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Model 8901B
Service
change of the HF VCO should be less than 100 kHz.
If
it is not, 14 will be displayed in digit
positions 3 and 4.
N=4
Test 15. Undefined.
N=4
Test
16.
Undefined.
LF VCXO Lock Range Tests
N=5
Test 17. LF VCXO Range Test. See Service Sheets 19 and 20.
Tests to see if the DAC moves the LF VCXO within the proper frequency limits. Since the
LF VCXO frequency cannot be measured directly, an indirect process is used. The Controller
sends a hexadecimal 00 to the DAC, which drives the LF VCXO to its minimum frequency.
This frequency change causes a proportional change in the HF VCO frequency, which
is
measured by the Counter. The Controller then sends a hexadecimal FF to the DAC, driving
the LF VCXO to its highest frequency. The
HF
VCO output is again counted. The difference
between the highest and lowest frequencies from the H F VCO should be between 2.95 and
5.5 MHz. If the frequency difference does not fall within this range, 17 will be displayed in
digit positions 1 and 2.
Test 18. Gain of the LF VCXO Drive. See Service Sheets 19 and 20.
Tests the gain of the LF VCXO. This is the hardest test in this series for the instrument to
pass. The Controller sends a hexadecimal 55 to the most-significant DAC (MSDAC) and a
hexadecimal AA to the least-significant DAC (LSDAC) and then counts the frequency of the
HF VCO. The Controller then changes the MSDAC to a hexadecimal
AA
and the LSDAC to
a hexadecimal 55, and again counts the frequency of the HF VCO. The difference between
the first and second frequencies should be within the range of 1.05 and 2.4 MHz. If it is not,
18 will be displayed in digit positions 3 and 4.
N=5
N=5
Test 19. Undefined.
N=5
Test 20. Undefined.
55.0 Sweep Doubler Band. Sweeps the LO slowly back and forth across the doubler band. See Service
Sheet
17.
56.0 Sweep Bands 4 through 8. Sweeps the LO slowly and sequentially across bands 4 through 8. See
Service Sheet 17.
57.0 Sweep Bands DBLR through 3.
Sweeps the LO slowly and sequentially across bands
DBLR
through 3. See Service Sheet 17.
Special Fbnctions
8B-11
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