Agilent Technologies E1439 manual Frequency and filtering

Using the Agilent E1439

Frequency and filtering

Frequency and filtering

The Agilent E1439’s center frequency is normally set at zero (baseband path) and 70 MHz for the IF signal path. However, you may set the center frequency to a non-zero value in order to examine a narrower span away from baseband (zoom measurement).The frequency band of interest, represented by digitized time data samples from the ADC, is mixed with the E1439 digital LO, a complex exponential, at the desired center frequency. As a result, the frequency band of interest in the input signal is shifted to a complex signal centered around dc. See “Synchronizing changes in multi-module systems” on page 39 for special considerations with respect to changing the center frequency in multi-module systems.

The default filter for E1439 measurements is an analog anti-alias filter. However, you may further isolate the frequency band of interest for more detailed analysis by using digital filtering. A decimating digital filter simultaneously decreases the bandwidth of the signal and decreases the sample rate. The built-in digital filters conform to the Nyquist sampling criterion, which guarantees that the output sample rate may be reduced by the same factor as the signal bandwidth reduction while still maintaining a complete representation of the underlying bandlimited signal.

For each octave step in bandwidth reduction (except for the first octave), the E1439 digital filters automatically reduce the data rate by discarding alternate output samples. This process, called decimation, results in an output sample rate that is nominally four times the signal bandwidth whenever sigBw>0. This is still double the theoretical rate necessary to fully characterize the band limited signal. However, because the digital filters do not have a perfectly abrupt cutoff, the sample rate cannot be reduced to the theoretical limit without some aliasing of signals in the transition frequency band of the filters. In many applications, this limited aliasing potential is not important. For this reason you may optionally choose to apply a final factor-of-two decimation. See the Technical Specifications for detailed information on the digital filter shapes.

The decimation process used to reduce the output sample rate is driven from a "decimation counter" that keeps track of which samples to save and which ones to discard for each of the octave bandwidth reduction filter stages. In multi-module systems where synchronous sampling is required, the decimation counters in all the modules must be synchronous with each other. See “Synchronizing changes in multi-module systems” on page 39.

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