Tektronix 2715, Руководство пользователя

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Dedicated Controls
4–12
2715 User Manual
Set the 2715 parameters as follows to match the preceding settings box:
1. Press [UTIL] [1] [1] to recall factory default settings.
2. Press [CHAN/FREQ] [1] [0] [0] [X] to set the center frequency to
100 MHz.
3. Press [SPAN/DIV] [2] [0] [X] to set the span/div to 20 MHz.
4. Press [REF LEVEL] [3] [8] [.] [8] [Y] to set the reference level to
38.8 dBmV.
5. Press [INPUT] [9] to enable the calibrator.
6. Press [AUTO] to set the resolution bandwidth to 5 MHz.
7. Press [MKR/ /OFF] twice to disable the markers.
Arrow Keys. Until now, resolution bandwidth has been left in the AUTO mode,
which enables you to make measurements without worrying about where the
resolution bandwidth is set. The 2715’s default is a fixed resolution bandwidth of
300 kHz. AUTO mode is available, but it is not the normal operating mode.
There are circumstances in which you will want to control the resolution
bandwidth yourself. For instance, if you look at the time domain representation
of a TV video signal using zero span, you will want to use the 5 MHz (maximum
bandwidth) filter to ensure enough bandwidth for the video signal. In other cases,
you may want to select a very narrow resolution bandwidth in order to resolve
signal sidebands or intermodulation distortion products. The RES BW arrow
keys enable you to select resolution bandwidths of 300 Hz, 1 kHz, 3 kHz,
10 kHz, 30 kHz, 100 kHz, 300 kHz, 1 MHz, and 5 MHz in the 2715.
It is important to know how far signals must be separated before we can see them
as separate. The answer depends on the particular filters, bandwidths, signal
levels and other factors, but the following rules apply:
H
If the signal amplitudes are less than 3 dB different, they are resolved when
their frequency separation equals the resolution bandwidth.
H
For signals more widely separated in amplitude and frequency, let A be the
amplitude difference. Then:
F
+
(1
)
A
22
) resolution BW
where F is the required frequency separation. This rule is based on the fact
that the 2715’s 60 dB filter bandwidths tend to be approximately 7 times the
6 dB bandwidth and assumes the filter roll-off is approximately linear in dB.
Using this condition, if the signals are 30 dB different in amplitude (A), then
they have to be separated by approximately 2.4
×
resolution BW.