Fluke PM6690 Operator's Manual page 51

n
Prescaling May Influence
Measurement Time
Prescalers do affect the minimum measure-
ment time, inasmuch as short bursts have to
contain a minimum number of carrier wave
periods. This number depends on the
prescaling factor.
I n p u t
s i g n a l
A f t e r
p r e s c a l e r
Fig. 4-11 Divide-by-16 Prescaler
Fig. 4-11 shows the effect of the 3 GHz
prescaler. For 16 input cycles, the prescaler
gives one square wave output cycle. When the
counter uses a prescaler, it counts the number
of prescaled output cycles, here f/16. The dis-
play shows the correct input frequency since
the microcomputer compensates for the effect
of the division factor d as follows:
´
n d
=
f
t
g
Prescalers do not reduce resolution in recipro-
cal counters. The relative quantization error is
100 ps
still:
t
g
See Table 4-1 to find the prescaling factors
used in different operating modes.
n
LF Signals
Signals below 100 Hz should be measured
with manual triggering, unless the default set-
ting (100 Hz) is changed. See page 2-11. The
low limit can be set to 1 Hz, but the measure-
ment process will be slowed down consider-
ably if auto triggering is used in conjunction
with very low frequencies.
f
f / 1 6
1 6 p e r i o d s
Function
FREQ A/B (300 MHz)
BURST A/B (<160 MHz)
BURST A/B (>160 MHz
PERIOD A/B AVG (300 MHz)
PERIOD A/B SGL (300 MHz)
FREQ C (3 GHz)
FREQ C (8 GHz)
All other functions
Table 4-1 Prescaling factors.
When measuring pulses with a low repetition
rate, for example a 0.1 Hz pulse with a
non-prescaled function like PERIOD SGL, the
measurement will require at least the duration
of one cycle, that is 10 seconds, and at worst
nearly 20 seconds. The worst case is when a
trigger event took place just before the begin-
ning of a measurement time (Fig. 4-12). Mea-
suring the frequency of the same signal will
take twice as long, since this function involves
prescaling by a factor two.
P o s s i b l e t r i g g e r e v e n t s
S e t m e a s u r i n g
t i m e
T i m e f o r o n e m e a s u r e m e n t
Fig. 4-12 Measurement Time
Even if you have chosen a short measurement
time, this measurement will require between
20 and 40 seconds (for this example).
Theory of Measurement 4-11
Measuring Functions
Prescaling
Factor
2
1
2
2
1
16
256
1
G a t e t i m e