HP 70427A User Manual page 499

Block Diagram
YIG-Tuned-Filter (YTF) and GaAs FET Amplifier Driver Assembly
limit, the Main Frame will current-overload, and shut down. This problem is
eliminated by turning off the 10.00 volt reference to the DACs.
The 10.00 V precision reference must be present for the DACs to output a
signal. Removing the 10.00 volt reference sets the output to zero, regardless
of the data latched in the DAC at power-up. After the microprocessor
initializes the DAC data to zero, it turns on the 10 volt reference. This
process also protects the YTF and GaAs FETs from being held at maximum
power dissipation indefinitely during an MSIB power-on failure, which will
prevent the microprocessor from initializing the module.
The power-on control is implemented using a supply voltage supervisor
(U25) to monitor the +5 V supply. The supply voltage supervisor produces a
200 μs pulse as soon as the supply voltage rises to the minimum 3.5 V level.
The pulse sets the R-S flip-flop made from U24, which turns on the control
switch U21A in the +10 volt reference, which shorts out the 10 volt
reference signal. The +10 volt reference will remain off until the flip-flop is
reset by addressing to latch 4.
+10 Volt Precision Reference
The +10 volt reference (U12) is a monolithic, low drift (less than 1.5
°
ppm/ C), low initial error (less than 1.0 mV) reference. It offers 12-bit
absolute accuracy without any user adjustment. This performance is
obtained by an ion- implanted buried zener diode reference cell and
laser-wafer- drift-trimming of the high stability thin-film resistors.
The reference also contains two low temperature drift op-amps. The first
amplifier is used to buffer the reference cell's +10 volt output. The buffered
output drives a 300 Hz one pole R-C filter, which is followed by the second
op-amp configured as a voltage follower. The noise filter reduces the noise
beyond 300 Hz, and provides shunt switching access. The voltage follower
input bias current is typically 20 nA which corresponds to 100 μV voltage
drop across the filter. The +10 volt reference is turned off by shorting the
filter output to ground with a FET switch (U21d). The shunt switch is used
to minimize the voltage drop when the reference is on. The output voltage is
reduced to less than 0.07 volts due to internal switch resistance when the
reference is off.
Tune-Span Generator
The tune-span generator and output level control are both controlled by a
dual 12-bit DAC (U7). Each 12-bit DAC section uses the +10.0 volt
reference to produce an output from 0.0 volts to –10.0 volts. The output is
then shifted and gain adjusted to produce the desired output.
The tune-span generator drives an auxiliary rear panel output (not currently
used). The output voltage range is from +4.33 V to +9.95 V. This is the same
range covered by the Tune-Span Output on the HP 70900 A/B Spectrum
13-64 HP 70427A/HP 70428A User's Guide