Tektronix 491 Maintenance Manual page 13

FINE-A one turn control that operates in conjunction with
the IF control to provide a fine adjustment of the IF center
frequency.
CAL-With the IF CENTER FREQ control centered, it cali-
brates the IF center frequency to 200 MHz.
V I D E O F I L T E R - W i t h t h e s w i t c h i n t h e u p p o s i t i o n , t h e
video bandwidth is restricted, to reduce high frequency
video components such as noise, and reduce zero beats
when viewing signals near minimum resolution.
VERTICAL DISPLAY-Selects logarithmic, linear or square-
law display. In the LOG position, signal display amplitude
is a logarithmic function with a ³40-dB dynamic range.
In the LIN position, signal display amplitude is a linear
function with a ³26-dB range. In the SQ LAW position
signal display amplitude is a square law function and the
display is a function of signal power, The SQ LAW posi-
tion has a ≥13 d B d y n a m i c r a n g e .
POWER-Turns power off and on to the Type 491.
INDICATOR LIGHT-indicates when POWER is applied to
the Type 491.
RF CENTER FREQUENCY-Tunes RF center frequency from
10 MHz to 40 GHz. With the IF CENTER FREQ control in
the 0 position, the RF CENTER FREQUENCY dial indicates
the center frequency of the display.
B A N D S E L E C T O R - S w i t c h s e l e c t s R F I n p u t s a n d b a n d s ;
A (10-275 MHz), B (270-2000 MHz) and C (1.5-40 GHz).
Fig. 2-4. Front panel controls and connectors.
F I N E R F C E N T E R F R E Q - A 1 0 t u r n c o n t r o l t o p r o v i d e a
f i n e a d j u s t m e n t o f t h e R F l o c a l o s c i l l a t o r f r e q u e n c y .
Especially useful in tuning the oscillator to a phase lock
condition with the reference frequency.
M I X E R P E A K I N G - A t w o p o s i t i o n c o n t r o l t h a t o p t i m i z e s
the conversion of the first local oscillator for bands B and
C. Does not affect band A. In the SEARCH position the
mixer current is swept through a range by the sweep volt-
age. This insures an optimum mixer conversion or sensi-
tivity point within the dispersion range of the analyzer, so
all signals within a given dispersion pass through this opti-
mum sensitivity point as the signals are tuned across the
screen. The manual (0 to 10) position of the control pro-
vides an adjustment to optimize mixer conversion for any
fixed center frequency setting.
L O C K C H E C K - A p u s h b u t t o n s w i t c h t h a t a p p l i e s t h e
phase lock output beat signal (between the local oscillator
and reference frequency) to the vertical display system.
Provides a visual indication to the operator of phase lock
operation.
INT REF FREQ-A switch and control. The control varies
the internal 1 MHz Reference Frequency over a range of
approximately 1 kHz. With the control in the OFF posi-
tion, the Internal Reference Frequency is turned off, and
a n e x t e r n a l l y a p p l i e d s i g n a l t o t h e E X T R E F F R E Q I N
(1 MHz MARKERS OUT) connector becomes the reference
frequency.
2-5