Syllabus Sections:-

Measurements

9A6   56 Understand Recall the uses and limitations of 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".

CRYSTAL CALIBRATORS

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 TRANSMISSIONS

A suitable accurate frequency standard has to be found to check the accuracy of any of the above mentioned meters.

At present, standard frequency transmissions of guaranteed accuracy are emitted by the U.S. National Bureau of Standards continuously on the eight frequencies 2·5, 5, 10, 15, 20, 25, 30 and 35 MHz from a station with call sign  WWV.

The European standard frequencies were shut down to save costs but recently agreed that transmissions on  2·5, 5, 10, 15, 20 and 25 MHz should be available world wide.

The standard frequency can be used by tuning to the standard frequency and comparing it with the indicated frequency on your radio receiver where any error would show up as a difference from the standard frequency and your dial frequency.

Limitation - atmospheric conditions may make it impossible to hear a standard frequency transmission at all !!