Mixer Peaking
As
the
analyzer sweep rate is increased, the amplitude
of the
CW
signal
will decrease and the bandwidth becomes
wider; which signifies that both the sensitivity and resolution
have been degraded by the analyzer sweep rate,
The MIXER PEAKING control has a switch and variable
control position and is used to optimize mixer conversion in
the harmonic bands of Band C. The switched position (fully
C C W
) is referred to as the SEARCH mode. In this position, an
optimum mixer conversion or sensitivity point is provided
within the dispersion window so signals that are tuned across
The loss of the analyzer sensitivity due to sweep rate
and dispersion can be expressed mathematically as:
w h e r e S / S
O
is the ratio of the effective sensitivity to the
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-
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
each RF CENTER FREQUENCY setting. It has no affect on
band A and is usually broad through the B and C band fre-
D is the dispersion in hertz
B is the -3 dB bandwidth of the analyzer in hertz
T is the sweep time in seconds, or
is the sweep rate.
quency range.
Relative amplitude and sensitivity measurements must be
made after the MIXER PEAKING is adjusted because the dis-
play response is not flat when the central is in the SEACH
These same variables also determine the resolution of
the analyzer. The loss in resolution can be expressed as
follows:
position.
Dispersion
W h e r e R / R
O
is the ratio of the effective resolution of the
analyzer to the analyzer measured resolution bandwidth
Dispersion is the swept frequency range, or screen window.
The dispersion is adjustable from 10 MHz/div to 1 kHz/div in
a 1, 2, 5 sequence with an added zero dispersion position
at very slow sweep speeds. R
O
is somewhat arbitrary and
is taken as the displayed width of the CW signal at the
-6 dB point.
for fixed frequency operation. Band A is limited to a maxi-
mum usable dispersion of 5 MHz/div (±25 MHz), because
of the added 235 MHz low pass filter.
T h e r e s o l u t i o n o f t h e T y p e 4 9 1 S p e c t r u m A n a l y z e r i s
o p t i m i z e d f o r m o s t s e t t i n g s o f t h e D I S P E R S I O N s e l e c t o r
when the RESOLUTION control is in the coupled position.
Resolution however, can be varied from approximately 100
k H z t o l e s s t h a n 1 k H z b y u n c o u p l i n g t h e R E S O L U T I O N
control and changing it as an independent function of the
DISPERSION selector.
Dispersion accuracy is a function of the IF CENTER FREQ
control position
and the DISPERSION RANGE switch set-
ting. See Characteristics section. The dispersion accuracy
f a r t h e k H z / d i v s e l e c t i o n s i s g r e a t e r t h a n t h e M H z / d i v
selections, because the range of the IF CENTER FREQ is ten
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-
tude loss in the signal.
The front panel DISPERSION-CAL adjustment may be used
to recalibrate dispersion for specific IF CENTER FREQ con-
trol settings if a high degree of accuracy is desired. The pro-
cedure is described in step 4 for front panel calibration.
As previously shown in the above formula the effective
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
1/10 of the sidelobe frequency width or the reciprocal of
the pulse width. The RESOLUTION control is usually set,
after the sweep rate has been adjusted, for optimum main
lobe detail. See Fig. 2-9.
Resolution
Resolution is the ability of the spectrum analyzer to dis-
play adjacent signal frequencies discretely. The measure of
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
function of sweep speed, dispersion and bandwidth of the
most selective (usually the last IF) amplifier in the signal path.
Front Panel Calibration Adjustments
Three front panel screwdriver adjustments are provided,
to enable the operator to calibrate the dispersion and IF CEN-
TER FREQ controls, and balance the MHz and kHz positions
of the DISPERSION RANGE selector.
Resolution bandwidth is approximately the -6 dB band-
width (with Gaussian response) of the analyzer, with the dis-
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
the CW signal will decrease and the bandwidth becomes
wider; which signifies that both the sensitivity and resolution
have been degraded by the analyzer sweep rate.
a. Turn the INT REF FREQ control to OFF position, then
tune a signal on screen with the RF CENTER FREQUENCY
control.
2-10
Summary of Contents for 491
Page 4: ...i i Fig 1 1 The Type 491 Spectrum Analyzer ...
Page 24: ...2 16 Fig 2 17 Control set up chart ...
Page 34: ...Fig 3 1 Function block diagram of the Type 491 3 2 ...
Page 42: ...Fig 3 10 Block diagram of the video detector and vertical amplifier 3 1 0 ...
Page 48: ......
Page 53: ...Fig 4 4 Power Supply Circuit board assembly with wiring color code 4 5 ...
Page 54: ...Fig 4 5 Horizontal Display circuit board assembly showing color code to pin connectors 4 6 ...
Page 55: ...Fig 4 6 IF Control board assembly Wiring color code to pin connector 4 7 ...
Page 64: ...Fig 4 20A Tube subassembly removal procedure 4 16 ...
Page 65: ...Fig 4 20B Tube subassembly installation procedure 4 17 ...
Page 68: ...Fig 4 22 Power supply board assembly with component call out 4 2 0 ...
Page 69: ...Fig 4 23 Vertical Amplifier and Blanking board assembly with component call out 4 21 ...
Page 70: ...Fig 4 24 IF control board assembly with component call out 4 22 ...
Page 71: ...Fig 4 24 IF control board assembly with component call out 4 23 ...
Page 72: ...Fig 4 25 Horizontal display board with component call out 4 24 ...
Page 73: ...Fig 4 25 Horizontal display board with component call out 4 25 ...
Page 74: ...Fig 4 26 Phase lock board with component call out 4 26 ...
Page 88: ......
Page 90: ...Fig 6 1 Test equipment recommended for calibration of the Type 491 6 2 ...
Page 138: ......
Page 192: ......
Page 195: ...SECTION II B 3 ...
Page 196: ......
Page 200: ......
Page 201: ...SECTION 9 DIAGRAMS MECHANICAL PARTS LIST ILLUSTRATIONS ACCESSORIES ...
Page 202: ......
Page 203: ...9 1 ...
Page 204: ...9 3 ...
Page 205: ...9 5 ...
Page 206: ...9 7 ...
Page 207: ...9 9 ...
Page 208: ...9 11 ...
Page 209: ...9 13 ...
Page 210: ...9 15 ...
Page 211: ...9 17 ...
Page 212: ...9 19 ...
Page 213: ...9 21 ...
Page 214: ...9 23 ...
Page 215: ...9 25 ...
Page 216: ...9 27 ...
Page 217: ...FIG 1 9 29 ...
Page 218: ...FIG 2 REAR 9 31 ...
Page 219: ...9 33 FIG 3 IF CHASSIS PHASE LOCK ASSEMBLIES ...
Page 220: ...FIG 4 POWER CHASSIS 9 35 ...
Page 221: ...FIG 5 TIME DIV SWITCH OSCILLATOR ASSEMBLIES 9 37 ...
Page 222: ...FIG 6 CRT SHIELD ASSEMBLY 9 39 ...
Page 223: ...FIG 7 CABINET ASSEMBLY HANDLE 9 4 1 ...
Page 224: ...FIG 8 491 STANDARD ACCESSORIES FIG 8 491 STANDARD ACCESSORIES 9 43 ...
Page 225: ......
Page 226: ...PIN 028017 000 ...