Tektronix 491, Maintenance Manual

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1)
Set
the front panel controls as follows:
R F C E N T E R F R E Q U E N C Y
10 MHz
D I S P E R S I O N R A N G E M H z / D I V
D I S P E R S I O N 5 MHz/div
I F A T T E N U A T O R 20 dB
V E R T I C A L D I S P L A Y
L I N
T I M E / D I V 5 ms
Band Selector A
2) Apply the output signal from a signal generator
within the frequency range of band A, to the band A
RF INPUT connector.
3) Set the generator frequency and the RF center fre-
quency to the frequencies that are listed in Table 6-7.
Adjust the signal generator output attenuator and the
Type 491 GAIN control for a signal amplitude of 6 divi-
sions.
4) Check band A display flatness by tuning the signal
from the display screen left edge to the right edge with
the RF CENTER FREQUENCY control. (Frequency range +
and -25 MHz of the RF center frequency.) Signal ampli-
t u d e s h o u l d n o t c h a n g e m o r e t h a n ± 1 . 5 d B f r o m i t s
average amplitude or 3 dB total.
T A B L E 6 - 7
R F C e n t e r A p p l i e d S i g n a l
Frequency G e n e r a t o r F r e q .
10 MHz-60 MHz 35 MHz
50 MHz-100 MHz 75 MHz
100 MHz-150 MHz 125 MHz
150 MHz-200 MHz
175 MHz
2 0 0 M H z - 2 5 0 M H z 225 MHz
250 MHz-275 MHz
275 MHz
5) Remove the signal to band A, RF INPUT and apply
a signal within the frequency range of band B to RF INPUT
B. Set
the band selector to B and set the DISPERSION to
10 MHz/div.
T A B L E 6 - 8
6) Check display flatness as per Table 6-8. 3 dB maxi-
mum amplitude variation over 100 MHz
window (±50 MHz
from
RF center frequency). Maximum signal amplitude
with the MIXER PEAKING control before measuring flat-
ness.
7) Remove the signal from band B INPUT and apply
the output from signal generators, that cover scales 4
through 6 frequency range, to band C Coaxial Mixer.
8) Check response flatness through the frequency range
of the Coaxial Mixer. Maximum amplitude variation over
100 MHz dispersion window must not exceed 3 dB. Adjust
MIXER PEAKING for maximum signal amplitude before
measuring flatness.
9) Replace the Coaxial Mixer with the Waveguide Mixer
Adapter.
10) Apply the output from o signal generator, within
the frequency range of scale 8 and 9 through one of the
Waveguide Mixers and the 2 foot cable (with the BNC
connectors) to band C Waveguide Adapter.
11) Check response flatness for the frequency range
above 12.4 GHz. Maximum amplitude variation over 100
MHz dispersion window must not exceed 6 dB. Adjust
M i x e r P E A K I N G f o r m a x i m u m s i g n a l a m p l i t u d e b e f o r e
measuring flatness.
6 - 3 8