1250 Frequency Response Analyzer Making Measurements: The Analyzers
AMK/1250/3 5-3
For measurements where the random noise level is high, however, the number of cycles
of integration needs to be increased to improve the signal to noise ratio.
The minimum values that can be set are 10 ms or 1 cycle of the fundamental frequency,
whichever is the longer, up to 655 Hz. Above this, the values are 10 ms or 61 cycles of
the fundamental frequency, whichever is the longer (61 periods at 655 Hz = 93 ms).
If a time value is entered, the 1250 automatically rounds this up to give an exact
number of cycles, or blocks of 61 cycles, according to frequency. Hence the user would
normally enter a time value only as an approximate setting.
If no entry is made after initialising, a default value will be assigned automatically,
depending upon the frequency of the Generator. This value will be as per the minima
specified above.
If the user enters an integration time of less than the period of 1 cycle of the Generator
frequency, the 1250 will not reject it, but will automatically set the relevant minimum
integration time for the frequency concerned.
When the Analyzer is set to measure the nth harmonic, over x cycles of integration, the
1250 always interprets this as meaning x cycles of the fundamental frequency, and not
the harmonic.
One technique for evaluating the effect of different integration times, once
measurements are being made, is to observe the scatter on the displayed readings, with
the Analyzer on RECYCLE. If the current integration time is too short to allow
observation of the readings, add in a delay, as explained under Section 2.2 'DELAY'. If
the scatter is due to noise, or other spurious components, increasing the ∫ TIME value
should cause a decrease in the scatter.
5.2.2 DELAY
This facility enables a time delay to be interposed between applying the Generator
waveform to the system under test and taking a reading.
Two applications of this are:
1. To allow the system under test to 'settle' after a change to the input stimulus, e.g.
after each change of frequency when performing a sweep.
2. To enable scatter on readings to be observed on the Display, when using only a short
∫ TIME value.
Note: The 1250 itself does not require any delay to allow it to settle. However, on
receipt of a command to start, either from the front panel or by remote control, it takes
approximately 10 ms to action the command.
As with ∫ TIME, the required delay is entered as a number, plus units of time or cycles,
but the 1250 converts all delays into time values. See Section 4 'Measurement Start' for
the implications of this. There is no minimum delay, and the default value is zero.
5.2.3 HARMONIC
The 1250 can be set to analyze the response of the system under test at any harmonic
of the fundamental up to the 16th, provided that the frequency of the harmonic does not
exceed 65.535 kHz. Enter the required harmonic number, between 1 and 16, in the
round brackets in the Display. The default value is 1, i.e. the fundamental frequency.
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