As the analyzer sweep rate is increased, the amplitude
Mixer Peaking
The MIXER PEAKING control has a switch and variable
wider; which signifies that both the sensitivity and resolution
control position and is used to optimize mixer conversion in
have been degraded by the analyzer sweep rate,
the harmonic bands of Band C. The switched position (fully
The loss of the analyzer sensitivity due to sweep rate
C C W) is referred to as the SEARCH mode. In this position, an
and dispersion can be expressed mathematically as:
optimum mixer conversion or sensitivity point is provided
within the dispersion window so signals that are tuned across
the screen will pass through this optimum sensitivity point.
This insures thot most signals within a given dispersion win-
dow will be observed as the RF CENTER FREQUENCY con-
w h e r e S / SO is the ratio of the effective sensitivity to the
trol is tuned.
analyzer measured sensitivity, at very slow sweep rates or
zero dispersion.
Mixer conversion becomes a manual adjustment with the
control in the VARIABLE position and should be optimized at
D is the dispersion in hertz
each RF CENTER FREQUENCY setting. It has no affect on
B is the -3 dB bandwidth of the analyzer in hertz
band A and is usually broad through the B and C band fre-
quency range.
T is the sweep time in seconds, or
is the sweep rate.
Relative amplitude and sensitivity measurements must be
made after the MIXER PEAKING is adjusted because the dis-
These same variables also determine the resolution of
play response is not flat when the central is in the SEACH
the analyzer. The loss in resolution can be expressed as
position.
follows:
Dispersion
W h e r e R / R O is the ratio of the effective resolution of the
Dispersion is the swept frequency range, or screen window.
analyzer to the analyzer measured resolution bandwidth
The dispersion is adjustable from 10 MHz/div to 1 kHz/div in
at very slow sweep speeds. RO is somewhat arbitrary and
a 1, 2, 5 sequence with an added zero dispersion position
is taken as the displayed width of the CW signal at the
for fixed frequency operation. Band A is limited to a maxi-
-6 dB point.
mum usable dispersion of 5 MHz/div (25 MHz), because
of the added 235 MHz low pass filter.
The resolution of the Type 491 Spectrum Analyzer is
optimized for most settings of the DISPERSION selector
Dispersion accuracy is a function of the IF CENTER FREQ
when the RESOLUTION control is in the coupled position.
control position
and the DISPERSION RANGE switch set-
Resolution however, can be varied from approximately 100
ting. See Characteristics section. The dispersion accuracy
kHz to less than 1 kHz by uncoupling the RESOLUTION
far the kHz/div selections is greater than the MHz/div
control and changing it as an independent function of the
selections, because the range of the IF CENTER FREQ is ten
DISPERSION selector.
times greater for the MHz/div ranges (25 MHz in the MHz/
DIV range).
The sweep rate, as previously mentioned, should be set
below the sweep rate at which there is no noticeable ampli-
The front panel DISPERSION-CAL adjustment may be used
tude loss in the signal.
to recalibrate dispersion for specific IF CENTER FREQ con-
trol settings if a high degree of accuracy is desired. The pro-
As previously shown in the above formula the effective
cedure is described in step 4 for front panel calibration.
resolution of the analyzer is a function af the IF bandwidth.
To adequately resolve pulsed spectrum information, the reso-
lution bandwidth of the analyzer should be on the order of
Resolution
1/10 of the sidelobe frequency width or the reciprocal of
the pulse width. The RESOLUTION control is usually set,
Resolution is the ability of the spectrum analyzer to dis-
after the sweep rate has been adjusted, for optimum main
play adjacent signal frequencies discretely. The measure of
lobe detail. See Fig. 2-9.
resolution is the frequency separation (in Hz) of two equal
amplitude signals when the notch or dip between these sig-
nals is 3 dB down. The resolution for a given display is a
Front Panel Calibration Adjustments
function of sweep speed, dispersion and bandwidth of the
most selective (usually the last IF) amplifier in the signal path.
Three front panel screwdriver adjustments are provided,
to enable the operator to calibrate the dispersion and IF CEN-
Resolution bandwidth is approximately the -6 dB band-
TER FREQ controls, and balance the MHz and kHz positions
width (with Gaussian response) of the analyzer, with the dis-
of the DISPERSION RANGE selector.
persion and sweep time adjusted for the minimum displayed
bandwidth to a CW signal. Resolution and resolution band-
width become synonymous at very long sweep times.
1. Balance and Calibration Check
As the analyzer sweep rate is increased, the amplitude of
a. Turn the INT REF FREQ control to OFF position, then
the CW signal will decrease and the bandwidth becomes
tune a signal on screen with the RF CENTER FREQUENCY
wider; which signifies that both the sensitivity and resolution
control.
have been degraded by the analyzer sweep rate.
2-10
