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Old 8th Feb 2019, 9:47 pm   #1
FERNSEH
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Default Unusual sound demodulator. Baird T5.

The Baird T5 and T23 TV receivers have an unusual sound demodulator.

All later production T5 models have an interesting sound demodulator. Instead of the usual diode half wave detector, a Mullard FC4 octode valve performs the demodulator function. The first two grids of the FC4 operate as a Colpitts type oscillator. The 41.5mc/s TV sound signal is supplied to grid 4. The resultant audio is developed across the anode load and supplied to a Mullard 354V triode amplifier.
Has any forum member encountered this type of demodulator in a radio receiver?


Here's my suggestion how the Baird T5 sound demodulator works. The 41.5mc/s TV sound is supplied to the signal grid of the FC4 octode, G4. Along with the 100,000 ohm resistor and 0.00005mfd capacitor normal leaky grid detection takes place. The resultant audio signal developed between G4 and the cathode is modulated by the 200Kc/s local oscillator. The 200Kc/s component is filtered out by the pi circuit comprising of the 20mH coils and 0.0005mfd capacitors. It could be said the circuit operates in a similar manner as the bias oscillator in tape recorders.

Seems to me as an overly complicated circuit when one considers a single diode can perform the function of a demodulator.

DFWB.
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Old 8th Feb 2019, 10:33 pm   #2
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Default Re: Unusual sound demodulator. Baird T5.

Quote:
Seems to me as an overly complicated circuit when one considers a single diode can perform the function of a demodulator.
Not only that, the leaky grid detection would likely be much more efficient if the signal was applied to G1, and not use the 200kHz oscillator. It definitely looks like there is detection at G4, due to the 500pF and 100k resistance which has the right time constant for an audio detector.The load that G4 applied though would be low compared to a detector diode.

I'm not sure of the advantage multiplying the leaky grid signal with the 200KHz signal, perhaps the amplitude of the filtered signal increases. It would be really interesting to study this working on the scope, and see just how it worked. I have not seen an audio detector set up like this in any set I have. In theory I think, apart from the sum & difference frequency products in the output, there is also 2 x A1(f)1 x A2(f)2, or at least there is if there is any non linear or square law like amplification, so perhaps it is just a gain lifting measure.

Last edited by Argus25; 8th Feb 2019 at 10:46 pm. Reason: typo
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Old 8th Feb 2019, 10:54 pm   #3
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Default Re: Unusual sound demodulator. Baird T5.

My Baird T23 has the same sound demodulator circuit and it performs very well. However, the sound unit in the Baird T5 needs to be force fed from the signal generator to produce any audio from the loudspeaker.

DFWB.
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Old 8th Feb 2019, 11:22 pm   #4
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Default Re: Unusual sound demodulator. Baird T5.

It has the look of a super regerative detector with the 200Kc/s being the quenching frequency. Very often used at one time in VHF circuits for extra gain.

It’s just a thought, I have virtually no experience with this type of circuit but to me it resembles one.
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Old 9th Feb 2019, 10:52 am   #5
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Default Re: Unusual sound demodulator. Baird T5.

Hi Frank
Super regenerative detector is the term I've always used for this detector.
The 'scope shows a 30volt P - P sine wave on the oscillator grid of the FC4.
When the oscillator is disabled there is almost no audio signal across the anode load resistor.

DFWB.
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Old 9th Feb 2019, 11:17 am   #6
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Default Re: Unusual sound demodulator. Baird T5.

See:

https://www.vintage-radio.net/forum/...ad.php?t=82422

Cheers,
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Old 9th Feb 2019, 12:46 pm   #7
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Default Re: Unusual sound demodulator. Baird T5.

I'm not so sure it is a super regenerative detector, or could rightly be called that (but I'm happy to be proved wrong and could be convinced otherwise)

Generally with either regenerative or super-regenerative detectors, the high gain comes about from the small input signal effectively increasing the feedback so that the oscillation amplitude increases. Blocking (quenching) the supersonic oscillations in the super-regen help this process and huge amounts of gain can be attained with more stability than a regenerative detector.The incoming signals influence the oscillations.

