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Pulse Modulation
Video Feedt hrough
Video feedthrough is a video signal at the modulation rate that is
superimposed on the RF envelope (see Figure M-6). If large enough,
video feedthrough can disturb mixer balance, amplifier bias, crystal
detector output, etc. Because it is low frequency energy, it can
disturb systems that are not intended to deal with it, especially
demodulation systems. High band
(>2.3
GHz) employs a tracking
YIG filter that essentially eliminates video feedthrough except in
Option 006(b
ecause
the pulse modulator is after the YIG filter).
Attempts to measure high band video feedthrough can turn out to
be measurements of ground currents in coaxial cables. Low band
(<2.3
GH
)
z em o
pl y
s
a low-level mixer followed by a high gain
amplifier. At high power levels
(>-
-10
dBm),
the bias levels in the
amplifier shift slightly as the RF is turned on or off. The slew of the
bias from one level to another couples to the output and produces
the video feedthrough waveform. At low ALC levels (-10
dBm),
another mechanism dominates. Mixer imbalance produces DC at the
output of the mixer, and its magnitude varies with RF amplitude and
modulator state. This shifting DC level couples through the amplifier
as video feedthrough spikes. In percentage terms, this mechanism
gets worse at low levels.
RF ENVELOPE
I
VIDEO
FEEDTHROUGH
RF ENVELOPE
WITH
VIDEO FEEDTHROUGH
Figure M-8. Video Feedthrough
Slow Rise Time Pulse Modulation for Scalar Network Analyzers
For use with Hewlett-Packard scalar analyzers, the synthesizer
offers a scalar pulse modulation mode that provides approximately
2
ps rise and fall times. An internal oscillator provides the 27.778
kHz square wave with no external connections necessary. The slow
waveform reduces the spectral width of the output, improving
measurements made on filters with steep skirts. A slow pulse rise
time (approximately 2
ps)
is available for externally generated pulse
inputs as well.
M-22 Operating and Programming Reference
HP 8360
User’s Handbook