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Old November 25th 17, 11:12 PM posted to,,
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Default [FOAR] How can you measure what frequency your radio is on?

Foundations of Amateur Radio

How can you measure what frequency your radio is on?

Posted: 25 Nov 2017 09:00 AM PST

Foundations of Amateur Radio The frequency you listen and transmit on in a
modern radio is derived from a crystal master oscillator, in my case 22.625
MHz. That master frequency is multiplied and divided to determine the final
frequency. To get to 2m you need to multiply by 6. To get to 70cm, multiply
by 20. Similarly, to get to 40m, divide by 3. Any slight variation of
crystal frequency has an impact. 100 Hz variation in the master oscillator
causes the radio to be off by 600 Hz in 2m, or 2000 Hz in 70cm. The higher
you go the bigger the error. This leaves us with two problems. If the
crystal changes frequency over time, your radio wanders with that change
which is especially noticeable on the higher frequencies. I've previously
discussed how you can deal with the variation by correcting for
temperature. The other problem is the actual absolute frequency. If the
radio is set-up for a crystal with one frequency and you replace the
crystal with a different one, how do you know what frequency you're
actually on? Your dial says one thing, but is that the actual frequency?
How do you measure any difference? Is a new radio the same as an old
radio, does the frequency change over time? Measurement is the act of
comparing two things. Think of a ruler, wooden stick with markings on it.
If the lines on the stick are not drawn in the right place, anything you
measure with that stick will not match other sticks. That won't matter if
you only ever use your stick to build everything, but typically you use
parts supplied by someone else with their own measuring stick. In your
radio the same is true. What the actual frequency is doesn't matter until
you need to compare it to the frequency of someone else. Like say, another
radio station. The first thing we need is something to compare with, a
reference frequency. As it happens there are several of those around. As an
example, you'll find reference broadcasts on 5 MHz, on 10 MHz, 15 MHz and
20 MHz. There are countless other frequencies where you'll find radio time
signal stations. These stations broadcast on a steady frequency with a
defined signal that you can use to do measurements against, even your local
broadcast stations have a carrier that you can get started with. A typical
radio time signal will be an AM station with all manner of information
encoded on the transmission. You can tune your radio to the station and
hear a talking clock, second marks etc. Unless your radio is seriously out
of whack you're unlikely to be able to notice any frequency errors. If you
tune to the same station with side-band you'll hear some artefacts, but
essentially you'll hear nothing. However, if you tune slightly off
frequency, you'll hear a tone. This tone is the central carrier frequency
and it's very accurate. At this point you can do many things. I'll cover
one of them. I'll explain this with 10 MHz. If you set your radio to
Upper Side Band and tune to 9.999 MHz on your radio, you should hear a 1
kHz tone. Similarly if you set your radio to Lower Side Band and tune to
10.001 MHz you'll also hear a 1 kHz tone. In essence you're listening to
the carrier as a 1 kHz audio tone. You can swap between the two
frequencies, by setting one on VFO-A and the other on VFO-B and switching
between them with the A/B switch on your radio. If the tone changes, your
radio is off frequency. How much off frequency is determined by the
difference between the two tones. By lowering both frequencies by the same
amount, or raising both by the same amount, one of the tones will go up
while the other one goes down and vice versa. Once you've got both the
tones the same, write down both frequencies. Split the difference and
you'll know what frequency your radio thinks 10 MHz is on. You'll need a
radio with both Upper and Lower Side-Band and the ability to switch between
two frequencies and before you get started, you need to make sure that your
radio doesn't have any frequency changing stuff turned on, RIT,...
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