9A3 Understand the effect of measurement tolerances, calibration accuracy and time drift on frequency measurements and the allowance to be made for transmission bandwidths.
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.
Even with well calibrated 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.
If you take the figure of 3kHz as the basic bench mark and if your 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.
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.
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??
The origin of some of the text on this page is from the RSGB with additions by the web master