Agilent Technologies Switch/Measure Manual de usuario descargar pdf (Pagina 15)

Spectrum due to modulation and wideband noise

Purpose of measurement—what it proves

This measurement and the next “spectrum due to switching,” are often grouped

together and called “output RF spectrum” (ORFS).

The modulation process in a transmitter causes the continuous wave (CW) carrier

to spread spectrally. The “spectrum due to modulation and wideband noise”

measurement is used to ensure that modulation process does not cause excessive

spectral spread. If it did, other users who are operating on different frequencies

would experience interference. The measurement of spectum due to modulation

and wideband noise can be thought of as an adjacent channel power (ACP)

measurement although several adjacent channels are tested.

This measurement, along with the phase error measurement, can reveal numerous

faults in the transmit chain, for example, faults in the I/Q baseband generator,

filters and modulator.

As defined, the measurement also checks for wideband noise from the transmitter.

The specification requires the entire transmit band to be tested. Again, if the

transmitter produces excessive wideband noise, other users will experience

interference.

Theory in pictures

The measurement is defined and designed as follows. The analyzer is tuned to

a spot frequency and then time-gated across part of the modulated burst. Power

is then measured using this mode and then the analyzer is re-tuned to the

next frequency, or offset of interest. This process continues until all offsets are

measured and checked against permissible limits. What results is the “spectrum”

of the signal, however, spectral components that result from the effect of

bursting do not appear because the ramps are gated out.

Note: the result of the measurement is a set of frequency/power points, this is

not a swept measurement (with the exception of offsets beyond 1800 kHz in the

BTS case).

The test limits are mostly expressed in relative terms (dBc) so the first step of the

measurement is to take a reading at the center frequency to which the transmitter

is tuned. Because this measurement is gated and a different bandwidth is used,

this reading will not be the same as the mean transmitted RF carrier power

measurement. In practice the latter is approximately 8 dB higher but this does

depend on the spectral shape of the signal.

50%

90%

Measure carrier power in pre-defined

bandwidth (gated from 50–90% of burst)

Tune to offset frequency

Repeat through offset list

Amplitude

Frequency

Time

Measure power at offset in pre-defined

bandwidth (gated from 50–90% of burst)

Subtract offset power from carrier power

Report relative (dBc) result

Figure 12. Theory of spectrum due to modulation and wideband noise

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 35 36

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