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Blanking circuit
During transitions from one frequency to another, the RF source power is
briefly interrupted. As a result, the IF disappears. When the IF is lost, the
normal tendency of the leveling loop is to try to recapture it, by driving the
RF source to maximum power. This tendency needs to be counteracted
during frequency changes; otherwise, power spikes would occur
immediately after every frequency change (because the RF source would be
at maximum power when it was turned back on).
In order to prevent these power spikes, a blanking circuit is placed between
the output of the multiplying DAC and the dc output to the loop amplifier.
During each frequency change, a blanking strobe is received from the
8510C, and the blanking circuit drives the dc output to a maximum. This
gives the level control circuit a false indication that the RF level is too high,
rather than too low. The result is that the loop amplifier and integrator drive
the modulator so as to reduce RF power to a minimum.
When the next frequency point is reached, blanking is turned off. The RF
output of the source is reactivated, and the detector signal is allowed to pass
through to the loop amplifier. The RF level is brought up from its temporary
minimum-power setting to the requested level. The integrator insures that
the RF level is brought up smoothly and rapidly.
Spurious error messages
When the blanking circuit drives the modulator so as to minimize the RF
level, this event is interpreted by the RF source as an error condition
("unleveled" and/or "overmod"), and an SRQ is sent over the GPIB to the
8510C. If this error-detection system were not modified during sweep
operation, it would generate a large number of spurious error messages.
Therefore, during a sweep, SRQ triggering for "unleveled/overmod" errors
is disabled at the source. Triggering of "overmod" errors remains disabled
throughout the sweep (detection of this type of error is incompatible with
8510XF sweep operation). However, the network analyzer is able to poll the
source after each blanking interval, and determine the status of the
"unleveled" condition flag (if there is an actual error, it will then be
detected).
Because the polling process adds from 3 to 6 milliseconds of delay for each
frequency point, many users will want to control the amount of polling that
takes place. The menu called up by
four different ways to regulate the error-polling process during a sweep:
ALWAYS
If this option is selected, the source is polled for errors during
every sweep.
SMART
If this option is selected, the source is polled for errors during
the first sweep, and thereafter only if a genuine error condition
was detected during the first sweep. (This is the default mode.)
System Maintenance
Theory of Operation
[SYSTEM] {MORE} {RF POWER CONFIG}
8510XF Network Analyzer Systems 5-17
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