VHF (146-174MHz) Receiver
2-3
2.2
Receiver Back End
The receiver back end is a dual conversion design. High IF selectivity is provided by FL51, a 4-pole
fundamental mode 44.85 MHz crystal filter with a minimum 3 dB bandwidth of + 6.7 kHz, a
maximum 20 dB bandwidth of
±
12.5 kHz, and a maximum insertion loss of 3.5 dB. The output is
matched to IF amplifier stage Q51 by L53 and C93. Q51 provides 16 dB of gain and a noise figure of
1.8 dB. The dc operating current is 1 mA. The output of Q51 is applied to the input of the receiver
IFIC U51. Diode CR51 limits the maximum RF level applied to the IFIC.
The IFIC is a low-voltage monolithic FM IF system incorporating a mixer/oscillator, two limiting IF
amplifiers, quadrature detector, logarithmic received signal strength indicator (RSSI), voltage
regulator and audio and RSSI op amps. The second LO frequency, 44.395 MHz, is determined by
Y51. The second mixer converts the 44.85 MHz high IF frequency to 455 kHz.
Additional IF selectivity is provided by two ceramic filters, FL52 (between the second mixer and IF
amp) and FL53 or FL54 (between the IF amp and the limiter input). The wider filter FL53 is used for
20/25 kHz channel spacing, and the narrower filter FL54 is used for 12.5 kHz channels. When the
BW_SEL line is high, the two upper diodes in packages D51 and D52 are forward biased, selecting
FL53 for 20/25 kHz channels. When the BW_SEL line is low, the two lower diodes in packages D51
and D52 are forward biased, selecting FL54 for 12.5 kHz channels.
Ceramic resonator Y70 provides phase vs. frequency characteristic required by the quadrature
detector, with 90 degree phase shift occurring at 455 kHz. Buffer Q70 provides a lower driving
impedance from the limiter to the resonator, improving the IF waveform and lowering the distortion
of the recovered audio signal. The recovered audio level at the DEMOD output is 100 mV rms (25
kHz channel, 3 kHz deviation) or 50 mV rms (12.5 kHz channel, 1.5 kHz deviation). An additional
RSSI output provides a DC voltage level that is proportional to RF signal level. This voltage is
measured by an A/D converter contained in the microprocessor (PE4_AN4, U401 pin 63).
FL52
FL53
FL54
Number of Elements:
4
6
6
Insertion Loss:
4 dB
4 dB
4 dB
6 dB Bandwidth:
15 kHz
15 kHz
9 kHz
50 dB Bandwidth:
30 kHz
30 kHz
22 kHz
Stopband Rejection:
27 dB
47 dB
47 dB
Summary of Contents for CP 140
Page 1: ...Commercial Series CP140 CP160 CP180 Portable Radios Detailed Service Manual 6866550D20 O ...
Page 2: ...ii ...
Page 4: ...iv ...
Page 12: ...viii ...
Page 28: ...iv ...
Page 40: ...1 12 THEORY OF OPERATION ...
Page 48: ...2 8 TROUBLESHOOTING TABLES ...
Page 50: ...3 2 CONTROLLER KEYPAD SCHEMATICS ...
Page 58: ...3 10 Keypad Board Parts List 8466565A01 O ...
Page 62: ...iv ...
Page 66: ...1 4 MODEL CHART AND TECHNICAL SPECIFICATIONS ...
Page 90: ...3 16 TROUBLESHOOTING TABLES ...
Page 92: ...4 2 VHF PCB SCHEMATICS PARTS LISTS ...
Page 108: ...iv ...
Page 112: ...1 4 MODEL CHART AND TECHNICAL SPECIFICATIONS ...
Page 136: ...3 16 TROUBLESHOOTING TABLES ...
Page 138: ...4 2 UHF1 PCB SCHEMATICS PARTS LISTS ...
Page 154: ...iv ...
Page 158: ...1 4 MODEL CHART AND TECHNICAL SPECIFICATIONS ...
Page 182: ...3 16 TROUBLESHOOTING TABLES ...
Page 184: ...4 2 UHF2 PCB SCHEMATICS PARTS LISTS ...
Page 200: ...iv ...
Page 204: ...1 4 MODEL CHART AND TECHNICAL SPECIFICATIONS ...
Page 228: ...3 16 TROUBLESHOOTING TABLES ...