Quote:
Originally Posted by jr_tech
Interesting, thanks for posting! I suspect that they are driving the first two stages of the if amp with a different level of agc (through the dc amplifier, v11) than the rf stage (direct from the local/distant switch) to optimize signal to noise ratio at a given total gain. This is just a WAG. 
jr |
I haven't traced it through, but if what you surmise is the case, it's quite normal for TV AGC design.
This is called tuner AGC "delay." This doesn't mean a time delay, but that the tuner runs at full gain for weak signals so that the noise figure is maximized [EDIT: I meant minimized, of course]. The tuner AGC gain reduction starts only when the IF AGC gain reduction has already had some effect. In the low signal range (I seem to recall, typically below 1 millivolt RF) the IF AGC is active by itself. Then, above that level, AGC gain reduction is also applied in the tuner.
An alternative, which was never used in TVs as far as I know, is to run both the tuner and IF at max gain for as large a range as possible and use a PIN diode attenuator at the input to adjust signal level.
TVs actually get pretty poor noise figures when the signal is strong and tuner AGC is active, but as long as the video signal-to-noise ratio is great enough to make the noise invisible, it's OK.