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Old 2nd Dec 2017, 3:10 am   #28
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Interesting Armstrong AM detector circuit.

An example of a current-driven diode demodulator may be found in the envelope demodulator part of the Brook outboard synchronous demodulator unit described in Wireless World 1989 September, page 856ff.

This was described as having a low-distortion, with the diode driven by a grounded-base transistor at constant current. Here is the circuit:

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Brook quoted a maximum THD of 0.6% for both envelope and synchronous demodulation modes over the frequency range 20 Hz to 7 kHz.

Whilst this type of demodulator was evidently chosen for its low distortion (say as compared with a conventional diode circuit), it does seem to be an interesting choice for the application. The envelope demodulator was required for use during initial tuning, before switching to the synchronous mode. As well as having selectable sidebands in the synchronous mode, there was also a “window” facility in which the receiver’s passband could be offset to minimize interference, in which case the outboard demodulator was receiving an asymmetric sideband signal. The optimum offset in any situation would be found during tuning, when the envelope demodulator was in use. But even perfect diodes distort when presented with a corrupted AM signal, such as one with sideband asymmetry. So, for example a quasi-synchronous demodulator with wideband reference channel might have been preferable for the “envelope” mode, given that it produces much less distortion when operating with asymmetric sidebands.

Certainly, Hershberger used a quasi-synchronous envelope demodulator in his broadly similar outboard synchronous demodulation unit described in Popular Electronics for 1982 April, page 61ff. Here is the block schematic:

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Hershberger claimed that this form of envelope demodulator got around the diode limitations of diagonal clipping and diode threshold distortion. He also provided a diagram showing the difference between synchronous and envelope demodulators when presented with less-than-ideal AM signals:

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In context, envelope demodulator in this diagram is taken to refer to the rectifying types.

Also, Sherwood used a quasi-synchronous demodulator for “envelope” demodulation in SE3 outboard synchronous demodulator unit. This relied upon offset rather than phasing and matrixing for sideband selection, so I imagine that had it used a diode demodulator, it would have sounded very distorted during the setup and offset selection. And Drake had used a quasi-synchronous AM demodulator in its R7 HF receiver, I imagine in connection with its claim that the R7 could provide a 3 kHz AM audio bandwidth with a 4 kHz IF filter, which clearly required significant offsetting and so asymmetric sidebands at the demodulator.

But these examples were from the solid-state era, where from c.1970, active device count was not much of an issue except perhaps for battery-operated equipment. Back in the valve era, more sophisticated forms of AM demodulator were probably too complex, too costly and required too many valves for consumer equipment in general, although used professionally, hence the efforts put into getting better results from the diode and its relatives. In that regard, I wonder if constant-current drive might have been obtained by using a grounded grid triode ahead of the diode. A double diode-triode (DDT) valve of the 6AJ8 type, with all three units separate, might have worked in this role, with one diode as demodulator and the second used to provide AGC delay. Whilst such DDTs were plentiful in the American valve range, they seem to have been unknown in the European range.

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Cheers,
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