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Pulse Envelope
The pulse envelope produced by the HP 8340B/41B is not a perfect rectangle, rather it has finite rise
time, overshoot, and video feedthrough. Below 2.3 GHz the rise time and overshoot are essentially
independent of frequency, but above 2.3 GHz they are strongly influenced by the passband shape and
centering of the HP 8340B/41B's tracking YIG filter. Best envelopes are normally obtained by using
the PEAK function. The envelopes will change with frequency and slightly with power level.
Source Match
In the absence of attenuation with a resistive pad, a leveled microwave source generally provides a
50ft source match at its operating frequency only. The source match at other frequencies is that of the
unleveled RF hardware. In this case this is a YIG filter which is a good reflection for anything more than
about 20 MHz off center, and not much better within its passband. The match is good only for signals
at the output frequency plus or minus the leveling loop bandwidth. Thus a CW reflection will be
absorbed. A time varying reflection, as from a reflective modulator, may contain modulation side¬
bands outside the leveling loop bandwidth. If so, these sidebands may be re-reflected. When pulse
modulating, any reflection of the pulse will contain sidebands far outside the loop bandwidth, so the
leading edge of the reflection will be re-reflected. In this manner a poorly matched system can gener¬
ate very poor envelopes (anywhere in the system, not only at the source). Performance is improved by
padding between the reflections. At the source, if output powers above -10 dBm are in use, coupled
mode results on 0 dB RF attenuation. If enough power is available, decoupled mode may be used to
improve the HP 8340B/41B's source match by inserting 10 dB attenuation and using a 10 dB high ALC
level.
Video Feedthrough
Video feedthrough is a low frequency signal, at the modulation rate, which is superimposed on the RF
envelope. See Figure 3-37. If large enough, video feedthrough can disturb mixer balance, amplifier
bias, crystal detector output, etc. Since it is low frequency energy, it can distrub systems which are not
intended to deal with it, especially demodulation systems.
HP 8340B/41B
Operating Information
3-115
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