Chapter 5 283
Concepts
Frequency Counter Measurements Concepts
Frequency Counter Measurements Concepts
Purpose
Frequency Counter measurement is fundamental to characterization of all signal
generators that have periodic RF/microwave outputs. Frequency Counter
measurements are used to accurately tune to and measure the carrier frequency of
the signal. The Frequency Counter measurement and its result are the fundamental
to all of the other measurements in the Measure Receiver System.
Measurement Technique
In automatic operation, the Frequency Counter automatically adjusts attenuation as
the input level changes.
The frequency counter can accurately measure the carrier frequency by searching
the full span of the spectrum analyzer as follows:
1. RBW 1 kHz is used for searching through the signal range from 1 kHz to 1
MHz with DC coupling.
2. RBW 10 kHz is used for searching through 1 MHz to 100 MHz with DC
coupling.
3. RBW 3 MHz is used for searching above 100 MHz with AC coupling for PSA
models E4440A, E443A, E4445A with DC coupling for PSA models E4446A,
E4447A, E4448A (these models don’t support AC coupling). Using these
settings, the PSA can find signals above 100 MHz for all supported PSA
models.
4. When the PSA finds a signal, the PSA searches the span=106*RBW and uses
marker frequency counter to assure the resolution. RBW can be set from the
front panel. The default setting is 1 kHz.
The default gate time of 100ms is long enough to make the counter resolution as
low as 0.001Hz. For E4440A, E4443A, E4445A models, if the PSA fails to find a
signal through 1 kHz to 100 MHz, the PSA will try AC coupling from 100 MHz
above. The PSA never goes back to DC coupling even though the frequency is
re-tuned to below 100MHz in order to protect the switch.
For FM/PM modulated measurements, you should turn off the modulation and
measure the carrier frequency first and then turn on the modulation and make other
modulation measurement. For AM modulated measurement, it is also
recommended to first turn off the modulation when measuring the Frequency
Counter. If the AM modulation is not turned off, make sure that the RBW setting
lower than the Modulation Rate.
The result of Frequency Error can be used to measure the frequency drift of input
signals, or to compare input frequencies against a keyboard-entered reference.
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