User's Manual

The VCO operates at the output frequency and is controlled by
applying a voltage to the voltage variable capacitor. The oscillator is
tuned to the center of the operating frequency range so the voltage on
the capacitor is compatible with the output range of the low pass
filter.
Phase Locked Loop "PLL"
The PLL compares the VCO frequency, divided by the dual-modulus
divider and internal dividers, to the reference frequency and outputs a
series of correction pulses to the low pass filter to correct the VCO
frequency.
Dual Modulus Pre-Scalar & Buffer
Dual Modulus operation will not be described here, since
operation of this circuit is explained in great detail in common
technical literature.
A simplified explanation of dual modulus circuit operation is
that it is an additional frequency divider used by the PLL. The buffer
provides isolation to prevent switching spikes from getting back into
the VCO and erroneously modulating the output frequency.
Modulation Filter
The modulation signal is passed through an active filter which
attenuates all audio signals above the cutoff frequency, resulting in
an improved signal to noise ratio.
Modulation Attenuation Circuit
The Modulation Attenuation Circuit consists of several resistors
that may be switched in and out of the circuit to control the
modulation level across the operating bandwidth of the Transmitter.
Encoder
Microcontroller
The microcontroller includes Eprom, EEprom, Ram and various
peripheral devices to accomplish the task required of the encoder.
Additional Support Circuitry
The following are some of the support circuits associated with
the encoder:
* Sensor Interface circuit.
* Power Supply circuit.
* User interface circuit (Switches and Pushbutton).
* External state of health timer.