HP E4418A, Руководство по обслуживанию

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Theory of Operation
A6 Measurement Assembly
4-10 HP EPM-441A/442A Service Guide
A6 Measurement Assembly
There is one measurement assembly in the HP EPM-441A and two in the
HP EPM-442A.
The measurement assembly amplifies and converts the chopped AC signal
produced by the power sensor (either HP 8480 series power sensors or
HP E-series power sensors) into a 32 bit digital word. This digital word is
proportional to the input RF power level applied to the power sensor. The
measurement assembly also measures the sensor resistor voltage to detect
whether a power sensor is present. If a power sensor is connected it
determines the model, and whether it’s connected to the front or rear
panel. This assembly produces the differential drive signal for the power
sensor’s chopper. When an HP E-series power sensor is present this
assembly converts and corrects the power sensor’s temperature dependent
voltage.
With reference to the block diagram at the end of this chapter, the chopped
AC input signal is amplified by the input amplifier. This operational
amplifier in combination with a single bipolar transistor in the power
sensor, produce a feedback amplifier which has a gain of approximately
500. The “Feedback” line shown sets the gain and also biases the
transistor. The differential amplifier removes any common mode noise or
interference. It uses a special ground wire (“Sensor Ground”), connected
close to the RF bulkhead in the power sensor as it’s second input.
The band pass filter represents the filtering action of the input amplifier.
When the power meter is working with an HP E-series power sensor the
equalizer is switched on to reduce the high pass cut off frequency of the
input amplifier. This improves the settling time in fast mode operation.
The output of the equalizer is split into two paths. One path is amplified
by a gain of 100. Both signals are each converted to digital words by the
dual analog to digital converter (ADC). The ADC’s sampling rate is set to a
frequency 200 times above the chop rate for the HP 8480 series power
sensors and 100 times above the chop rate for the HP E-series power
sensors.
The Digital Signal Processor (DSP) converts the chopped signals into
digital words proportional to the input level. It compares both levels,
calibrates the gain and decides which level is output to the host processor
(after additional filtering) via the “Internal Serial Bus”.
4402serv.book Page 10 Monday, March 11, 2002 11:34 AM