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Syllabus Sections:-
Measurement
9A3 54
Understand the effect of measurement tolerances,
calibration accuracy and time drift on frequency
measurements and the allowance to be made for transmission
bandwidths.
Measurement
tolerance
Unless
you have very accurate and calibrate equipment to check your
equipment then the ability to measure frequency accurately is
always a matter of how far out your assessment of the
frequency measurement you could be. At worst you should not be
more that say +/- 3kHz else with speech you could be out side
the band edge.
Calibration
accuracy
Even with
well calibrate equipment over time the frequency calibration
can "drift" or change frequency so what you think is
calibrated is in fact in error. Further the amount of
variation is dependent upon how good the equipment is in its
design and built to have a stable frequency generator
etc.
Transmission
bandwidth
If you
take the figure of 3kHz as the basic bench mark and if you
equipment is variable by 3kHz then you will not know the band
edge position within 6kHz. so it is all a matter of knowing
your equipment and its calibration standard.
What
is the purpose behind this section?
This is
where we stated from at the top of the page - this section of
the syllabus is to ensure that you know how to assess your
frequency of operation.
As the
frequency counter is probably the more readily accessible and
easiest to operate let's look at an example.
Frequency
counter example
If your
transmitter is operating in the 14MHz band has a frequency
tolerance of 100 parts per million and an AM band width of 6
kHz and the digital
frequency display is accurate to 10 parts per million, what
is the lowest frequency that the licensee can use to ensure
that no emission is below 14 MHz ?
a. 14,004.54 kHz
b. 14,045.4 kHz
c. 14,090.8 kHz
d. 14,005.31 kHz
The tolerance build up is
as follows:-
| BAND EDGE |
14,000,000 |
14,000.00
kHz
|
| FREQUENCY
TOLERANCE (100ppm) |
1,400
|
1.40
kHz
|
| FREQUENCY METER
TOLERANCE (10 ppm) |
140
|
0.14
kHz
|
| HALF OF A3E
BANDWIDTH (6 kHz) |
3,000
|
3.00
kHz
|
|
TOTAL
|
14,004,540 |
14,004.54 kHz |
This is the closest to
the band edge you would be able to transmit, so know your
equipment??
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