Agilent Technologies FieldFox MW N9913A, Service Manual

Page: 74 / 156

5-24 N9927-90003
Theory of Operation FieldFox Analyzer
Operation in Modes
CAT, NA, and VVM Modes
CAT, NA, and VVM modes have some similar capabilities but are still separate operating modes.
Measurement similarities:
• Return Loss = S11 magnitude
• Insertion Loss = S21 magnitude
Unlike S11 and S21, the values displayed below zero for Return Loss and Insertion Loss are not
negative.
There is user control of output power level, which is high, low or manual setting on the RF Out Atten, 35 dB
in or out.
SA and CPM Mode
SA Mode measures signals at the SA RF IN connector.
SA Mode measurements require NO calibration.
The signal path for SA mode is the same as the input path for two port CAT/NA modes.
The RF In Atten can be set from 0 through 30 dB in 5 dB steps. The Attenuator Slope Adjustment
determines the correction factors for each step in each frequency band.
The RF In Atten can be controlled manually or automatically. When in auto mode, the Reference Level
setting determines the attenuator setting.
The Pre-amp can be in or out. Nominal gain of the pre-amp is 22 dB.
The “Attn” and “PA” indicators in the display header give current status. The Scale/Amptd hard key
provides access to control for them.
The displayed measured value for a given signal is automatically compensated for in the current Attn and
PA settings. For instance, if an incoming signal is measured at -70 dBm with the RF IN Atten set to 5 dB
and PA “out”, changing to PA “in” will give a measured value of approximately -70 (+/- because the 22
dB for the PA is only nominal). The difference seen in the trace will be that the noise floor of the trace
will be lowered by approximately 22 dB. If the RF In Atten is changed from 5 dB to 15 dB, the measured
value will not change significantly (still approximately -70 dBm) but the noise floor will raise by 10 dB.
SA has two basic modes: step and FFT.
Step mode reads a narrow slice of the spectrum and the LO steps for each slice. In this mode, a trace on
the display is composed from multiple slices. Generally spans greater than 100 kHz operate in step
mode.
FFT mode reads a wide slice of the spectrum and the LO is fixed. In this mode, a trace on the display is
based on a single LO setting.
The LO minimum step size is 2 MHz; this restriction limits spurs.
The hardware operation of the A/D’s is essentially the same for both step and FFT modes. The difference
is the digital signal processing.
The SA adjustments generally correct the gain with 11 frequency bands. See page 5-11 for a listing of
the 11 bands. See the TME help file for details on SA adjustments.