3/6/08
888-2463-001
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WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
Diamond Series™
With no current in PIN diodes CR2 and CR3 or CR4 and CR5, R24 and R26 and R30
and R32 provide a nominal impedance termination for T2 and T3 ports 1 and 4 and
therefor a minimum of signal transmission from the input (6) to the isolated (3) port.
As current is applied to the PIN diodes, their resistance goes from high to low and the
overall termination of T2 and T3 is lowered from it’s nominal 50 ohms. This reduces
the attenuation between the hybrids input (6) and output (3) ports.
The output of the attenuators is summed by in phase combiner T1.
As a result of varying the current in the PIN diodes, the phase and amplitude of the
signal at the output of T1 (6) may be adjusted with respect to the input signal applied to
T4 (6). Minimum attenuation through this assembly is 5 dB.
Output from the Vector Modulator circuit is applied to a CATV modular amplifier U1.
This circuit provides a nominal 17 dB of gain with an output 1 dB compression point of
1 watt.
Output from the CATV amplifier is applied to discrete distributed amplifier stages Q5
and Q6. These stages combined provide a nominal 20 dB of gain. Amplifier output is
available at J3. Q1 and Q2 and Q3 and Q4 and associated components provide closed
loop constant current bias control for Q5 and Q6.
OP-Amp U2 provides the linearization interface between the Vector Modulator control
input at J1-8 and J1-10 and it’s attenuator PIN diodes.
There are two identical circuits, one for the modulator’s 0 degree path and one for the
90 degree path.
The first op-amp in each path operates as a variable gain stage in response to the control
signal level. There are four levels of gain, the transition thresholds set by reference
regulator U5 and divider resistors in the anodes of CR7 through CR10, CR14 and
CR15. When the back bias reference applied to these diodes is exceeded by the applied
control signal at each threshold, the net input resistance in the op-amp negative input
path is reduced. The net input resistance in relation the feedback resistor (R67, R81,
R101 & R102) sets the stage gain. Gain is lowest at low control input and highest at
high input. Thresholds are chosen to provide the best linearization of the relationship
between control voltage and Vector Modulator attenuation.
The second op-amp inverts the output of the first to provide an overall increasing
module RF gain for increasing control voltage. It also provides voltage to current
conversion in conjunction with the resistance in the path from it’s output to the PIN
diodes. Reference offset applied to the op-amp input sets the minimum PIN diode
current and resulting maximum attenuation level through the Vector Modulator.
