Gain Compression

Gain compression
In our discussion of dynamic range, we did not concern ourselves with how
accurately the larger tone is displayed, even on a relative basis. As we raise
the level of a sinusoidal input signal, eventually the level at the input mixer
becomes so high that the desired output mixing product no longer changes
linearly with respect to the input signal. The mixer is in saturation, and the
displayed signal amplitude is too low. Saturation is gradual rather than
sudden. To help us stay away from the saturation condition, the 1-dB
compression point is normally specified. Typically, this gain compression
occurs at a mixer level in the range of 5 to + 5 dBm. Thus we can determine
what input attenuator setting to use for accurate measurement of high-level
signals 3. Spectrum analyzers with a digital IF will display an IF Overload
message when the ADC is over-ranged.

Actually, there are three different methods of valuating compression. A
traditional method, called CW compression, measures the change in gain of
a device (amplifier or mixer or system) as the input signal power is swept
upward. This method is the one just described. Note that the CW compression
point is considerably higher than the levels for the fundamentals indicated
previously for even moderate dynamic range. So we were correct in not
concerning ourselves with the possibility of compression of the larger
signal(s) .

A second method, called two-tone compression, measures the change in
system gain for a small signal while the power of a larger signal is swept
upward. Two-tone compression applies to the measurement of multiple
CW signals, such as sidebands and independent signals. The threshold of
compression of this method is usually a few dB lower than that of the CW
method. This is the method used by Agilent Technologies to specify spectrum
analyzer gain compression.

A final method, called pulse compression, measures the change in system
gain to a narrow (broadband) RF pulse while the power of the pulse is swept
upward. When measuring pulses, we often use a resolution bandwidth much
narrower than the bandwidth of the pulse, so our analyzer displays the signal
level well below the peak pulse power. As a result, we could be unaware of
the fact that the total signal power is above the mixer compression threshold.
A high threshold improves signal-to-noise ratio for high-power, ultra-narrow
or widely chirped pulses. The threshold is about 12 dB higher than for
two-tone compression in the Agilent 8560EC Series spectrum analyzers.
Nevertheless, because different compression mechanisms affect CW, two-tone,
and pulse compression differently, any of the compression thresholds can
be lower than any other.

Display range and measurement range
There are two additional ranges that are often confused with dynamic range:
display range and measurement range. Display range, often called display
dynamic range, refers to the calibrated amplitude range of the spectrum
analyzer display. For example, a display with ten divisions would seem
to have a 100 dB display range when we select 10 dB per division. This is
certainly true for modern analyzers with digital IF circuitry, such as the
Agilent PSA Series. It is also true for the Agilent ESA-E Series when using
the narrow (10 to 300 Hz) digital resolution bandwidths. However, spectrum
analyzers with analog IF sections typically are only calibrated for the first
85 or 90 dB below the reference level. In this case, the bottom line of the
graticule represents signal amplitudes of zero, so the bottom portion of
the display covers the range from 85 or 90 dB to infinity, relative to the
reference level.

3. Many analyzers internally control the combined
settings of the input attenuator and IF gain so that
a CW signal as high as the compression level at
the input mixer creates a deflection above the top
line of the graticule. Thus we cannot make incorrect
measurements on CW signals inadvertently.