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2-22
Theory of Operation
: VHF Receiver
The IF signal coming out of the mixer is transfered to the crystal filter (Y3200) through a resistor pad
(R3321 – R3323) and a diplexer (C3320 and L3309). Matching to the input of the crystal filter is
provided by C3201 and L3200. The crystal filter provides the necessary selectivity and
intermodulation protection.
2.13.2 Receiver Back-End
The output of crystal filter Y3200 is matched to the input of IF amplifier transistor Q3200 by L3203.
Voltage supply to the IF amplifier is taken from the receive 5 volts (R5). The IF amplifier Q3200 is
actively biased by a collector base feedback provided by R3202 and R3203. The gain controlled IF
amplifer provides a maximum gain of about 16dB. A dual hot carrier diode (CR3201) limits the filter
output voltage swing to reduce overdrive effects at RF levels above -27dBm. The amplified IF signal
is then coupled into U3220 (pin 1) via L3202, C3207, and C3200 which provides the matching for the
IF amplifier and U3220.
The IF signal applied to pin 1 of U3220 is amplified, down-converted, filtered, and demodulated, to
produce the recovered audio at pin 7 of U3220.
Within U3220, the first IF 44.85 MHz signal mixes with the 44.395 MHz second local oscillator (2nd
LO) to produce the second IF signal at 455 kHz. The 2nd LO signal frequency is determined by
crystal Y3201. The second IF signal (455 kHz) is then filtered by an external ceramic filter Y3205
before being amplified by the second IF amplifier within U3220. Again, the signal is filtered by a
second external ceramic filter Y3203 or Y3204 depending on the selected channel spacing. Y3203 is
used for 20/25 kHz channel spacing whereas Y3204, for 12.5 kHz channel spacing. The simple
circuit consisting of U3221, CR3202, CR3203 and resistors R3209, R3212, R3211 and R3205 divert
the second IF signal according to the BW_SEL line. The filtered output of the second IF signal is
applied to the limiter input pin of U3220 (Pin 14).
The IF IC (U3220) contains a quadrature detector using a ceramic phase-shift element (Y3202) to
provide audio detection. Internal amplification provides an audio output level around 120mVrms
(@60% deviation) from pin 8 of U3220. This demodulated audio is fed to the ASFIC_CMP IC (U404)
in the controller section.
The IF IC (U3220) also performs several other functions. It provides a received signal-strength
indicator (RSSI) with a dynamic range of 70 dB. The RSSI is a dc voltage monitored by the
microprocessor, and used as a peak indicator during the bench tuning of the receiver front-end
varactor filter.
2.13.3 Automatic Gain Control (AGC)
The front end automatic gain control circuit provides automatic reduction of gain, of the front end RF
amplifier via feedback. This action is necessary to prevent overloading of backend circuits. This is
achieved by drawing some of the output power from the RF amplifier output. At high radio
frequencies, capacitor C3327 provides the low impedance path to ground for this purpose. CR3302
is a PIN diode used for switching the path on or off. A certain amount of forward biasing current is
needed to turn the PIN diode on. Transistor Q3301 provides this current.
Radio signal strength indicator, RSSI, a voltage signal, is used to drive Q3301 to saturation i.e.
turned on. RSSI is produced by U3220 and is proportional to the gain of the RF amplifier and the
input power to the radio.
Resistors R3304 and R3305 are voltage dividers designed to turn on Q3301 at certain RSSI levels.
In order to turn on Q3301 the voltage across R3305 must be greater or equal to the voltage across
R3324, plus the base-emitter voltage (Vbe) present at Q3301. Capacitor C3209 is used to dampen
any instability while the AGC is turning on. The current flowing into the collector of Q3301, a high
current gain NPN transistor, will be drawn through the PIN diode to turn it on. Maximum current
flowing through the PIN is limited by the resistors R3316, R3313, R3306 and R3324. C3326 is a
feedback capacitor used to provide some stability to this high gain stage.
Содержание PR860
Страница 1: ...PR860 Two Way Portable Radio Detailed Service Manual ...
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Страница 5: ...iv Notes ...
Страница 9: ...viii Table of Contents Notes ...
Страница 13: ...Notes xii List of Figures ...
Страница 17: ...1 4 Introduction Radio Model Information Notes ...
Страница 51: ...2 34 Theory of Operation Voltage Control Oscillators VCO Notes ...
Страница 81: ...4 4 Schematic Diagrams Overlays and Parts Lists Flex Layout Notes ...
Страница 193: ...4 116 Schematic Diagrams Overlays and Parts Lists Low Band Section Notes ...
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