But looking at that circuit, the oscillator is just solidly running in the G1 circuit with typical grid self bias and by the look of it not significantly influenced by the signal voltage at G4.

Also, the modulated carrier voltage that results at G4 will be asymmetrical with the value of the positive going modulation reduced (due to grid detection), meaning, its average value will have the audio component on it. That signal is then multiplied by the 200KHz signal and both the 200kHz signal and 41.5MHz signal filtered off at the plate circuit to recover the audio.

So I think the circuit is the equivalent of an amplitude modulator, converted into a detector by grid leak detection at one of its input ports and a carrier filter on its output port.

Last edited by Argus25; 9th Feb 2019 at 12:53 pm.
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Old 9th Feb 2019, 1:07 pm   #8
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Default Re: Unusual sound demodulator. Baird T5.

Lots of interesting posts in the thread pointed out by Synchrodyne, post 6.
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Old 9th Feb 2019, 7:37 pm   #9
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Default Re: Unusual sound demodulator. Baird T5.

It's actually a deeply-cunning bit of detector design - a phase-locked synchronous demodulator (with harmonic injection). It reinserts the [locally generated] harmonic, high-level phase-locked-to-the-original carrier into what would nowadays be considered a sort of 'product detector' and so by recovering from both sidebands in-phase gains a couple of dB of S/N ratio.

"Exalted Carrier" detection was another name for the technique; I experimented with this on MW in the 1960s/1970s to try and avoid the horrible fading-distortion on dark-hours Radio Luxembourg.
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Old 9th Feb 2019, 8:19 pm   #10
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Default Re: Unusual sound demodulator. Baird T5.

Its 200Kc/s oscillator, wouldn’t the harmonics fall at 41.4 and 41.6 Mc/s?
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Old 10th Feb 2019, 1:10 am   #11
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Default Re: Unusual sound demodulator. Baird T5.

Quote:
Originally Posted by G6Tanuki View Post
what would nowadays be considered a sort of 'product detector' and so by recovering from both sidebands in-phase gains a couple of dB of S/N ratio.
I think it is a type of product detector too.The only thing is whether the incoming carrier on G4 and the oscillator at G1 really do have to be phase locked for it to work, in other words it could be an asynchronous product detector. G3 and G5 are well bypassed at RF frequencies. It would be really interesting to look at the 41.5MHz signal and the 200KHz signal with a good two channel scope and see if they wanted to phase lock or not and if they did, if the detection improved.
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Old 10th Feb 2019, 9:29 pm   #12
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Default Re: Unusual sound demodulator. Baird T5.

The near identical sound receiver from the Baird T23 is now on the bench so that it can be compared with the unit from the T5.
The T23 unit performs much better than the one from the T5.
The oscillator frequency is 190.725Kc/s and is unaffected by either no signal input or the presence of the 41.5Mc/s TV sound signal. The sine wave amplitude on G1 of the MX40 is 30volts P - P, same as the T5 sound unit. Once again an FC4 octode was fitted in place of the MX40 and once again the result was no sound output. For some reason or other the demodulator in the T23 sound receiver will not operate with the Mullard FC4 octode.
When the X10 'scope probe was connected to the signal grid of the MX40 (G4) there was a complete loss of sound output.

Interestingly, the audio output is sensibly constant irrespective of signal input level. Seems the demodulator offers a degree of AVC.

DFWB.
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Old 10th Feb 2019, 11:05 pm   #13
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Default Re: Unusual sound demodulator. Baird T5.

Quote:
When the X10 'scope probe was connected to the signal grid of the MX40 (G4) there was a complete loss of sound output.

Interestingly, the audio output is sensibly constant irrespective of signal input level. Seems the demodulator offers a degree of AVC.
A x10 proble often has far too much input capacitance for the job, especially connecting it to a tuned circuit in the 10's of MHz region. One way to get around that is to clip the scope probe onto the insulation of a wire in the grid circuit, it makes a fraction of a pF gimmick capacitor and won't disturb the circuit to any extent, but the waveform can still be seen winding up the scope's gain. I often do this to look at waveforms in sensitive RF or osc circuits where I want to see what is there without disturbing it or pulling the frequency. Of course the amplitude data is meaningless (but you can get that with a diode detector probe) it is just good to be able to look at the waveform without altering the circuit under test.

