10b Frequency Checking
10b.1 Recall the uses and limitations of absorption wavemeters, heterodyne wavemeters, crystal calibrators, digital frequency counters and standard frequency transmissions.
What is behind this part of the syllabus ??
It is a requirement that anyone holding an amateur radio licence must be able to prove that the equipment they are using is working correctly and does not interfere with others - this section of the syllabus is to ensure that you know how to assess your frequency of operation.
Well that's a pretty all embracing statement, but what does it mean??
It means - that anything you may use in your hobby of amateur radio whether it is used for self training or not, must comply with the terms and conditions of your licence.
You can't make statements like "Well I bought it from a reputable dealer and it's new so therefore it must be ok".
THE ABSORPTION WAVEMETER.
The absorption wavemeter consists of a parallel tuned circuit with the addition of a rectifying diode and a meter used as an indicator of resonance. A version of this type of meter is shown below with approximate values of components.
The variable capacitor of the tuned circuit is adjusted so that a resonant frequencies may be obtained.
The wavemeter absorbs power from a nearby transmitter and shows a maximum reading on the meter when it is tuned to the same frequency.
Calibration of the unit is by setting a known good source of transmissions to different stop frequencies and marking up a scale. The obvious choices would be 1.8MHz and its harmonics so that the meter could also be easily used to check for harmonic output from a transmitter you were building (for instance).
By tuning the variable capacitor across its range and the use of the switch a large range of frequencies can be covered looking for relative harmonic outputs.
Limitation - the wavemeter can only give you an idea as to what frequency you are transmitting on and where there are any harmonics it cannot tell you precise frequencies.
THE HETERODYNE WAVEMETER
This operate in a completely different way. Rather than absorbing the RF from a Tx this gives out RF at very low levels such that the a Receiver may be calibrate by beating against the know frequency.
Such an experience was common place say 30 years ago to check your equipment but today few will have ever tried to check out a receiver in this manner. With the Heterodyne wavemeter set to a specific frequency output the receiver is tuned across the band and the signal will be received as a tone. The tone lowers in frequency as you reach the "tuned" position and then at the beat frequency point disappears. As soon as you pass through the null position the signal reappears - assuming you are receiving on AM else on side band only one side of the wavemeters signal is heard.
The BRATS club have been donated an old heterodyne wavemeter and this will be used at field weekends to demonstrate its operation.
Limitation - A little more accurate than the wavemeter but again accuracy is limited to the components used and at best would be within +/- 2 kHz
The crystal calibrator is again an item used about 30 years ago to ensure that your receiver was on frequency at stop frequencies. The calibrator had a 1 MHz crystal that was so arranged to be rich in harmonics and could then check the frequency up so say the 30th harmonic ( 30MHz ).
The Heterodyne wavemeter mentioned above also has a crystal calibrator output.
Limitation - The crystal calibrator can only give single frequency output / and harmonically related outputs.
DIGITAL FREQUENCY COUNTERS
Most radio equipment these days is fitted with a digital frequency meter, but the question always arises as to how accurate the meter actually is.
Digital frequency meters sample or count the number of complete sine waves in a given period, ie: 1 second or 1 µsecond, and displaying the count on a LED or LIQUID CRYSTAL display.
The digital frequency meter consists of 4 basic parts.
The display, the counter, the clock and the gate.
THE CLOCK produces pulses from a stable oscillator, usually 1 or 10 MHz, the pulses have a time period ranging from 1 µsec to 1 sec. The pulses are applied to :-
THE GATE which allows converted sine wave pulses through the gate for a given time period and then shuts.
The pulses are then COUNTED and DISPLAYED as frequency.
Limitation - The accuracy of the frequency counter is dependent upon the accuracy of the internal crystal oscillator sometimes called the "clock oscillator", if the gate is switched on too long then the pulses counted will be wrong for the frequency being measured.
STANDARD FREQUENCY TONES
A suitable accurate frequency standard has to be found to check the accuracy of any of the above mentioned meters.
Until relatively recently there were world wide transmissions of standard frequency tones on 2.5, 5, 10, 15, and 25 MHz but now the only ones know to exist are radiated from the USA and China the European standard frequencies were shut down to save costs.
By comparing the received standard with the indicated frequency any error would soon show up.
Limitation - atmospheric conditions may make it impossible to hear a standard frequency tone!!
10b.2 Understand the effect of measurement tolerance, calibration accuracy and time related drift on frequency measurements and the allowances 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 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.
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:-
This is the closest to the band edge you would be able to transmit, so know your equipment??