Multitone BSR 113, Техническое руководство, Страница: 20 / 63

BSR 113
TM1194A Issue B Page 3 - 6
NOISE SQUELCH PATH
Filtering and Amplification
15. The audio signal from IC1 (pin 14) is de-emphasised by R365 and C154. It is then
filtered by the active filter stages TR41 and TR44 to pass only frequencies above
approximately 6kHz. These stages give approximately 60dBs of rejection of low
frequency audio relative to the high audio frequencies used by the squelch circuitry.
Adjustment of the noise Squelch signal is provided by RV2.
16. From the active filtering stages the audio is amplified by the successive stages
TR40, TR39 and TR38 to produce a high level noise signal at the collector of
TR38. All of these stages are designed to have very stable gain and frequency
response over the operational temperature range.
17. After passing through C163 the signal is fed to the final gain stage TR42, TR43,
which has been configured to have a very stable DC operating point so that the
peak-to-peak noise on the output is very stable with temperature change. The
resulting high level signal is then rectified and buffered by D9, TR37 and TR28,
again with special care being taken of temperature performance. The rectified
signal is smoothed by C167.
Schmitt Monostable
18. The following stage (comprising TR45, TR46, TR47, TR48, TR49 and TR50) is
configured as a Schmitt monostable. When the DC level goes above the level
determined by R373 and R414, then TR36 and TR47 are turned on thus holding
TR45 on and providing hysteresis. As the DC level decreases again R417 stops
TR45 turning off until a lower level is reached than that required to turn it on.
When TR36 collector goes low (i.e. during high noise/low Rx signal strength
conditions), TR48 turns on charging C106 and turning off TR49. This then turns
off TR50 and produces the noise Squelch signal on the collector of TR50.
19. The high levels of amplification are required to enable the noise Squelch to work
on strong RF signals where there is a great deal of quieting. This ensures that
sub-standard audio signals are not output to the system.
Signal Logic Levels
20. The time constants provided by C106 and R420 ensure that the audio is not
enabled until the carrier has been detected for more than approximately 50ms. This
ensures the minimum of squelch bounce in noisy conditions. The logic levels at the
collector of TR50 for the noise Squelch signal are as shown in Table 1.