So there is something to the theory about sawtooth oscillators being
better for sidetones. Now here's another question. If we use a sawthooth
osc. to produce the sidetone and a low pass filter to smooth it out,
which system would result in the least delay?
1. Filtered sawtooth
2. Twin T
I'm assuming I would use an op amp for the LP filter.
-----Original Message-----
From: "Dr. Gerald N. Johnson, electrical engineer" <geraldj@isunet.net>
To: Steve Ellington <n4lq@iglou.com>
To: <tentec@contesting.com>
Date: Sun, 14 Apr 2002 21:37:06 -0500
Subject: Re: [TenTec] Sidetone revisited
> The sawtooth oscillator should be at full amplitude the first half
> cycle
> of the tone. And the start up chirp done in that time period. It won't
> hang over either when unkeyed.
>
> The twin-T undoubtedly takes up to a couple dozen cycles to start and
> changes frequency during that.
>
> Any filter causes some delay, the more samples the more delay in a DSP
> filter, but the better the relative time response. E.g. the less
> difference of time delay according to frequency.
>
> I sometimes call CW with my DSP-599+ in the speaker line. I've not
> looked for delay, but something seems not quit right though the tone
> quality is super, even on SSB mode. Maybe next time I rattle off a call
> at 25 wpm, I'll see if there's some time delay to bug me. I know
> there's
> enough time delay that I'd rather tune the band with the DSP bypassed.
>
> A more optimum filter would be a passive lowpass filter in the speaker
> line. It could have a cutoff of 2.5 KHz or so, and would clean up the
> side tone with less delay than a DSP and would contribute NO noise to
> the signal while cleaning up hiss from the radio. I have created a
> couple that have switched cutoffs and published them locally. They
> work.
> When on CW using a tone of about 400 Hz and cranking the LP filter down
> to about 450 Hz cutoff, makes signals off the air sound like a super
> high quality local code practice oscillator. No harmonics, and very
> little noise. I should maybe post that design to my web page someday
> along with my power supply design and other good ideas.
>
> 73, Jerry, K0CQ
> --
> Entire content copyright Dr. Gerald N. Johnson. Reproduction by
> permission only.
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