166 Keysight N9038A MXE Service Guide
RF Preselector Section
RF Preselector Section Purpose
RF Preselector Section Assembly Descriptions
A21 RF Preselector Input Assembly
The main functions of the A21 RF Preselector Input assembly are to provide:
— Signal path switching
— Input protection
— Prefilters for both the conducted and radiated filter paths
— Preamplifiers for both the conducted and radiated filter paths
— Overload detection for both the conducted and radiated filter paths
— A23 Limiter Assembly control
—SW1 & SW2 control
—Alignment signals
Signal Path Switching
When the receiver is switched to one of the RF Preselector filter paths,
signals will enter A21 RF Preselector Input assembly first before being
switched into either the A24 Conducted Filter assembly or the A22
Radiated Filter assembly. Output signals from both the A24 Conducted
Filter assembly and the A22 Radiated Filter assembly are routed back
through the A21 RF Preselector Input assembly before being routed to the
down-converter section of the instrument.
Input Protection
The purpose of the limiter is to provide overvoltage protection to the filter
boards which is useful while making electromagnetic emissions
measurements where the signals are often impulsive in nature. The input
signals will encounter the limiter circuit before being routed to either of the
filter assemblies.
Conducted Filter Path Prefilter
This is a lowpass filter to reduce the high frequency components of the
input signals before going to A24 Conducted Filter assembly. It is designed
with a 3 dB roll-off at around 55 MHz. The purpose of this filter is to only
allow input signals within the operating frequency range of the A24
Conducted Filter assembly to be sent to it.
Refer to Chapter 15, “Block Diagrams.”.
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