Manual - 8500A Series Peak Power Meter - Giga-tronics

4
Theory of Operation
4.1 Introduction
This chapter presents a functional description of the electrical assemblies in the Model 8501A and 8502A
Peak Power Meters (PPM). Table 8-1 in Chapter 8 lists the circuit assemblies by reference designation
and includes the schematic diagram and assembly number for each board. Schematic diagrams and
component layouts are in Chapter 8.
The method to properly ground all PC boards is common throughout the PPM. Each PC board contains
both input and output isolator stages to transfer the reference or ratio common ground when signals are
passing between the PC boards to an on-board common ground. When any signals leave the boards, they
pass through another isolator stage, which transfers them back to the between-the-boards reference
ground.
4.2 System Description
The following is a general description of the PPM circuit, with reference to Figure 4-1.
The detected RF signal enters the PPM as an analog voltage and is received by the circuits of the Analog
PC board (A6/A7). The single channel Model 8501A uses only A6 whereas the dual channel 8502A uses
both A6 and A7. Both boards are identical.
The Analog board circuits use an RC network to compensate the signal for frequency dependent losses in
the delay line. The signal is sampled and held until the completion of the data conversion cycle. A/D
conversion occurs to supply digital data to the Central Processing Unit (CPU).
The Trigger Amplifier of the Analog board senses the detector output voltage before it reaches the Delay
Line. It uses the signal to provide an Internal Trigger, controlled and conditioned by the A5 Digital Delay
board, when the internal trigger is selected.
The Peripheral Interface circuits interface read and write signals under the control of the CPU board.
The Analog board also receives temperature information sensed by the thermistor in each detector.
The programmable delay function of the A5 Digital Delay board establishes the time delays when
measuring RF pulses. Trigger inputs can come from either the CPU (internal), through the Trigger
Amplifier, from the pulse itself as it is detected, or from an external source connected to the rear panel of
the instrument. The Delay board also supplies a synchronized output signal to a rear panel connector for
an external meter to read the pulse at the point where the sample is taken.
The A4 CPU board provides (through the CPU Bus Line) the interface between the other PC boards, and
generates all of the control signals for timing and controlling the various functions of the boards. Apart
from the bus, the CPU has two other lines from the power supply and the delay board. These inputs tell
the CPU if there has been a power failure of any kind, and when it should halt its activity until a sample
of the incoming pulse is taken. This allows the sample to be unaffected by CPU generated interference.
The A9 Keyboard and A10 Display PC boards interface keystroke and hand wheel movement information,
and display indication signals to and from the front panel components. These functions are controlled by
the A8 Front Panel Interface board, which is controlled by the CPU. The front panel interface also has a
Manual No. 20790, Rev C, November 1998 4-1
Superceded by Revision D, March 2009