4 Theory of Operation
82 E4416A/E4417A Power Meters Service Guide
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 Agilent 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.
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