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Model 35601A
6-65. Summing Amplifier
If the phase locked loop is to be operated, the switch U7B is closed. Two gains are available
(depending on whether or not U17C is actuated) at the summing junction of op-amp U19A.
This Summing Amplifier combines the error-signal for the phase-locked loop (which comes
from U15A) with the signal from the Digital-to-Analog Converter. This provides a DC off
set at the output of U19A. Also summed at the Summing Amplifier is the Noise Source from
the -hp- 3582A (if desired) or the Tracking Generator signal from the -hp- 3585A. The addi
tion of the noise source or the tracking generator permits the measurement of the error func
tion of the closed phase-locked loop. This enables the calculation of the correction factors
used in Phase Noise measurements. The calculations are based on the actual dynamic condi
tions of the phase-locked loop when it is closed.
At the output of the Summing Amplifier U19A, a signal is sent to the 1MQ input of the -hp-
3585A for transfer function measurements at frequencies above a few hundred Hz. The out
put of the Summing Amplifier also provides a signal for channel B of the -hp- 3582A for
transfer function measurements below a few kHz.
6-66. Wein Bridge Oscillator
Also connected around the summing amplifier is the Wein Bridge Oscillator U15D. The
Wein Bridge Oscillator operates at a frequency of approximately 1.6Hz. When the phase-
locked loop is open, sufficient gain exists in U19A for the Wein Bridge Oscillator to operate.
The Wein Bridge Oscillator provides a searching signal for the external oscillator. When the
phase-locked loop closes, U19 appears to the Wein Bridge Oscillator to have very low gain;
therefore the Wein Bridge Oscillator does not operate.
6-67. Oscillation Detector
If the Wein Bridge Oscillator is operating, its output is split into two components, each 90
degrees apart from the other. These components are converted to provide a four-phase
signal to diodes CR14, 15, 16 and 17. This signal is rectified by U18C and used as an indica
tion that the loop is unlocked with the Wein Bridge Oscillator searching. This signal also sets
the Out-of-Lock Flip-Flop U11A.
6-68. Lag/Lead Network
The output of Summing Amplifier is fed to a Lag/Lead frequency compensation network
consisting of R25, C8 and various resistors selected by the CMOS multiplexer U4. The
Lag/Lead resistor is selected by U4; this is determined by the software. The software deter
mines this by the characteristics of the external oscillator within the phase-locked loop.
U15B is a buffer for the Lag/Lead network.
6-69. 0 to 20 dB Attenuator/Output
The Lag/Lead Network is followed by 0 to 20dB attenuator. The phase-locked loop error-
signal is finally sent to the external oscillator via pin 2 of Jl on the A5 Board.
6-66
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