Service Model
8901B
NOTE
Test
6
is
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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