Service Manual UHF1 (400-470MHz) Information
37
7.3.2 Working Principle of VCO
VCO employs Colpitts oscillator circuit (the RX oscillator circuit is composed of D102, D103, D106, D107
and L112; the TX oscillator circuit is composed of D108, D109, D110, D101 and L117). It obtains
different output frequencies by changing the varactor's control voltage (i.e. CV voltage).
There are two types of VCO: TX VCO and RX VCO. Both types control EMD22 to switch operating
status via OMAP. RX VCO is composed of the oscillator loop and Q104, to provide LO signal. TX VCO is
composed of the oscillator loop and Q108, to provide carrier for TX signal.
7.3.3 Two-point Modulation
In TX mode, the two-point modulation technology is employed, to obtain higher modulation accuracy and
lower 4FSK bit error rate. MOD-VCO and MOD-XO send the modulation signal to the modulation end of
VCO and the reference crystal oscillator of PLL respectively to modulate TX VCO and the reference
crystal oscillator.
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