Measuring Audio Frequencies with a Guitar Tuner

Here’s a little hack that serves no real purpose.  I’ll tell you about it anyway.

I recently built the T-Tone Code Practice Oscillator (CPO) kit from Morse Express.   It’s a handy little addition to the shack for adjusting straight keys or testing keyers.  After building it, I just adjusted the audio frequency for a pleasing tone.  Most people would have just left it alone at that point.  I’m not most people.

I started to do some thinking, which is a dangerous practice that can sometimes lead to unexpected consequences.  I wondered how the frequency of CPO compared to the sidetone of my FT-817.  There was no particular point to this mental exercise other than idle curiosity.

Now, I certainly could have keyed both the CPO and the FT-817 and done a comparison by ear.  I could have just adjusted the CPO by ear to match the FT-817.  But what fun would that be?  I was curious about the exact audio frequency of the FT-817’s sidetone, so I opted to do some experimenting.

Having been playing guitar for more than 50 years, I have acquired a gadget or two over the years.  One of those gadgets is a clip-on guitar tuner.  I can clip this clever device on the headstock of my guitar and, by sensing vibrations, it will tell me what note I’m playing and whether the pitch is sharp or flat.  I figured I could use this thing as an audio frequency meter of sorts.

First, I laid the guitar tuner on top of the FT-817’s speaker and keyed up.  That indicated that the pitch of the sidetone was an F note.  Consulting a conversion chart I found on the Internet, that equates to 699Hz.  I seemed to recall that the FT-817’s sidetone was somewhere around 700Hz, so that seemed about right.  I was sure I was in the right octave.

Guitar tuner on top of the FT-817

Guitar tuner on top of the FT-817

Next I took the lid off of the CPO and clipped the guitar tuner on it.  It initially indicated that the CPO was tuned to F#.  That equates to a frequency of 740Hz.  I tweaked the CPO’s frequency adjustment pot to F, matching the FT-817.  A side-by-side comparison of the CPO and the FT-817 showed that I was successful.

Guitar tuner clamped onto the lid of the code practice oscillator

Guitar tuner clamped onto the lid of the code practice oscillator

So, what’s the point of all this?  None really.  Is there a practical use for this?  Probably not.  Does it really matter that my CPO matches the sidetone of my radio?  Nope.  I just had one of those “I wonder what would happen if…” moments.  Now I know.

72, Craig WB3GCK

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