One thing that would be interesting to find out about this detector, would be how it behaved with very low level input signals compared to a standard valve diode detector or grid leak detector. I think it would probably be better, which would make it a very appealing option for a home built radio.
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Old 14th Feb 2019, 11:57 am   #14
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Default Re: Unusual sound demodulator. Baird T5.

In the March/April 2002 GFGF bulletin an article about the synchronous detector employed in the Korting Royal Syntektor 55 FM radio receiver. In this receiver an ECH81 performs the function as the synchonous detector.
The IF is the usual 10.7Mhz and the oscillator in the Syntektor operates in a Meissner circuit at 2.14Mhz +/-30Khz. Corresponds to 1/5 of the IF.
Reading though the text it appears that the syntektor function is to achieve optimum selectivity (trennscharfe)
Through the Synchro-Detektor circuit alone a selectivity improvement of 1:100 to 1:1000 can be achieved.

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Old 15th Feb 2019, 11:55 pm   #15
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Default Re: Unusual sound demodulator. Baird T5.

In the valve era, locked oscillator type synchronous (quadrature) demodulation does seem to have been used more for FM than for AM.

Possibly the first such circuit used in production receivers was that developed by Bradley of Philco c.1946. For this Sylvania developed a purpose-designed heptode valve, namely the FM1000. Philco used this demodulator for a few years, but it did not appear to have sustained success.

In part it was the outcome of a quest for a single-valve circuit that combined limiting and FM demodulation, and preferably with AF gain as well, and was one of several approaches that appeared following WWII.

Another was the gated beam-valve circuit, a joint Zenith and GE effort, and for which a special valve, the 6BN6 was developed. This was a non-oscillating self-limiting quadrature demodulator, founded upon the earlier work by Kalmus and Zakarias. I think that it was used in some American TV receivers through the 1950s.

And yet another was the Philips enneode circuit, also of the non-oscillating, self-limiting quadrature type, based upon the earlier work of Okrent. For this the EQ40/EQ80 enneode valve (misnamed by Mullard as a nonode) was developed. It was not used long-term.

In 1954 RCA introduced its own locked-oscillator FM demodulator, based upon a dual-control pentode valve, the 6DT6. This was actually a hybrid circuit, acting as a locked-oscillator demodulator with lower level input signals, but as a quadrature demodulator, with oscillation suppressed, at higher signal levels. I think one of the objectives was to do the job with a lower cost and more conventional valve than the 6BN6. As best I can determine this was used through to the end of the valve era, and improved dual-control pentode valves were developed, such as the 6GX6 in 1961.

Quite late on, early 1960s I think, Philips/Mullard introduced a locked-oscillator FM demodulator based upon the EH90 heptode valve, essentially for TV sound applications. To some extent this appears to have been a reincarnation of the Philco circuit. The EH90 was the Pro-Electron designation for the American 6CS6. This, a sharp cutoff short grid-base heptode, had been introduced c.1983-54 for use as a noise-gated sync separator in TV receivers, as had the similar 6BY6. Zenith had originally developed the basic noise-gated sync separator circuit around the 6BN6 gated bean valve FM demodulator, but as this was an expensive valve, had looked at alternatives and found that the 6BE6 radio pentagrid worked reasonably well. The US valve industry must have seen that a purpose-designed valve was justified, hence the 6BY6 and 6CS6. So it’s somewhat ironic that the 6CS6 ended up in FM demodulator service.

The Korting FM demodulator appears to have been another resurrection of the locked-oscillator type, this time with a sub-harmonic oscillator using a separate valve section. Possibly there were other implementations as well. But these may not have been well enough known to be mentioned in the available histories. I had not previously seen anything on the Korting circuit.


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Old 16th Feb 2019, 7:33 pm   #16
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Default Re: Unusual sound demodulator. Baird T5.

Thanks to Synchrodyne for this rather-good-and-suucinct summation of the various "Non-Traditional Demodulators" and their associated specialist valves.

It's always interesting to see what different manufacturers got up to in the urge to get one-over on their competitors!
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