Agilent Technologies 89410A Operator's Manual page 201

Extending Analysis to 26.5 GHz with 20 MHz Information Bandwidth
With two instruments, there are actually two sets of input attenuators that need
to be adjusted. Obviously, the input to the microwave receiver is adjusted
based on the level of the signal to be measured. The vector signal analyzer
inputs are adjusted to reflect the I and Q signal levels at the output of the IF
module. This setting is fixed and has been chosen as a compromise between
noise and distortion performance. A variable in the example program can be
adjusted to provide different levels of performance.
When you set the reference level via the program, that level should be the
expected peak level of the signal you are measuring. Keep in mind that the
peak-to-average ratio for your signal may need to be taken into account. The
program assumes a value of 4 dB. If the signal exceeds the level you specify,
distortion of the signal or possible damage to the system may result.
To provide better measurements of low-level signals, the program lets you set
the RF attenuator to zero dB. If you set the reference level below –10 dBm, a
prompt appears asking you to confirm the removal of the last 10 dB of
attenuation. You should consider your answer carefully since removing all
RF attenuation could degrade the measurement. In particular, it could result in
increased distortion, possible exposure of the 70910A RF module's input mixer
to damage, and reduced return loss. If you are not sure your signal level will
always be below –10 dBm, you should never set the RF attenuation to 0 dB
(answer 'NO'). Leaving in 10 dB of attenuation will help protect the receiver's
input circuitry, with the consequence that there is a small signal-to-noise
degradation of your measurement.
The maximum reference level allowed is +30 dBm; the minimum is –60 dBm.
Signal levels above +30 dBm could damage the 70910A RF module, while levels
below –60 dBm will exhaust the available IF gain resulting in poor S/N
performance. The program warns you when you request a signal level outside
of this range.
To optimize the overall signal-to-noise ratio of the measurement the IF step
gain is used to 'interpolate' between RF attenuator values. This allows the
overall signal level to be set where the best S/N can be achieved for all
reference level settings.
16 - 15