Anritsu MA24106A Guia do Utilizador descarregar pdf (Página 146)

11-3 Measurement Considerations Using the MA24108A, MA24118A, and MA24126A

11-6 PN: 10585-00020 Rev. C PowerXpert UG

Noise and Averaging

When there is a need to achieve a required reading resolution, particularly at low power levels, averaging is

often needed to reduce noise and steady the displayed power reading. Use the noise vs. resolution table in

“Optimizing the Readings” on page 11-3 to determine the number of averages that will typically be required for

a given resolution. Alternatively, determine the number of averages through calculation by using the noise

specifications and the fact that noise will be proportional to the square root of N, where N is the number of

averages. For example, a CW tone at –25 dBm is to be measured to 0.05 dB resolution. Using Table 3-7, the

required number of averages is 2 averages. The same measurement to 0.01 dB resolution would require 31

averages.

Average Value of Time Varying Signals

When measuring the average Power of a time varying or modulated signal with a modulation rate which is

much greater than the signal channel BW of the sensor, averaging of the power is performed in the sensor

hardware (detectors and or preamplifiers). For the case of the MA24108A / MA24118A the signal channel BW

is 50 KHz, so signals modulated at MHz rates will be averaged in the hardware, and no special considerations

are required.

When measuring signals with modulation frequency components near, or below, the signal channel bandwidth,

average power readings may be seen to fluctuate over time. This fluctuation may be reduced through careful

selection of the aperture time and averaging number. Ideally, the aperture time should be chosen to be an

integer multiple of the modulation frequency. If this can be done, then the average power reading will be stable

for each measurement update. For modulations with multiple frequencies present, or with significant

modulation components with periods longer than the maximum 300 ms aperture time, averaging will have to

be increased to obtain a stable reading. If the measurement update rate is very close to the period of the

modulation, a low frequency “beat note” can result. If the frequencies are very close, the beat note can be very

low in frequency, and therefore require very long averaging times to remove. In this case it is suggested that

the aperture time be changed to result in a higher frequency beat which is easier to average out.

Settling Time

The signal channel bandwidth of the power sensor supports a rise time of about 8 us. The ADC sample period

is a bit more than 7 us. Thus it will take more than one ADC sample for the signal channel hardware to

completely settle in response to a step change in input power. The hardware settling time can be estimated by

assuming a single pole response with the 50 kHz bandwidth:

where:

For small settling percentages, it is quite likely that the noise per ADC sample will be larger than the desired

settling percentage, thus averaging or decimation of ADC samples will have to be used to reduce the noise.

Averaging will, of course, increase the settling time of the measurement in direct proportion to the averaging

number used.

It is important to note that the settling time described above strictly applies only to increasing power steps

(rise time). Settling to decreasing power steps is typically slower. For settling decreasing power steps to

1%or0.1%, the settling will typically be within a factor of 2 or 3 of the calculation above. Settling

to 0.01 % or less may take considerably longer.

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