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3. TECHNICAL BRIEF
B. OPLL
The offset mixer down converts the feedback Tx RF signal using LO to generate a IF modulating
signal. The IF signal goes via external passive bandpass filter to one port of the phase detector.
The other side of the phase detector input is LO signal. The phase detector generates an error
current proportional to the phase difference between the modulated signal from the offset mixer and
the reference signal from the LO.
The error current is filtered by a second order low-pass filter to generate an output voltage which
depends on the GMSK modulation and the desired channel frequency. This voltage controls the
transmit VCO such that the VCO output signal, centered on the correct RF channel, is frequency
modulated with the original GMSK data. The OPLL acts as a tracking narrowband band pass filter
tuned to the desired channel frequency. This reduces the wideband noise floor of the modulation
and up-conversion process and provides significant filtering of spurious products.
C. Synthesizer
The CX74017 includes a fully integrated UHF VCO with an on-chip LC tank.
A single sigma-delta fractional-N synthesizer can phase lock the local osillator used in both transmit
and receive path to a precision frequency reference input. Fractional-N operation offers low phase
noise and fast setting times, allowing for multiple slot applications such as GPRS.
The generated frequency is given by the following equation
f
where :
= Generated VCO frequency
VCO
N = N-divider ratio integer part
FN = Fractional setting
R = R-divider ratio
f
= Reference Frequency
VCO
=
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