Agilent Technologies e4418b, e4419b manual A6 Measurement Assembly

Theory of Operation

A6 Measurement Assembly

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”.

4-10 Agilent E4418B/E4419B Service Guide