[Elecraft] Need comparison values on sidetone mod

[Bob - AG5Q]

Hi Lenny,

I got the following before and after data using Spectrogram and a 600Hz
sidetone:

(a) Before:  2nd  -24db,   3rd  -21db, 4th -33db
(b) After  :  2nd  -41db,   3rd -32db,  4th >-50db

The even harmonics are reduced a lot because the symmetry of the waveform is
closer to an ideal square wave. In the original circuit, the impedance at Q5
is different for rising edges and falling edges so the signal into the low
pass filter is not quite square (in an ideal square wave the even harmonics
would be zero).

My modification was to put a 150K resistor in series with C24 and put a
0.022uf cap across RP7 pins 7 and 8.  I didn't use an inductor.  This also
reduced the amplitude of the sidetone which originally was too loud for me.
Before, I had to set the ST Level to about 4 or 5. After the mod, I set ST
Level to 20.

73/ Bob - AG5Q



----- Original Message -----
From: "Lenny Wintfeld" <w2bvh@comcast.net>
To: <elecraft@mailman.qth.net>
Sent: Sunday, January 12, 2003 4:34 PM
Subject: [Elecraft] Need comparison values on sidetone mod


> Hi
>
> I tried the sidetone mod over a year ago using the 47k resistor and a
> 20mh choke I had laying around. The result was disappointing. I didn't
> make any measurements back then, but with a "listening test", I came to
> the conclusion that the mod was ineffective and backed it out.
>
> I just did the mod again. This time with a 47k resistor and a 68 mh
> choke. I was a little more scientific this time. I used spectrogram for
> before and after measurements. The results are as follows
>
> a) Before the mod: 2nd harmonic -23 db, 3rd harmonic -21 db both wrt
> 680h Hz tone
> b) After the mod:   2nd harmonic -32 db, 3rd harmonic -27 db both wrt
> 680h Hz tone
> c) The all important "listening test": sounds a LOT cleaner.
>
>
> Has anyone else done spectrogram before and after measurements? I'd be
> curious to know if my numbers are in the same ballpark as others'.
>
> 73,
> Lenny W2BVH
> K2  sn 1520
>
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