RELEASED: 10/2/2006
Page 30
COMMUNICATI0NS
HSD SERIES OPERATION AND MAINTENANCE
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The internal oven controlled oscillator provides a 1volt peak to peak sine wave
reference output at 10 MHz, and R12 provides a fine tune adjustment for the output
of the internal reference frequency. The oven controlled oscillator provides for a
stability of .1~.2ppm over the specified operating temperature range. This internal
reference is used for the master frequency reference for the transmitter unless an
external 10MHz reference signal is present at a sufficient level.
The external 10MHz reference input connects at J2 on the module. This input
requires a minimum signal level of 0dBm. The output of J2 splits and one side feeds
the network of CR2 and CR3 which provide limiting of the external reference input.
The other side of the split feeds a detector circuit formed by L1 through R2. The
detector provides a DC output which feeds the op amp U1B exhibiting a voltage gain
of 2. This then feeds U1A and its supporting circuitry which acts as a comparator.
The comparator circuit compares a reference voltage and the level of the detected
external reference signal. If the detected input signal is of a sufficient level to
exceed the reference voltage, U1A outputs a control voltage to activate switch U10,
effectively connecting the external reference through to the next stage of the
module. The output of U1A also turns on the associated External 10 MHz indicator
on the display board, through Q1.
The switched output of U10 then drives a pin diode attenuator circuit formed by CR4
and CR5 and the associated circuitry. The attenuator circuit is used to control the
level of the selected 10 MHz reference signal. The pin diode attenuator is
connected to U5, a 20dB gain amplifier, through a low pass filter network. The
amplifier output is fed through a second low pass filter network and this signal is
split. One side of the U5 output is connected through a two-way (3dB) Wilkinson
splitter to the two 10 MHz reference outputs at J3 and J4 which each have an output
of 1Vpp.
The other side of the U5 output feeds a detector circuit formed around CR6, which is
the first stage of an ALC loop, to control the reference level at the module outputs.
The detector circuit drives op amp U6A which exhibits a gain of 1.37. The circuitry
at U6B forms the integrator of the ALC loop, its output controls the base voltage of
Q2 which levers the action of the pin diode attenuator.
