Motorola CP150TM - Manual (Page 144)

June, 2005 6880309N62-C
8-2 403-440 MHz UHF Theory Of Operation: UHF Receiver
The output of the RF amplifier is applied to the interstage filter, a fixed-tuned 4-pole Butterworth
shunt-coupled resonator design having a 3 dB bandwidth of 68 MHz centered at 462 MHz, and
insertion loss of 3.5 dB. This filter yields an image rejection of 58 dB at 350 MHz, assisted by a
transmission-zero at 300 MHz implemented by C524 for the reasons mentioned above.
The output of the interstage filter is connected to the passive double-balanced mixer consisting of
components T41, T42, and CR41. This mixer has a conversion loss of 7.2 dB. Low-side injection from
the frequency synthesizer is filtered by L40-L41 and C41-C45 to remove second harmonic energy
that may degrade half-IF spurious rejection performance. The injection filter has a 3 dB bandwidth of
100 MHz centered at 376.15 MHz, and an insertion loss of 2.7 dB. The second-harmonic rejection is
typically 45 dB or greater. The filtered injection signal is applied to T42 at a level of +6 dBm.
The mixer output is applied to a diplexer network (L51-L52, C51, R51) which matches the 44.85 MHz
IF signal to crystal filter FL51, and terminates the mixer into 50
Ω at all other frequencies.
8.2.2 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.
The ceramic filters have the following specifications:
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 120 mV rms (25 kHz
channel, 3 kHz deviation) or 60 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