Hi Mike,
On 6/2/2015 8:39 AM, Michael Black wrote:
> Isn't part of the reason for split to make for a cleaner signal? Seems to
> me that would still hold true no matter where it converts to analog,
> wouldn't it?
Using Split mode is advantageous with most radios because it obviates
the need for Tx audio frequencies that might fall outside the width of
the Tx passband -- say, tones lower than 500 Hz or higher than 2500 Hz.
Tones near the low end can be especially problematic, because any
nonlinearities in the analog audio chain will create in-band harmonics,
e.g., 400, 800, 1200, ... Hz. With split mode the Tx audio frequency is
always between 1500 and 2000 Hz, so any harmonics are automatically
filtered out.
If the signal is up-converted from audio (say 1500 Hz) to RF by digital
means, harmonic distortion should not be an issue.
> Somebody is doing an inverse FFT which I assume is the problem area.
I don't know what you may be thinking of here. There are no FFTs in the
sequence of generating the Tx audio signal.
> I never quite understood the FFT reason for putting the signal in the
> 1500-2000 offset thought I am fairly literate in FFT effects.
-- 73, Joe, K1JT
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