CIRCUIT DESCRIPTION
AUDIO/LOGIC DESCRIPTION (ALL MODELS)
The passband of this filter is controlled by the Q1 output (pin 18) of latch U111. When LTR or digital Call Guard data or
From U300C the data signal is fed to a DC restoration circuit formed by U300D and U300A. This circuit converts it from an analog signal floating at half supply to a digital signal at 0 and 5 volt levels that can be detected by the microcontroller. U300D is a standard noninverting amplifier with a gain set by R308, R316, and R317 (R317 is AC grounded by C309).
Two attenuation levels are selected by gate U307D similar to gate U307B described in Section
3.4.1.When wideband (25 kHz) channels are selected, the control input (pin 12) of U307D is high and R317 is effectively shorted which increases gain. The gain of U300D is approximately 3 with wideband channels and 2 with
The CR301 diodes charge and discharge C309 to establish a DC reference on pin 2 of comparator U300A. This reference voltage is the average of the positive and negative alternations of the data signal. When pin 3 of U300A rises above the reference on pin 2, the output goes high (8 volts) and vice versa. Voltage divider R304/R311 provides the
3.4.4 SQUELCH CIRCUIT (U304A, U304B)
The microcontroller uses the output from the
squelch circuit and also the RSSI output of the limiter/ detector (see Section 3.8.4) to determine when to mute and unmute the receive audio and also when valid data may be present. The squelch circuit is controlled by the amount of noise present in the receive audio signal. When no signal or a weak signal is being received, there is a large amount of noise present. Conversely, when a strong signal is received, there is very little noise present.
The receive audio, data, and noise signal from amplifier U301B is applied to a bandpass filter and amplifier formed by U304A and other components. This stage attenuates voice frequencies and LTR and Call Guard signaling so that only noise frequencies in the range of approximately
C304 charges through the forward biased diode in CR300 and discharges through R306. C303 discharges through the other diode in CR300. When the voltage on pin 6 of comparator U304B rises above the reference on pin 5, the output goes low. For example, when received signal strength is low, more noise is rectified which causes the voltage on pin 6 of U304B to increase and the output on pin 7 to go low (squelched condition). A voltage divider formed by R416 and R417 reduces the
The squelch threshold level is set by a D/A converter formed by shift register U305 and several resistors. The
NOTE: A block diagram of the audio and data processing circuitry is shown in Figure
3.5.1MICROPHONE AMPLIFIER (U303B),
The microphone audio signal is coupled by C349 to amplifier U303B which provides a gain of approximately two. R410, R414, and C354 provide a bias voltage of approximately 3.2 volts on the noninverting input. An
From U303B the microphone signal is coupled by C350 to a
| February 2001 |
Part No. |