Anritsu 680 C Series, Maintenance Manual

Page: 94 / 317

6. The difference between these two frequencies is
the actual peak-to-peak frequency deviation. It
should be between 190 MHz and 210 MHz. Re-
cord the difference on the Test Record.
Unlocked Narrow FM Mode
1. Set up the 691XXB as follows:
a. Reset the instrument by pressing SYSTEM ,
then Reset . Upon reset, the CW Menu is dis-
played.
b. Press Edit F1 to open the current frequency
parameter for editing.
c. Set F1 to 5.0 GHz.
2. Set up the Function Generator for a 99.8 kHz
sine wave with an output level of
±
1 volt
(2 volts peak to peak).
3. Set up the Spectrum Analyzer as follows:
a. CF: 5 GHz
b. Span/Div: 200 kHz (2 MHz across the display)
4. On the 691XXB, make the following settings:
a. Press MODULATION , then FM . At the result-
ing External FM Status menu, press More to
go to the additional External FM Status menu.
b. At this menu, press Unlocked Narrow , then
press Previous Menu to return to the main Ex-
ternal FM Status menu.
c. At the main External FM Status menu, press
Edit Sens and set the sensitivity to
0.240 MHz/V.
d. Press On / Off to turn FM on.
5. Since the 691XXB is now in Unlocked Narrow
FM mode, it is necessary to retune the Spectrum
Analyzer to center the display.
6. On the 691XXB, use the cursor control key to
place the cursor under the last 0 in the sensitivity
setting (0.240 MHz/V). While observing the first
Bessel null (Figure 3-9), increase/decrease the
sensitivity setting with the cursor control key to
reduce the carrier as low as possible.
690XXB/691XXB MM 3-33
PERFORMANCE FREQUENCY
VERIFICATION MODULATION TESTS
NOTE
You may need to adjust the RBW
setting on the Spectrum Analyzer in
order to see the >–48 dBc null.
CENTER FREQUENCY
BESSEL
NULL
Figure 3-9. Typical Spectrum Analyzer Display
of Bessel Null on FM Waveform