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Theory of Operation
A6 Measurement Assembly
4-10 Agilent E4418B/E4419B Service Guide
A6 Measurement Assembly
There is one measurement assembly in the Agilent E4418B and two in the
Agilent E4419B.
The measurement assembly amplifies and converts the chopped AC signal
produced by the power sensor (either Agilent 8480 series power sensors or
Agilent 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 Agilent 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 Agilent 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 Agilent 8480 series power
sensors and 100 times above the chop rate for the Agilent 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”.