User's Manual

XFIN-BLADE-SM
TECHNICAL DESCRIPTION PAGE 43
by U300B/C with D304 to D307 providing the voltage multiplying effect needed to achieve
-17V. The output of the negative supply is applied directly to the VCO varactor anodes as
the negative tuning voltage VCAP BIAS.
4.3.4.5 Phase Modulator
The modulation path for audio, data and higher frequency CTCSS signals is via D609 and
its associated components in the TX VCO. The reference input to the PLL (FXTAL)
provides the low frequency modulation path in conjunction with phase modulator Q714 to
Q716. U711A is a low pass filter that provides 6dB per octave attenuation to frequencies
above approximately 180Hz. Modulation balance adjustment is carried out using a
CODEC generated 100Hz square wave applied to TX MOD1. A DAC output from the
Alignment Tool is applied to buffer U711B and ramp generator Q711 to Q713 via the
TUNE BAL line to adjust the low frequency modulation level.
4.3.4.6 Reference Oscillator
TCXO U700 determines the overall frequency stability and frequency setting of the radio.
The frequency setting is achieved by adjusting its ADJ voltage with the Alignment Tool. In
addition, the ADJ input is used in a frequency control loop with the receiver I and Q signals
to provide receiver AFC. U700 operates at 14.4MHz and is specified at ±2.0ppm
frequency stability over the temperature range –25C to +75C.