Fluke PM6690 - Ratio A;B, B;A, C;A, C;B; Burst A, B, C; Triggering

RATIOA/B,B/A,
C/A, C/B
To find the ratio between two input frequen
-
cies, the counter counts the cycles on two
channels simultaneously and divides the result
on the primary channel by the result on the
secondary channel.
Ratio can be measured between Input A and
Input B, where either channnel can be the pri
-
mary or the secondary channel. Ratio can also
be measured between Input C and Input A or
between Input C and Input B. Here Input C is
the primary channel.
+
Note that the resolution calcula
-
tions are very different as com-
pared to frequency measure-
ments. See page 8-9 for details.
BURST A, B, C
A burst signal as in Fig. 4-2has a carrier wave
(CW) frequency and a modulation frequency,
also called the pulse repetition frequency
(PRF), that switches the CW signal on and off.
Both the CW frequency, the PRF, and the
number of cycles in a burst are measured
without external arming signals and with or
without selectable start arming delay. See
Chapter 5 “Measurement Control” for a fun
-
damental discussion of arming and arming de
-
lay.
The general frequency limitations for the re
-
spective measuring channel also apply to burst
measurements. The minimum number of cy
-
cles in a burst on Input A or Input B is 3 be
-
low 160 MHz and 6 between 160 MHz and
300 MHz. Burst measurements on Input C in
-
volve prescaling, so the minimum number of
cycles will be 3 x prescaling factor.The
3 GHz option, for example, has a prescaling
factor of 16 and requires at least 48 cycles in
each burst.
The minimum burst duration is 40 ns below
and 80 ns above 160 MHz.
Triggering
Bursts with a PRF above 50 Hz can be mea
-
sured with auto triggering on.
The out-of-sync error described under heading
“Possible errors” on page 4-6may occur more
frequently when using Auto Trigger.
When PRF is below 50 Hz and when the gap
between the bursts is very small, use manual
triggering.
Always try using AUTOSET first. Then the
Auto Trigger and the Auto Sync functions in
combination will give satisfactory results
without further tweaking in most cases. Some-
times switching from AUTO to MANual trig-
gering in the INPUT A/B menus is enough to
get stable readings. The continually calculated
trigger levels will then be fixed.
Input C has always automatic triggering and
AUTOSET only affects the burst synchroni
-
zation.
Measuring Functions
4-4 RATIO A/B, B/A, C/A, C/B
C W
P R F
B u r s t
S i g n a l
Fig. 4-2 Burst signal.