Why does my FieldFox spectrum analyzer preamplifier increase the noise floor when engaged?

The FieldFox spectrum analyzer preamplifier selection will normally reduce the displayed noise floor when activated. However, if the FieldFox RF attenuation AUTO selection is set to ON then the preamp may not reduce the noise floor. With the FieldFox auto attenuation ON the spectrum analyzer preamplifier engagement may increase the displayed average noise level due to automatic changes in the RF input attenuator.

To verify the preamp is working properly, please perform the following:

Perform a factory preset on the FieldFox via the Esc - Preset - Preset (Factory) keys.
As applicable, set the mode to Spectrum Analyzer via the Mode - SA keys.
Set the RF attenuator to Manual and a value of 0dB via the Scale / Amptd - RF Atten – Manual – 0dB – Enter.
To activate the preamplifier, within the RF Atten menu, press More – Preamp – ON.
The FieldFox spectrum analyzer noise floor should drop (lower) as expected.

If the FieldFox preamp is activated while the RF attenuation is set to AUTO the FieldFox may auto increase the input attenuation. Thus the rise in the noise floor with the preamp activated with the RF attenuator in auto mode.

For a preset condition, with the spectrum analyzer input connected to a 50Ω termination, with the RF attenuator in auto mode, the RF attenuation increased from 10dB with the preamp off to 30dB with the preamp on. The increase of the RF attenuator is the cause of the increase in displayed average noise, despite the improvements due to the preamplifier on setting.

To minimize the FieldFox noise floor set the preamp to ON and the RF Atten to Manual and 0dB.

The FieldFox Spectrum Analyzer is optional on FieldFox models N9912A, N9913A, N9914A, N9915A, N9916A, N9917A and N9918A. FieldFox spectrum analysis is standard on FieldFox models N9935A, N9936A, N9937A and N9938A. The FieldFox preamplifier requires option 235.

Spectrum analysis is not available on FieldFox models N9925A, N9926A, N9927A and N9928A.