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Theory of Operation
2715 Spectrum Analyzer Service Manual
3-23
This supply consists of VR160, U166, U174, Q170, Q674, and associated
components. VR160 is a temperature compensated reference diode that defines
the output voltage. U166 and Q170 constitute a buffered scaling amplifier to
provide the desired --5 V output at sufficient current. C660 makes the closed loop
bandwidth very low to minimize noise from the reference diode on the output.
U174 and Q674 convert --5 V to +10 V to provide stable low noise current to the
reference diode through R171. The +8.5 V path through CR175 is used to
guarantee start up of the supply. CR175 disconnects the potentially noisy start up
source once the emitter of Q674 goes sufficiently positive.
- 5 V Supply. This supply is not to be confused with the --5 V Reference Supply.
This supply is regulated by U470 and serves solely as a reference for the
remaining four supplies on the board. U470 is a programmable shunt regulator
whose output voltage is defined by an internal reference and by R472 and R473.
+8.5 V Supply. U282A and Q781 constitute a buffered operational amplifier used
in the inverting configuration to provide +8.5 V from an input of --5 V. (Note that
Q781 provides a phase inversion, so the polarities of the operational amplifier IC
input pins have to be reversed from the usual practice.)
+15 V Supply. U380B and Q880 operate in the same fashion as the corresponding
parts in the +8.5 V regulator.
- 8.5 V Supply. U282B and Q780 serve as a buffered operational amplifier in the
non inverting configuration to provide --8.5 V.
+5 V Supply. U380A and Q881 serve as a buffered operational amplifier used in
an inverting configuration to provide +5 V. (Note that Q881 is used in noninvert-
ing fashion, so that the normal polarities of the operational amplifier IC
terminals apply here.)
The regulators in the VCO module will be discussed later in this section on
page 3--25.
It is desirable to drive the 1st LO coils from high impedance sources so that
temperature dependent changes in winding resistance have negligible effect on
stability.
Main Coil Driver. U921 and Q521 constitute a buffered operational amplifier used
in an inverting configuration. Inputs to the summing junction are from a number
of sources and all pass through Q920. R900 (a current sense resistor in the return
lead of the main coil) develops a voltage proportional to main coil current.
1
st
LO Coil Drivers
Current Source