14 Chapter 1
Making Basic Measurements
Resolving Signals of Equal Amplitude
Resolving Signals of Equal Amplitude Two equal-amplitude input signals that are close in frequency can
appear as a single signal trace on th e analyzer display. Responding to a
single-frequency signal, a swept-tuned analyzer traces out the shape of
the selected internal IF (intermediate frequency) filter. As you change
the filter bandwidth, you change the width of the displayed response. If
a wide filter is used and two equal-amplitude input signals are close
enough in frequency, then the two si gnals will appear as one signal. If a
narrow enough filter is used, the two input signals can be discriminated
and will appear as separate peaks. Thus, signal resol u tio n is
determined by the IF filters inside the analyzer.
The bandwidth of the IF filter tells us how close together equal
amplitude signals can be and still be distinguished fr om each other. The
resolution bandwidth function selects an IF filter setting for a
measurement. Typically, resolution bandwidth is defined as the 3dB
bandwidth of the filter. However, resolution bandwidth may also be
defined as the 6 dB or impulse bandwidth of the filter.
Generally, to resolve two signals of equal amplitude, the resolution
bandwidth must be less than or equal to the frequency separation of the
two signals. If the bandwidth is equal to the separation and the video
bandwidth is less than the resolution bandwidth, a dip of
approximately 3 dB is seen between the peaks of the two equal signals,
and it is clear that more than one signal is present. See Figure1-7.
In order to keep the analyzer measurement calibrated, sweep time is
automatically set to a value that is inversely proportional to the square
of the resolution bandwidth (1/BW2 for resolution bandwidths ≥ 1kHz).
So, if the resolution bandwidth is reduced by a factor of 10, the sweep
time is increased by a factor of 100 when sweep time and bandwidth
settings are coupled. Sweep time is also a function of the type of
detection selected (peak detection is faster than sample or average
detection). For the shortest measurement times, use the widest
resolution bandwidth that still permits discrimina tion of all desired
signals. Sweeptime is also a function of which Detector is in use, Peak
detector sweeps more quickly than Sample or Average detector. The
analyzer allows you to select from 10 Hz (or 1 Hz with Option 1D5) to
3 MHz resolution bandwi dt hs in a 1, 3, 10 sequence and select a 5MHz
resolution bandwidth. In addition you can select the three CISPR
bandwidths (200 Hz, 9 kH z , an d 120 kHz) for maximum measur ement
flexibility.