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 HF 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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