Tektronix 2715, Руководство пользователя

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Spectrum Analyzer Menus
6–8
2715 User Manual
In this case, you select the dBm unit via [INPUT] [3] [0] and the 50
W
source
because the 2715 is matched to the 75
W
side of the minimum loss pad and the
signal really is being terminated in a 50
W
impedance. Further, it is the attenua-
tion of the pad that is entered under item 6 and not its insertion loss. The 7.5 dB
accounts for both the insertion loss and the fact that the signal at the input to the
minimum loss pad is 1.9 dB less than it would be if the 50
W
source were
connected directly to the 2715.
Source Minimum Loss Pad
2715
Input
50 43.3
86.7 75
Figure 6–3: 75/50 Matching Minimum Loss Pad
If in doubt whether the pad is needed, compare a measurement with the pad to
the same measurement without the pad. If there is no significant difference, omit
the pad. In some cases, such as carrier-to-noise measurements, the use of a pad
may drop your system noise level below the 2715 noise floor. In such cases, the
pad cannot be used. Removing the pad typically does not distort carrier-to-noise
measurements.
When a 50
W
source is frequently used, you can store the settings above in one
of the STORED SETTINGS registers (see the UTIL menu discussion on
page 6–60); then you merely have to recall the settings each time a 50
W
source
is used with the 2715.
Overdriving the 2715’s first mixer circuit can generate spurious signals and cause
inaccurate measurements. As the signal amplitude increases past the maximum
linear range of the circuit, its output amplitude becomes less than it should be.
This creates lower-than-actual amplitude measurements. On the other hand, if the
signal amplitude at the mixer is too low, signals may become lost in the 2715’s
internal noise. An optimum compromise is achieved within the 2715 by making
the top graticule line represent a –30 dBm level at the input to the first mixer.
However, in cases where total signal energy is large, it may be beneficial to
restrict the input to the mixer to a smaller value. By resetting the first mixer
input level to, say, –40 dBm, you increase the RF attenuation by 10 dB while
simultaneously increasing the IF gain 10 dB. This provides additional protection
to the first mixer. In other cases, you may want to examine a low-level signal
adjacent to a high-level signal. A –20 dBm level at the first mixer input allows
Setting the First Mixer
Input Level