Fluke PM6690 - Input Amplifiers Troubleshooting

n
RF Signals
As mentioned before, a prescaler in the C-in
-
put divides the input frequency before it is
counted by the normal digital counting logic.
The division factor is called prescaler factor
and can have different values depending on
the prescaler type. The 3 GHz prescaler is de
-
signed for a prescaling factor of 16. This
means that an input C frequency of, e.g.,
1.024 GHz is transformed to 64 MHz.
Prescalers are designed for optimum perfor
-
mance when measuring stable continuous RF.
Most prescalers are inherently unstable and
would self-oscillate without an input signal.
To prevent a prescaler from oscillating, a
“go-detector” is incorporated. See Fig. 4-13.
The go-detector continuously measures the
level of the input signal and simply blocks the
prescaler output when no signal, or a signal
that is too weak, is present.
The presence of a burst signal to be measured
makes certain demands upon the signal itself.
Regardless of the basic counter’s ability to
measure during very short measurement
times, the burst duration must meet the fol
-
lowing minimum conditions:
Burst presc factor inp cycle time
min
(. )(. )>´ ´3
or at least 80 ns
Normally the real minimum limit is set by
other factors, like the speed of the GO-detec
-
tor. This speed depends on the specific input
option used.
PERIOD
Single A, B
Average A, B, C
From a measuring point of view, the period
function is identical to the frequency function.
This is because the period of a cyclic signal
has the reciprocal value of the frequency (
1
f
).
In practice there are two minor differences.
1. The counter calculates FREQUENCY
(always AVG) as:
f
number of cycles
actual gate time
=
while it calculates PERIOD AVG as:
p
actual gate time
number of cycles
=
2. In the PERIOD SINGLE mode, the coun-
ter uses no prescaler.
All other functions and features as described
earlier under “Frequency” apply to Period
measurements.
Measuring Functions
4-12 PERIOD
+ N
&
G o - d e t e c t o r
T o c o u n t i n g
l o g i
c
Fig. 4-13 Go-detector in the prescaler