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The HP 8340B/41B's high band (>2.3 GHz) employs a tracking YIG filter which essentially eliminates
video feedthrough. Attempts to measure it can turn out to be measurements of ground currents in
coaxial cables. The HP 8340B/41B's low band (<2.3 GHz) employs a low level mixer followed by a
high gain amplifier. At high power levels, 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. For this reason the 5% specification is only valid for power levels up to +8
dBm. At low ALC levels (—10 dBm), another mechanism predominates. Mixer imbalance produces dc
at the output of the mixer, and its magnitude varies with RF amplitude and/or modulator state. This
shifting dc level couples through the amplifier, which is AC coupled, and emerges as video
feedthrough spikes. In percentage terms this mechanism gets worse at low levels. The lowest per¬
centage video feedthrough is probably found at ALC levels around 0 dBm.
SLOW RISE TIME PULSE MODULATION FOR SCALAR NETWORK
ANALYZERS
For proper operation of HP 8755C, 8756A, and 8757A scalar analyzers, the HP 8340B/41B offers a
pulse modulation mode which provides approximately 2us rise and fall times. Press [SHIFT] [PULSE]
to enter this mode. The scalar analyzer's modulation output is connected to the HP 8340B/41B PULSE
input. The slow waveform reduces the spectral width of the output, improving measurements made
on filters with steep skirts. This mode may be used for other purposes, and functions properly for
pulse widths wider than about 7us.
FURTHER INFORMATION
This completes Section III of the HP 8340B/41B Synthesized Sweepers Operating and Service Man¬
ual. For further information, there are several Programming Notes, Operating Guides, and general-
reference Application Notes that are applicable to the HP 8340B/41B.
3-116
Operating Information
HP 8340B/41B
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