Frequency Measurements; Freq A, B; Freq C - Fluke PM6690 Operator's Manual

Frequency Measurements

FREQ A, B

The counter measures frequency between
0 Hz and 300 MHz on Input A and Input B.
Frequencies above 100 Hz are best measured
using the Default Setup. See page 2-13. Then
Freq A will be selected automatically. Other
important automatic settings are AC Cou-
pling, Auto Trig and Meas Time 200 ms.
See below for an explanation. You are now
ready to start using the most common function
with a fair chance to get a result without fur-
ther adjustments.
n
Summary of Settings for Good
Frequency Measurements
–
AC Coupling, because possible DC offset
is normally undesirable.
–
Auto Trig means Auto Hysteresis in this
case, (comparable to AGC) because super-
imposed noise exceeding the normal nar-
row hysteresis window will be suppressed.
–
Meas Time 200 ms to get a reasonable
tradeoff between measurement speed and
resolution.
Some of the settings made above by recalling
the Default Setup can also be made by activat-
ing the AUTOSET key. Pressing it once
means:
–
Auto Trig. Note that this setting will be
made once only if Man Trig has been se-
lected earlier.
Pressing AUTOSET twice within two sec-
onds also adds the following setting:
–
Meas Time 200 ms.

FREQ C

With an optional prescaler the counter can
measure up to 3 GHz or 8 GHz on Input C.
These RF inputs are fully automatic and no
setup is required.
H y s t e r e s i s b a n d ( S E N S )
T r i g g e r l e v e l o f f s e t
T r i g g e r p o i n t s
0 V
R e s e t p o i n t s
Fig. 4-1 Frequency is measured as the
inverse of the time between
one trigger point and the next;
1
=
f
t
Measuring Functions
J
FREQ A, B 4-3