Further Information - HP 8340A Operating Manual

Model 8340A
The HP 8340A's high band
eliminates video feedthrough. Attempts to measure it can turn out to be measurements of
ground currents in coaxial cables. The HP 8340A'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 de at the output
of the mixer, and its magnitude varies with RF amplitude and/ or modulator state. This
shifting de 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
percentage video feedthrough is probably found at ALC levels around 0 dBm.
RF
ENVELOPE
VIDEO
FEEDTHROUGH

FURTHER INFORMATION

This completes Section III of the HP 8340A Synthesized Sweeper Operating and Service
Manual. For further information, there are several Programming Notes, Operating Guides,
general-reference Application Notes, and a Manual Changes Supplement that are applic­
able to the HP 8340A; all of these are available from Hewlett-Packard Sales and Service
Offices (listed inside of back cover).
(>
2.3 GHz) employs a tracking YIG filter which essentially
Figure 3-37. Video Feedthrough
Scans by HB9HCA and HB9FSX
Operating Information
RF ENVELOPE
WITH
VIDEO FEEDTHROUGH
3-113/3-114