ARLA/CLUSTER: Foundations of Amateur Radio: Como você pode medir a frequência em que o seu transceptor está a Emitir/Receber?

[João Costa > CT1FBF]

Foundations of Amateur Radio

How can you measure what frequency your radio is on?

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, Clarifier, Offset, whatever it's called
on your radio. All of them need to be off.

There are countless other ways of doing this, a procedure called
zero-beating with a signal strength meter, using a tone and listening
for a wobble in the sound, using an external second receiver and zero
beating against that, using a computer to generate tones, using the
FMT software included with the WSPR software and likely many more.

The point of all of these processes is to detect a difference where
there shouldn't be one.

One final comment.

The most accurate process at this time without specialist measuring
equipment is by using your WSPR enabled computer and the FMT software
that's included. I'll look at that next time if I can understand what
Joe K1JT wrote on the subject.

Happy measuring!

I'm Onno VK6FLAB

Listen to the podcast: http://podcasts.itmaze.com.au/foundations/