A6 10 Mhz Vcxo A Assembly - HP 11848A Service Manual

Model 11848A Service
After passing through Buffer 2, the signal is high-pass filtered. The decade-spaced High-Pass Filters,
selected by H1 through H5, are set to block low-frequency components from dc to one-tenth the sweep
range of the FFT spectrum analyzer. This is necessary because the noise signal frequently has very
large, low-frequency components due to the nature of phase noise. (As with the Low-Pass Filters,
these filters match the default sweep ranges when other sweep segments are selected.) The AC/DC
Adaptive Coupler, when set to dc, restores the dc component of the noise signal when the need arises
to measure it with the FFT spectrum analyzer. In the ac mode, the AC/DC Adaptive Coupler is a
16 MHz high-pass filter.
The noise signal is finally sent to the front-panel TO HP 3561A INPUT connector via Floating
Amplifier 2 which has a gain of —1. Having a floating output improves rejection of line-related signals
resulting from ground loops.
The remaining circuits in the A3 Analyzer Interface Assembly do not directly relate to the measurement
of the noise signal by the FFT spectrum analyzer.
The Calibration Oscillator outputs a 100 kHz squarewave. It is switched on when a new measurement
is selected to compare the amplitude calibration of the RF spectrum analyzer relative to the FFT
spectrum analyzer. (The frequency ranges of the two spectrum analyzers overlap at 100 kHz.)
The 10 Hz/50 kHz High-Pass Filter, Peak Detector, and Peak Hold circuits have three functions:
(1) to sense the presence of a beatnote when measuring phase noise using a phase lock loop, (2) to
determine whether to insert the Low-Noise Amplifier in the A4 Phase Detector Assembly, and (3) to
provide signal level information for ranging the RF spectrum analyzer. The first two functions use the
10 Hz high-pass filter; the third function uses the 50 kHz filter (which matches filtering to the RF
analyzer input provided by the 0.068 fiF capacitor at J2 on A4.)
DAC 2, DAC 3, and the DAC SUMMING JUNCTION output a programmable dc voltage to tune three
VCOs: (1) the 400 MHz VCO on A8, (2) the 10 MHz VCXO on A7, and (3) the 10 MHz VCXO on A6.
DAC 3 has 1 mV resolution; DAC 2 has 50 mV resolution. Switch L11 enables the same three VCOs
to be tuned as the VCO in a measurement of phase noise using a phase-lock loop. Note that the VCOs
are all tuned simultaneously. (See the discussions below about the function of these VCOs.)
Attenuator 3, Floating Amplifier 1, and the Lag-Lead Network duplicate the function of Attenuator 1,
Buffer 3, and Lag-Lead Network 2 on the A4 Phase Detector Assembly. The path is enabled by switch
L10. Floating Amplifier 1 breaks up ground loops on the VCO tune voltage and makes the front-panel
TUNE VOLTAGE OUTPUT the preferred output. (See the discussion on Lag-Lead Network 2 on A4
for the function of the Lag-Lead Network.)
Switch L8 routes the rear-panel TONE INPUT or the NOISE INPUT FROM HP 3561A SOURCE
OUTPUT to the Phase Modulator in the A7 10 MHz Modulated VCXO B Assembly.
A6 10 MHz VCXO A Assembly
The 10 MHz ±100 Hz VCXO (voltage controlled crystal oscillator) provides a very clean, tuneable source
available at the front-panel 10 MHz A connector or the rear-panel 10 MHz SOURCE A OUTPUT OR
AMP INPUT connector. This source can be used as a tuneable VCO for measurement of 10 MHz
sources or in conjunction with the 10 MHz ±1 kHz VCXO in A7 for demonstration purposes. The
Power Amplifier boosts the signal level high enough that it can serve as the signal to the L port of the
input phase detectors.
The switch at the output of the oscillator allows the amplifier to be used by itself in other applications
such as boosting the power level of the 10 MHz ± 1 kHz VCXO in A7.
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