HP 70427A User Manual page 497

Block Diagram
HP 70427A AM Detector Filter
cannot be set. The input attenuation can be set manually using the
INPUT
softkey.
ATTN
The attenuator output level range can be changed using the
MIXER MAX LEV
softkey, but will change back to the default value whenever the module is
preset.
The peak-hold output is also monitored by an internal comparator. The
comparator threshold corresponds to an attenuator output level of about 10
dBm. If the threshold level is exceeded, the comparator output will be set
'low'. The comparator may be read through the diagnostic multiplexer
system.
13-62 HP 70427A/HP 70428A User's Guide