A6 Measurement Assembly; Average Path Operation - Keysight Technologies E4417A Service Manual

4 Theory of Operation

A6 Measurement Assembly

There is one measurement assembly in the E4416A and two in the E4417A. As the
E4416 and E4417 are peak and average power meters that are compatible with all
Keysight power sensors in the 848x Series, N8480 Series and E Series, the
measurement assembly has two measurement paths. One path deals with
average only measurements and this is the path that is compatible with the 848x
Series, N8480 Series and E-Series sensors. The other path provides the peak
measurements. In addition to these two measurement paths there is a DSP
subsystem which provides the signal processing of the digitized analogue signals
together with various ancillary support functions.

Average Path Operation

The measurement assembly amplifies and converts the chopped AC signal
produced by the power sensor (either 848x Series power sensors, N8480 Series
power sensors or 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.
With reference to the block diagram at the end of this chapter, the input amplifier
amplifies the chopped AC input signal. 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 its second input.
The band pass filter represents the filtering action of the input amplifier. When the
power meter is working with an 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.
94 Keysight E4416A/E4417A Power Meters Service Guide