HP 87114B, Руководство по обслуживанию

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Figure 6-2. Simplied HP 8711B/12B A4 Source Block Diagram
P ower level control of the source RF output is achieved by using a coupler/detector on the
source RF output signal, which controls a modulator on the RF1 signal. Digital correction is
applied to the RF2 signal to control the receiver LO power level. Both power levels can be
adjusted by running the appropriate service tests . See the \A djustments" chapter of this guide
for more details . The source also has a \dither" mode for spur avoidance . When dither is on,
the RF1 frequency changes from 2340 MHz to 2304 MHz, and the RF2 frequency changes
to (2304 MHz + IF) to maintain the 27.778 kHz oset between these two signals . Because of
these changes , the mixing products from the two source mixers will be at dierent frequencies ,
resulting in spurs that have shifted in frequency when compared to the dither o state . This is
used in two ways . Y ou can select
N
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Dither ON (under the SOURCE 4 MENU 5 key), which causes
the analyzer to shift the RF1 and RF2 frequencies to the \dither on" values for all subsequent
sweeps , until dither is switched OFF again. This is useful if you are only interested in a narrow
frequency span. Switching dither ON successfully moves a spur out of the span of interest.
Y ou can also select
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
Spur Avoid ON . In this case , the analyzer will approach a known spur
frequency , switch dither ON until it sweeps past the spur , then switch dither OFF again until
it gets to the next spur . This method is more suitable for wider frequency spans , but it slows
down the sweep because of the time required to switch dither function ON and OFF .
Theory of Operation 6-5