HP 24A, 83620A, 22A manual Figure M-8. Video Feedthrough

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 GHz) employs 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

WITH

VIDEO FEEDTHROUGH

RF ENVELOPE

I

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