Service Manual Part 5
123)
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Current .. <30mA
Operating Temperature Range .. -30 to +60°C ambient
Audio:
Input For 60% Deviation .. 1mVrms ± 0.1mV (1kHz, 600Ω)
Microphone .. uni-directional electret
Microphone sensitivity .. -67dB ±3db (0dB = 1V/µbar)
Distortion .. <5%
Delay From Audio To PTT Request .. 20ms (typical)
+ !7
Audio is fed into the preamplifier circuitry (Q1) from the electret microphone via pin 2
of the 9 way D-range (S21). The pre-amplified audio is then split and goes into the
speech bandpass filter and the noise filter.
The speech bandpass filter consists of a 4th order 250Hz high pass filter (IC1) and a gain
stage (IC1) with a 3kHz rolloff. Audio then goes into the compressor circuitry (IC1).
When PTT is requested by the logic circuitry, IC2 switches and the audio is driven into
the TX-LINE-IN at a low impedance. After the high pass filter, audio is tapped off into
the speech VOX filter (IC4), which provides a low pass cutoff of 700Hz. This, combined
with the previous high pass filter, gives the speech VOX filter cutoffs of 250Hz and
700Hz.
The second path of the audio after the preamplifier is to the noise filter (IC3). This con-
sists of a 2nd order high pass filter below 100Hz.
After the gain adjusting stages (IC3 & IC4) each filter goes into a precision rectifier (IC3
& IC4), where their appropriate DC level is presented to a comparator (IC3 pins 12, 13 &
14). The negative input has a potentiometer (RV67) providing a DC offset, which is used
to provide a minimum VOX threshold. This minimum threshold decreases with VOX
activity to provide some hysteresis. The negative input is also held high via IC2, pre-
venting the comparator switching until the synthesiser lock detect line (IN/LOCK) is
low. The positive input has a zener diode to prevent VOX activating when both inputs
are in saturation.
The switching circuitry after the comparator output has a slow decay provided by R69
and C31. IC5 will only allow a VOX PTT request if the AUX line has been taken high.
Two NAND gates in IC5 allow a handsfree PTT request (VOX or MOX) to proceed
unless the PTT-IN line has been taken low. A valid handsfree PTT request will mute the
ordinary microphone and switch IC2, whereas a PTT-IN signal will be mirrored on the
PTT line, without switching in the handsfree audio or muting the ordinary microphone.
The CALL line is used by trunking software to mute the handsfree audio when the
audio path has been invalidly switched in (i.e. on a control channel). Similarly, the BUSY
line is used to mute the audio if TX inhibit on busy is used.