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Old 22nd Mar 2022, 9:42 pm   #8
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: 6-gang FM stereo tuner heads

Quote:
Originally Posted by regenfreak View Post
I still struggle to understand concepts like quieting sensitivity and limiting sensitivity.
Perhaps these graphs of typical (albeit quite old) quieting curves will help.

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As may be seen from the upper curves in each case, full limiting (usually I think measured at the 3 dB before full limiting point) occurs before the 50 dB quieting points. And the stereo 50 dB quieting points are the expected 20 dB or so beyond the respective mono points.

The ratio detector (RD) had higher distortion than a Foster Seeley Discriminator (FSD) of the same bandwidth. But, the RD bandwidth could be widened considerably, for lower distortion, whereas that for the FSD could not. So the wide bandwidth RD (WDRD) was a pathway to hi-fi performance. But it then required more IF gain and more limiting ahead of it, which cancelled one of the main reasons for using it. Nonetheless, the WBRD was sometimes found in valve tuners, for example the Radford FMT1. I think that the extra limiting was required because with wide bandwidth, noise spikes well removed from the centre frequency would demodulate quite strongly.

In the solid state era, the equation changed. IF gain and limiting was much easier to obtain, and as RCA was wont to say, the integrated differential pair made an excellent limiter, with low phase shift across a wide variety of input levels. Early FM IF ICs were either designed to work with RDs, or actually included the requisite diodes. Thus some tuner makers, e.g. Leak, who had used FSDs in the valve era, switched to the WBRD for the early solid-state models. Then came the integrated quadrature type. Fairchild did it first in very simple form, but Sprague was first with the definitive, double balanced form using a six-transistor tree, which then became legion. RCA at first avoided this, and came up with its differential peak detector. But it capitulated with the CA3089 IF sub-system in 1971. For lowest distortion, double tuning of those quadrature detectors was desirable, until National came up with a feedback system that achieved the same low distortion with single tuning.

The 6BN6 and various other valve quadrature detectors (see: https://www.vintage-radio.net/forum/...6&postcount=15) were little used for FM receivers and tuners. They had higher distortion than the FSD and RD, and not only that, did not provide a bias output suitable for AFC and/or centre-channel tuning indicators. Rather they were used for intercarrier TV sound, where this basic system anyway limited sound quality. But as you say, the 6BN6 was used as a second limiter in some American high-performance tuners.


Cheers,
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