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Frequency Response
Tracking Generators
36
determined from its specifications. First check the distortion spec.
For example, this might be "all spurious products 70dB down for –
27dBm at the input mixer". Then, determine that adequate
sensitivity exists. For example, 70dB down from –27dBm is –97dB.
This is the level we must be able to detect, and the bandwidth
required for this sensitivity must not be too narrow or it will be
useless. Last, the display range must be adequate.
Notice that the spurious-free measurement range can be exten-
ded by reducing the level at the input mixer. The only limitation,
then, is sensitivity. To ensure a maximum dynamic range on the
CRT display, check to see that the following requirements are
satisfied.
1.The largest input signal does not exceed the optimum input level
of the analyzer (typically –27dBm with 0dB input attenuation).
2.The peak of the largest input signal rests at the top of the CRT
display (reference level).
The frequency response of an analyzer is the amplitude linearity of
the analyzer over its frequency range. If a spectrum analyzer is to
display equal amplitudes for input signals of equal amplitude, inde-
pendent of frequency, then the conversion (power) loss of the input
mixer must not depend on frequency. If the voltage from the LO is
too large compared to the input signal voltage then the conversion
loss of the input mixer is frequency dependent and the frequency
response of the system is nonlinear. For accurate amplitude
measurements, a spectrum analyzer should be as flat as possible
over its frequency range. Flatness is usually the limiting factor in
amplitude accuracy since it is extremely difficult to calibrate out.
And, since the primary function of the spectrum analyzer is to
compare signal levels at different frequencies, a lack of flatness
can seriously limit its usefulness.
The tracking generator (HM5014 only) is a special signal source
whose RF output frequency tracks (follows) some other signal
beyond the tracking generator itself. In conjunction with the
spectrum analyzer, the tracking generator produces a signal whose
frequency precisely tracks the spectrum analyzer tuning. The
tracking generator frequency precisely tracks the spectrum analyzer
Subject to change without notice
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