Response Time; Unleveled Mode Used With Pulse Modulation - HP 8340A Operating Manual

Operating Information
Since this may cause very long response times for narrow pulses, the integrate/hold switch is
held closed a minimum of 10 µsec per pulse, for pulses narrower than that This is not long
enough to cause overcorrections but speeds response time for 100 nsec pulses by a factor of
100.
During the period between pulses, the integrate/hold circuit is expected to hold the
modulator drive constant. Because of leakage currents, the output will in fact drift, causing
the pulse amplitude to be in error. This error will grow with off time and also temperature,
as leakage is strongly temperature dependent. The circuit is designed worst case for< 0.1 dB
droop in 10 msec at an ambient temperature of 55°C. At 25°C, a typical unit drifts about 1
dB per minute. The drift may be in either direction.

Response Time

A tradeoff exists between response time and drift between pulses. Reducing C in Figure 3-35
speeds response to a level change, but also increases drift rate. Some applications require
faster response than the HP 8340A's normal operating mode provides. The fast response
mode is activated by pressing
mode is also activated when sweeping with sweep times shorter than 5 seconds. In this mode
response is 20 times faster than normal, and worst case droop is 0.2 dB in 1 msec at 55°C.
The response time to a step change in level is a function of pulse width and rate and is
detailed in the specifications. The response time is a function of ALC loop bandwidth,
which varies with frequency and power as explained in the AM section. The listed response
characteristics apply at the minimum expected loop bandwidth. An intuitive feeling for
response may be gained by the following:
to respond to a change and settle to within 10% of the size of the change. In pulse mode, the
same degree of settling requires 70 µsec of closure of the integrate/hold switch. This may
come from one 70 µsec pulse, two 35 µsec pulses, etc. Any pulse less than 10 µsec wide is
treated as a 10 µsec pulse. If the pulse period is less than 10 µsec, the integrate/hold switch is
continuously closed, so the system responds in 70 µsec regardless of width or period. In fast
response mode the basic time is 4 µsec and the minimum integration time is 1 µsec.
Simultaneous AM and pulse modulation is provided by the HP 8340A. The AM is de
coupled and linear, just as with normal CW leveling. If AM is used to exercise the ALC
below -10 dBm, the narrow pulse leveling accuracy degrades as explained above. The
bandwidth is given by the equation:
a step change described above. The pulse leveling system is in its slow response mode when
the AM key is pressed. SHIFT AM activates the fast response mode as well as enabling the
AM circuits.
Pulse modulation while sweeping is also possible. The permissible sweep rates are a
function of response time, which is a function of pulse width and rate. Sweep times less than
5 seconds activate the fast response mode. The response time should be short compared to
the time needed to sweep across each unleveled ripple. See the sweep speed discussion in the
external leveling section. Note: If the start frequency is less than 400 MHz, the low band
detector will lock in its low bandwidth state for the entire band 0 portion of the sweep. This
effectively precludes leveling pulses narrower than 2 µsec.
Unleveled Mod� Used With Pulse Modulation
Decoupled operation may be used for dynamic range extension or any of the other uses
described for CW or AM. Several pulse related problems may be treated with the unleveled
([SHIFn [METER])
3-110
[SHIFn [AM],
3dB
operating mode already described.
Scans by HB9HCA and HB9FSX
which simultaneously turns on AM. This
At minimum bandwidth in CW it takes 70 µsec
BW 0.35/Tr, where Tr is the response time to
=
Model 8340A