Quick Start Guide N5511A-K26
25
Amplifier Measurement Example
Measurement Tips
Avoid adding an amplifier between the Stimulus and the stimulus signal input of the PNMI in
order reduce AM noise, unless unavoidable. Doing so may add correlated noise whose PM
component will mostly be canceled out, but the AM component will only be rejected by ~30 dB
and will not cross correlate out. This can raise the noise floor, potentially higher than DUT's PM
noise performance. Amplifiers with enough output to boost the signal before system losses are
inherently noisier than adding lower power amplifiers that can be inserted in the To LO paths of
the PNMI for each channel. This approach for dual channel measurements is highly preferred in
order to preserve dynamic range. Refer to the N5511A User’s Guide for more information on the
use of amplifiers in a dual channel measurement.
Adding accessory amplifiers at LO inputs is convenient, but should also not be used unless
necessary to meet power requirements. The amplifiers will add noise that needs to be cross
correlated out and can add to measurement time. Only use them to meet minimum
requirements to turn on the mixer diodes, or to increase the sensitivity of the test system.
Avoid using high 1/f amplifiers such as ones that are FET-based. Using amplifiers with such
characteristics will result in longer measurement times since the AM noise component needs to
be cross correlated out. Even when the amplifier's noise figure is low, the offsets are from the
carrier and the noise does not reflect 1/f. Bipolar-based amplifiers are preferred with a low offset
1/f corner.
Select amplifiers whose power output is matched to the detector LO In power range to avoid
having to add the complexity of attenuating the output of the amplifier.
