19-04-2018, 03:01 AM
No problem. It’s an interesting facet of receiver circuitry that does not appear to have received all that much attention in the literature.
Something else that occurred to me is that the use of the FM limiter as AM demodulator is a technique that could have been applicable to UK TV-FM receivers. But as far as I know, it was not used by any of the setmakers. Either a separate diode was used for AM demodulation, or, and I think more commonly, the FM diodes were switched to allow one or both of them to operate as an AM demodulator in the TV model
Pondering that, the best rationale that I can come up with is that it was more likely a question of IF strip gain than any objections to the technique per se. Economics typically indicated that two-stage rather than three-stage sound IF strips be used. At the TV sound IF of 38.15 MHz, I imagine that both amplification stages would have been needed to obtain sufficient overall gain. That effectively precluded AM grid demodulation at the second stage. Where the standard FM IF of 10.7 MHz was used, the second stage could be configured as a limiter or amplifier/limiter, i.e. with somewhat delayed limiting action, whilst still meeting overall gain requirements. Had a three-stage sound IF strip been used, then two stages could have been assigned to gain at both 38.15 and 10.7 MHz, with the 3rd stage as an FM limiter with AM grid demodulation.
The gain issue probably also explains why the limiter grid AM demodulation was not used in those TV-FM receivers that had a second conversion to a lower sound IF, such as Murphy, who used 6.31 MHz. Here, some of the required IF gain was obtained from the triode-heptode second frequency changer conversion gain, and the rest in a single pentode IF stage. (This meant that the valve count was no higher than for the conventional approach.)
In addition to the TV-FM case, limiter grid AM demodulation could be seen as something that might have been applicable to UK dual-standard TV receivers, which had to deal with both AM and FM sound. But again, IF strip gain was probably the issue, with the required gain at 38.15 MHz requiring full utilization of the available sound IF stages (say one or two) for gain, with no margin for using one as a grid demodulator.
I haven’t seen any evidence that Belgian valve era multistandard TV receivers, which had to deal with both AM and FM sound, used limiter grid AM demodulation, but then I have only found a small number of circuits for these. Earlier models apparently used a second conversion for sound, both AM and FM, to 7.0 or 11.8 MHz. Later models used triple-frequency sound IFs, 5.5 MHz (intercarrier) for FM, and 27.75 and 33.4 MHz for AM.
Looking back at the Jason case that triggered this thread, the original JTV had an EF89 IF amplifier followed by an EF80 limiter and a ratio detector for FM. Thus, on AM TV sound, it had just one 10.7 MHz gain stage before the demodulator. On FM, two IF stages before a ratio detector was generally viewed as being just sufficient, but in such cases the second stage was often an amplifier-limiter rather than a straight limiter. That the JTV was replaced by the JTV2 within a relatively short time suggests that higher IF gain was found to be highly desirable. The JTV2 had two EF89 IF gain stages followed by an EF80 limiter and Foster-Seeley discriminator for FM. Thus, on AM, there were two gain stages before the demodulator. The American AM-FM radio receivers that used limiter grid AM demodulation often had three-stage FM IF strips, with the third stage as limiter. So in those cases, no addition to the valve count – as determined by FM needs - was required to include limiter grid AM demodulation. On AM, at 455 kHz, one IF stage was enough for most purposes, and often the 2nd FM IF stage doubled as the AM IF stage. Even with two-stage FM IF strips, AM grid demodulation was feasible because then the 1st FM IF stage doubled as AM IF amplifier.
But a three-stage sound IF strip in a TV-FM or dual-standard TV receiver, as required to provide the needed two IF gain stages followed by AM grid demodulation, or for any other reason would probably have been viewed as excessive.
An interesting example from amongst the UK TV-FM receivers was provided by the Dynatron TV50. The sound IF strip was dual-frequency, 38.15 and 10.7 MHz, with two 6F23 amplifier stages. The second stage was operated with reduced anode and screen voltages on FM (10.7 MHz) to provide limiting action. In that mode, the grid of the second 6F23 was the source of AGC bias for the RF amplifier, so it was being used as a demodulator of sorts. FM demodulation was by a Foster-Seeley discriminator, using crystal diodes, that also provided AFC bias. AM demodulation was by 6D2 that appeared to function simultaneously as a discriminator, to provide AFC bias in the TV mode, and an AM demodulator. Half of another 6D2, fed from the demodulator output, provided delayed AGC bias for the 1st sound IF stage, and a crystal diode provided noise limiting.
