Service manual

NP88 FLEX RECEIVER
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Mixer Circuit
Mixer is to mix RF signal (Fr) and first local oscillator signal (Flo), and it will bring the first intermediate
frequency which is lower frequency (45MHz), the circuit composed of Q3, C17, R5, L3, C13,
R4, R6 is bias. L3, C13 are Tank circuit. Q3 mixes Fr and Flo, then take out 45MHz first inter-
mediate signal (IF1) by L3 and C13. IF1 = Fr – Flo. IF1 signal will couple to Crystal filter by through F1.
IF IC CircuitΚIF IC is an integrated circuit for dealing with intermediate frequency only. The function as
following.
• Second Local Oscillator(LO2)Κ˨̆˸ʳˇˇˁˈˇˈˠ˛̍ʳ˖̅̌̆̇˴˿.
•MixerΚMixer of the IC is double balance type, the first intermediate signal which is 45MHz input
through PIN24, after correct circuit by matching, IF1 will output with 455KHz second intermediate
frequency, which is produced by local oscillating signal mixer. And it will output through PIN3.
• LimiterΚSignal of IF2(455KHz) output from PIN3, it is usually received into ceramic filter for filtering out
noise which is out the channel, signal will back to inner IC through PIN5, it is amplified about 100dB and
limited the voltage amplitude for dealing with detector.
• DetectorΚIt is composed of quadrature type detector. Discriminator connects F3 to PIN8 in IC and R31.
The phase of the modulated FSK signal is shifted.
• Baud Rate FilterΚBefore data outputs to Decoder, for avoiding interference of noise to cause incorrect
message rate too high. Noise of data which is detected should be filter out. Circuit is composed of OP
amplifier which is inner IC, and match by C321, C322, C323.It will become a Butterworth
low-pass filter. Signal will be input from PIN9, 10, 11, then the internal part amplify to PIN15, 16,take out
data signal of NRZ through FSK comparing circuit from PIN18, 19, finaly connect PIN3 of J1.
• Regulating and stabling circuitΚFor avoiding characteristic abnormal this is caused by changing of RF
circuit. PIN22 of IFIC offers 1.0 regulating and stabling power. The power offers NP88 Flex 900MHzz bias
of the first local oscillator, mixer, PLL circuit and VCO circuit.
Power Saving Circuit
Power saving control is a control signal EC which sent by PIN17 of CONNECTOR J1.Turnitonwhenit
connect to IFIC PIN17 ”H”, RF circuit will start to receive message right away. Turn it off when it is “L”,
and RF circuit won’t work. At standby mode, it can save power.
June 21, 2004 3-3