Specifications

The operator can place the LNA in a “fixed” gain

configuration and select the amount of gain to

use (14 dB to 56 dB of gain in 14-dB steps), or the

operator can select “pause after auto LNA gain”

and manually adjust the LNA gain before the meas-

urement is started. It is also acceptable to manually

select an internal low-pass filter (LPF) to help

reduce the phase transients—being coincident with

the pulse, they will appear as PRF feedthrough

(which usually necessitates the use of a PRF filter).

When monitoring the phase transients of the base-

band signal (with a wide-band oscilloscope con-

nected to the monitor port of the 70420A), the

signal at the monitor port is 14 dB (5 times) lower

than the baseband signal.

PRF feedthrough

If the PRF feedthrough overloads the LNA for more

than 10 percent of the pulse width, measurement

accuracy can be degraded. If the magnitude of the

PRF feedthrough causes the LNA to be set to a

lower-than-appropriate gain or the baseband signal

analyzer to be set to a less sensitive range than

needed, overall system noise floor can be degraded.

The 70420A phase noise test set provides a group

of internal low-pass filters that can reduce the

magnitude of the PRF feedthrough signals. When

measuring pulsed carriers, it is recommended that

the operator place the test set LNA low-pass filters

in manual and select the filter that provides just

enough offset range for the pulsed measurement

(PRF/2). Refer to Table 2-1 for the proper selection

of the LNA low-pass filter. If these internal low-

pass filters are not sufficient for a particular PRF,

an external phase detector with external low-pass

filtering will be required.

Table 2-1. Selecting an Internal Low-Pass Filter

PRF LNA Low-Pass Filter

≤40 kHz 20 kHz

>40 kHz to ≤400 kHz 200 kHz

>400 kHz to ≤4 MHz 2 MHz

>4 MHz to ≤40 MHz 20 MHz

Figure 2-2. Phase transients (100 MHz LPF & max LNA gain) as observed on

a scope connected to the monitor port of the Agilent 70420