Cheers,
Steve
Something else that occurred to me is that the use of the FM limiter as AM demodulator is a technique that could have been applicable to UK TV-FM receivers. But as far as I know, it was not used by any of the setmakers. Either a separate diode was used for AM demodulation, or, and I think more commonly, the FM diodes were switched to allow one or both of them to operate as an AM demodulator in the TV model
Pondering that, the best rationale that I can come up with is that it was more likely a question of IF strip gain than any objections to the technique per se. Economics typically indicated that two-stage rather than three-stage sound IF strips be used. At the TV sound IF of 38.15 MHz, I imagine that both amplification stages would have been needed to obtain sufficient overall gain. That effectively precluded AM grid demodulation at the second stage. Where the standard FM IF of 10.7 MHz was used, the second stage could be configured as a limiter or amplifier/limiter, i.e. with somewhat delayed limiting action, whilst still meeting overall gain requirements. Had a three-stage sound IF strip been used, then two stages could have been assigned to gain at both 38.15 and 10.7 MHz, with the 3rd stage as an FM limiter with AM grid demodulation.
The gain issue probably also explains why the limiter grid AM demodulation was not used in those TV-FM receivers that had a second conversion to a lower sound IF, such as Murphy, who used 6.31 MHz. Here, some of the required IF gain was obtained from the triode-heptode second frequency changer conversion gain, and the rest in a single pentode IF stage. (This meant that the valve count was no higher than for the conventional approach.)
In addition to the TV-FM case, limiter grid AM demodulation could be seen as something that might have been applicable to UK dual-standard TV receivers, which had to deal with both AM and FM sound. But again, IF strip gain was probably the issue, with the required gain at 38.15 MHz requiring full utilization of the available sound IF stages (say one or two) for gain, with no margin for using one as a grid demodulator.
I haven’t seen any evidence that Belgian valve era multistandard TV receivers, which had to deal with both AM and FM sound, used limiter grid AM demodulation, but then I have only found a small number of circuits for these. Earlier models apparently used a second conversion for sound, both AM and FM, to 7.0 or 11.8 MHz. Later models used triple-frequency sound IFs, 5.5 MHz (intercarrier) for FM, and 27.75 and 33.4 MHz for AM.
Looking back at the Jason case that triggered this thread, the original JTV had an EF89 IF amplifier followed by an EF80 limiter and a ratio detector for FM. Thus, on AM TV sound, it had just one 10.7 MHz gain stage before the demodulator. On FM, two IF stages before a ratio detector was generally viewed as being just sufficient, but in such cases the second stage was often an amplifier-limiter rather than a straight limiter. That the JTV was replaced by the JTV2 within a relatively short time suggests that higher IF gain was found to be highly desirable. The JTV2 had two EF89 IF gain stages followed by an EF80 limiter and Foster-Seeley discriminator for FM. Thus, on AM, there were two gain stages before the demodulator. The American AM-FM radio receivers that used limiter grid AM demodulation often had three-stage FM IF strips, with the third stage as limiter. So in those cases, no addition to the valve count – as determined by FM needs - was required to include limiter grid AM demodulation. On AM, at 455 kHz, one IF stage was enough for most purposes, and often the 2nd FM IF stage doubled as the AM IF stage. Even with two-stage FM IF strips, AM grid demodulation was feasible because then the 1st FM IF stage doubled as AM IF amplifier.
But a three-stage sound IF strip in a TV-FM or dual-standard TV receiver, as required to provide the needed two IF gain stages followed by AM grid demodulation, or for any other reason would probably have been viewed as excessive.
An interesting example from amongst the UK TV-FM receivers was provided by the Dynatron TV50. The sound IF strip was dual-frequency, 38.15 and 10.7 MHz, with two 6F23 amplifier stages. The second stage was operated with reduced anode and screen voltages on FM (10.7 MHz) to provide limiting action. In that mode, the grid of the second 6F23 was the source of AGC bias for the RF amplifier, so it was being used as a demodulator of sorts. FM demodulation was by a Foster-Seeley discriminator, using crystal diodes, that also provided AFC bias. AM demodulation was by 6D2 that appeared to function simultaneously as a discriminator, to provide AFC bias in the TV mode, and an AM demodulator. Half of another 6D2, fed from the demodulator output, provided delayed AGC bias for the 1st sound IF stage, and a crystal diode provided noise limiting.
Cheers,
Steve
