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Service
Model 8901B
NOTE
Test
6 i s
not always conclusive. The test may not always detect a failure of
the VCO to tune to the bottom
of
the band.
If the VCO does fail to tune to
the bottom of its band, the instrument will not tune to certain frequencies
in the track mode.
N=2
Test 7. HF VCO Mid-Range Test. See Service Sheet 18.
Tests the DAC's ability to control the HF VCO near the center of its frequency range. The
DAC is programmed by the Controller to output a tune voltage near the center of the range.
If the HF VCO does not tune to between 454 and 575 MHz, 7 will be displayed in digit
position 6.
N=2
Test 8. Undefined.
DAC and HF VCO Incremental Range Tests
N=3
Test 9. Gain Test For Most Significant DAC. See Service Sheet 20.
Tests the gain of the most significant DAC. The Controller sends a hexadecimal 55 to
the most-significant DAC (MSDAC) and a hexadecimal
AA to
the least-significant DAC
(LSDAC). The Controller then counts the frequency of the HF VCO. The MSDAC is then
changed to AA. The Controller again counts the frequency of the HF VCO and then computes
the difference between the first and second frequencies. This difference should fall between
139 and 285 MHz. If it does not, 9 will be displayed in digit position 2.
Test 10. Gain Test For Least Significant DAC. See Service Sheet 20.
Tests the gain of the least significant DAC. The Controller sends a hexadecimal AA to
the most-significant DAC (MSDAC) and a hexadecimal 55 to the least-significant DAC
(LSDAC). The Controller then counts the frequency of the HF VCO. The LSDAC is then
changed
to
AA. The Controller again counts the output of the HF VCO and then computes
the difference between the first and second frequencies. This difference should fall between
1.95 and 4.5 MHz. If it does not, 10 will be displayed in digit positions 3 and 4.
N=3
N=3
Test 11. Undefined.
N=3
Test 12. Undefined.
LF VCXO Lock Acquisition Tests
N=4
Test 13. Phase Lock Loop Acquisition. See Service Sheets 18 and 20.
Tests the HF VCO's ability to lock to the LF VCXO. The Controller turns off the Sweep
Current Sources and the LF VCXO tune filter. It then programs the DAC to output a
tune voltage which causes the HF VCO
to
operate near the center of its frequency range.
The Controller rapidly switches the DAC output
to
the LF VCXO (with the DAC still
programmed
to
midrange). The sampler loop is then closed and the output of the HF VCO
is counted. If the HF VCO is operating properly, it will drift until it locks to a harmonic of
the LF VXCO (via the sampler). If the HF VCO frequency has moved more than 2 MHz, it
has failed to lock to a harmonic of the LF VCXO, and 13 will be displayed in digit positions
1 and 2.
Test 14. Phase Lock Loop Stability. See Service Sheet
20.
Tests the ability of the HF VCO to follow step changes in the LF VCXO. The Controller
sends the DAC a code which forces the LF VCXO to the bottom of its frequency range. The
frequency of the HF VCO is counted. The DAC is then instructed
to
quickly slew the LF
VCXO to the
top
of its frequency range and then back down
to
the bottom again. When
the DAC output voltage reaches minimum, the HF VCO is again counted. The frequency
N=4
8B-10
Special Fbnctions
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