RF Technology R70/R71 and R150
Page 13
5.5 Audio Signal Processing
5 CIRCUIT DESCRIPTION
1.1 GHz pre-scaler, programmable divider, reference divider and phase/frequency
detector. The frequency data is entered by a serial data link from the microprocessor.
The phase detector output signals of U4 are used to control two switched current
sources. The output of the positive and negative sources' Q10 and Q16, produce the
tuning voltage which is smoothed by the loop filter components to bias the VCO
varactor diode D4.
5.5
Audio Signal Processing
A 4 KHz low pass filter (U27b) is used to remove high frequency noise from the
signal. A 300 Hz high pass filter (U26a,b) then removes the sub-audible tones. A 240
Hz notch filter (U26c,d) is used to improve the rejection of tones above 200 Hz. The
high pass and notch filters can be bypassed by internal jumpers JP1 and JP3.
The audio frequent response can be set for either 750 uS de-emphasis or a flat
characteristic by JP2. JP2 selects the feedback network of amplifier U27c.
After de-emphasis and filtering, the audio signal is applied to the inputs of two
analog switches (U17a,b). These switches are controlled by the micro-controller and
squelch or mute the audio to the line and monitor output circuits. The monitor output
can be set for noise squelch only operation by S1.
The audio from U17a is adjusted by the volume control before connecting to the
monitor output amplifier U5. U5 drives the internal speaker and can also supply 3-5
watts to an external loudspeaker.
The audio from U17b is adjusted by RV3 before connecting to the line output IC
(U22a,b). U22 is a dual amplifier connected in a bridge configuration to drive the
600
Ω
line output transformer T1.
5.6
Noise Filter, Amplifier and Detector
The unfiltered audio from the discriminator is fed to trimpot RV4 which is used to
set the noise squelch threshold. From RV4 the audio goes to the noise filter (U27a).
This is a 10 KHz high pass filter and is used to eliminate voice frequency
components.
The noise signal is then amplified by U27d and fed to the noise detector. The noise
detector consists of D6, Q17 and U26c. D6 and Q17 are a charge pump detector and
pull the input to U26c low as the noise increases. U26c has positive feedback and
acts like a schmidt trigger. The output of U26c goes high when noise is detected. It
connects to the micro-controller and to analog switch U17d. U17d varies the gain of
the noise amplifier to provide approximately 2dB hysteressis.
5.7 Sub-Tone Filter and CTCSS
The discriminator audio is fed through cascaded low pass filters U28a
and U28b to filter out the voice frequency components. The filtered
sub-tone audio is supplied to the CTCSS hybrid and the rear panel
