Agilent Technologies 85032B/E 50 Specifiche Pagina 225

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Theory of Operation 8712ET/ES and 8714ET/ES
System Theory RF Network Analyzers
6-10 Service Guide
The power level of the Source RF output is controlled by the firmware on the CPU board.
The firmware digitally controls the level setting in the ALC/MODULATOR feedback loop.
The firmware combines the desired output level with data from Adjustment #104 to
determine the level settings for the ALC/MODULATOR feedback loop. (See Chapter 2,
Adjustments,for more details on the adjustment procedure.) The firmware is able to vary
the Source RF output level over a range of approximately 10 dB.
Controlling Spurs.
Spurs are inherent in the design of the system. Spurs are small unwanted spikes of signal
that occur in measurements. Most spurs are predictable, as they are a function of the LO
frequency of the receiver.
The source 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 offset between these two signals. Because of these changes, the
mixing products from the two source mixers will be at different frequencies, resulting in
spurs that have shifted in frequency when compared to the dither off state. This feature
can be used in two ways.
You can select (under ) , 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. You can also select . 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 the dither
function on and off.
8714ET/ES Source Assembly (A4)
The 8714ET/ES source assembly is similar to the previously described source assembly in
the 8712ET/ES. However, because of the wider frequency range of the 8714ET/ES, there
are some differences in theory. These differences are described below.
The source is designed with four different frequency bands:
Band 1= 0.31910 MHz
Band 2= 19102310 MHz
Band 3= 2310–2620 MHz
Band 4= 2620–3000 MHz
The differences between bands 2, 3, and 4 result from different filter changes. Filters are
automatically inserted or removed from the RF and LO signals to minimize spurs. The
major change is between bands 1 and 2.
Dither
MENU
Spur Avoid Options
Spur Avoid
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