Transcript Advanced Licence Course

Chelmsford Amateur Radio Society
Advanced Course
(10) Measurements
Part-2 - Frequency
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
1
Frequency Measurement
• It is important to know the frequency that your receiver is tuned to,
and the frequency of a signal radiated by a transmitter.
• In particular it is a requirement that the signal is within the operational
amateur band.
• Do not assume that your equipment cannot operate outside the band.
• There are several means to either measure frequency or to calibrate a
receiver or VFO.
• The syllabus requires you to have an understanding of all of these
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
2
Frequency Standards
• Even when a crystal oscillator is used as a calibration source it is
necessary to set it against a frequency standard.
• For a long time now national administrations have provided a
service by transmitting ultra stable frequencies to allow accurate
calibration of instruments.
• The transmissions can be found on one of the following
frequencies:
– 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, 20MHz and 25 MHz.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
3
Absorption Wavemeter
• An absorption wavemeter consists of a calibrated tuned circuit, which
absorbs power when the circuits are tuned to the same frequency as
that being tested.
• The power collected can be made to move a meter or light a bulb.
• If the wavemeter is held close enough to a RF circuit the current
induced will be high enough to light a bulb or move the meter.
• If the wavemeter has been calibrated it is easy to tune in to the
frequency in question, peak for maximum and read it off.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
4
Crystal Calibrator
• The absorption wavemeter is invaluable for confirming circuits are
tuned to the correct band, but they are not accurate enough for
frequency measurements.
• Thus it is necessary to check against an accurate frequency standard.
• The simplest is a 100 kHz oscillator that produces markers across the
bands.
• Another type is a highly stable 1 MHz oscillator with a divider circuit
to produce 100 kHz and 10 kHz markers.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
5
Heterodyne Wavemeter
• Before frequency counters this wavemeter represented one solution
for measuring frequency.
• It provides crystal control frequency markers with a calibrated highly
stable VFO to interpolate between the markers.
• Its use can be either;
To receive the unknown frequency and mix it with the oscillator to
produce an audible output.
Or to feed the unknown signal and the heterodyne frequency into a
receiver, zero beat the pair, and then read the frequency from the
meter scale.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
6
Digital Frequency Counter
• These can be purchased or constructed easily, they count a number
of cycles over a given time period.
• The accuracy of a meter depends upon the internal clock signal which
is derived from a crystal.
• This accuracy improves if the crystal is housed within a temperature
controlled oven.
• From time to time this crystal should be checked against a standard
frequency.
• The resolution of a instrument is the smallest digit displayed for the
frequency range in operation and the clock frequency stability.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
7
Measurement Tips
• Measurement Tolerance.
Is the percentage of the measured frequency and will vary from band to
band. Allow for such errors when close to band edges.
• Calibration Accuracy.
How well can you interpret your dial? In mechanical systems how good
is the tuning mechanism? When setting the instrument how close can
the reference oscillator be adjusted to the frequency standard.
• Frequency Drift.
All oscillators drift with voltage and temperature. Reference oscillators
must be well made, and be temperature or even oven controlled.
• Measurement Bandwidths.
Care should be taken when measuring on wide bandwidth that you are
in the centre. Always measure on a narrow bandwidth.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 22: v1.0, 1-Oct-2004
(10) Measurements-2 - Frequency
8