Theory Of Operation
A6 Measurement Assembly
The ADC has a bipolar range but the power signal is essentially unipolar.
To utilize the full ADC range an offset is summed in to the signal just
prior to the ADC in an offset summer circuit. The outputs of the summer
are arranged to give a full-scale negative input to the ADC, so it reads
circa (but greater than) -2048 when zero volts is applied to the normal
path input. (The ADC range is -2048 to 2047.) There is a further filter that
removes high frequency noise, originating in the amplifier chain, from the
ADC input. It has a 20 MHz theoretical cut-off (-3dB) frequency.
The ADC is a 12-bit converter running at 20MHz. Samples from the ADC
are combined with the normal path gain setting and the sensor range to
derive the power measurement. The ADC output and the range control
bits are fed to the FPGA. The FPGA controls a triggered acquisition into
SDRAM. The acquisition parameters (such as pre-trigger, post trigger and
trigger level) are controlled by the main processor.
The DSP transfers the acquired samples into its local memory where it
then performs the required processing to enable the demanded
measurements. This includes range correction, digital filters, linearity
correction, averaging together with display trace processing. The DSP
generates an interrupt to the processor when results are ready.
The processor assembly communicates with the DSP through the FPGA.
The DSP is loaded with the appropriate program by the processor
assembly depending on sensor type and required measurement mode. The
processor assembly loads the FPGA using a serial configuration bus.
Ancillary Functions
The measurement assembly also measures the sensor resistor voltage to
detect whether a power sensor is present. If a power sensor is connected
(or disconnected) an interrupt to the A2 processor assembly is generated.
Circuitry on the board is then used by the processor to determine the
sensor model, and whether it's connected to the front or rear panel. The
processor then configures the board?s ancillary functions, such as chop
frequency, chop voltage levels and also programmes the DSP with the
appropriate code for that sensor type. If the sensor was one of the E-series
then the processor reads calibration data from the sensor?s E2PROM
using the I2C bus. The processor then uses the board?s auxiliary ADC
(XADC) to measure the sensor temperature which is used in conjunction
with the calibration data to provide temperature corrected power
measurements.
4-12Agilent E4416A/E4417A Service Guide |
ACEC
HEWLETT-PACKARD
