Open a relay with 1750 Hz call-tone
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Difficulties
A dear friends had problems: He could not open a somewhat far away 70 cm FM relay. Once it was open, he could communicate via that relay all right. But to coax it into operation once it was dormant: Didn’t work.
That friend is spoiled by the closer relays he usually works. Those spring into operation if only a carrier is detected on the input frequency. This relay is different: It expects to be opened via an input signal containing an audio tone with a frequency 1750 Hz. These 1750 Hz audio activation tone was once ubiquitous, at least in Germany. My friend had forgotten about it.
He’s using a rather cheap FM mobile rig. (It has previously accumulated some amount of his displeasure.) According to the manual, a certain combination of keys is supposed to trigger the 1750 Hz tone: But pushing those buttons, nothing happened.
After further investigation of the manual we managed to control the frequency (1750 Hz was configured all right) and to configure the triggering of that tone to another button. Now, the rig did send a tone, loud and clear in the audio generated alongside the transmission, and I could also verify the tone was indeed transmitted, using my handheld I had brought along.
Only the relay still wouldn’t open.
The problem
I then recorded the tone received by my handheld via my computer’s
microphone and analyzed it with my trusted FLOSS sound manipulation
software audacity: The tone
deviated considerably from 1750 Hz. If I remember correctly, it was
about 100 Hz too low, maybe more.
The workaround
We reversed: I used audacity to produce a sound file containing
just an audio sine wave of 1750 Hz frequency. Lo and behold: Play
that via a computer or smartphone, bring the rig’s microphone close,
press PTT and - relay is open!
This one is for you!
Here comes that audio file, just one-and-a-half minutes of 1750 Hz sine wave: 1750.ogg.
If you’ve got a somewhat musical ear, you can also use this to easily check whether the tone sent by your rig has the right pitch.
If you want to discuss this and have a Fediverse account, leave your comments below https://mastodon.radio/@dj3ei/114700532869904791.