The purpose of this process is to find out
if there is a measurable signal present at
the input, and if so, fix the LO frequency
that gives rise to an IF signal above a cer-
tain threshold level. This is done by se-
quential
ly stepping the LO from the
highest value in the look-up table to the
lowest value and applying the resulting
comb generator spectrum to the mixer.
The process is stopped when the signal
detector outputs a status signal to the
processor.
Acquisition
We don't know yet which harmonic
generates the IF signal. First we measure
the IF with the counter. Then we decrease
the LO frequency by 1 MHz and measure
the IF once more. If, for instance, the
difference between the two values is 5
MHz, then we know that the fifth
harmonic is the origin. By examining the
sign of the difference, we can decide if
the original IF should be added to or
subtracted from the calculated harmonic
in order to arrive at
the final value.
Final RF calculation
Now we know the LO frequency, the
multiplication factor 'n' and the sign.
What remains to be done is to count the
IF during a measurement time
corresponding to the desired resolution,
and then the result is used for calculating
the final value to be presented on the
display as: f
x = n x f
LO
± IF
There are a number of conditions that can
complicate the acquisition process. All of
them are handled by measures taken by the
instrument firmware. Two examples:
— One of the step frequencies produces an
IF but not its shifted value. Action: go to
the next table value.
Frequency modulation causes an unstable
'n' value calculation. Action: increase the
measuring time.
Another feature in this instrument is the abil-
ity to measure power with high resolution and
moderate accuracy over the entire frequency
range, achieved by storing individual fre-
quency dependent power correction factors in
a memory located inside the conversion unit.
This memory is also used for storing other in-
f
ormation about the converter like identifi
ca-
tion data.
Single A, B & Avg. 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:
while it calculates PERIOD AVG as:
2. In the PERIOD SINGLE mode, the counter
uses no prescaler.
All other functions and features as described
earlier under "Frequency" apply to Period
measurements as well.
USER MANUAL ● CNT 9x Series ● Rev.22 February 2020
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