risk of thread-drift.

Dear Moderators/Moderatrices - before I deviate into the world of superregenerative-superhets and WWII RADAR gear - where should we take this??

[AC/HL]

Radar will definitely need a new thread, but as for superregenerative-superhets, it depends on how much there is to discuss, and whether or not you want it to be findable in the future.