A reason to do this is for efficency.
Since the ft8 waveform can can suffer non-linearity, a class C amp could be
driven until the phase distortion is approaching an unacceptable effect on
symbol recovery.
This would be highly efficent but would require filtering before radiating for
example.
There is a type of transmitter that is actually a RF DAC where the individual
devices that are the being switched between saturation and cutoff (to acheive
high eff.) are producing binary coded waveform segments. Think power AtoD. This
actually a very linear "amplifier" and can be made more so by weighting the
bits with a conical sine.
A waveform like ft8 could be processed using the info from a characterization
of such a RF DAC's transfer data to acheive high eff. With minimal filtering.
DE N2LO~>
Sent from Xfinity Connect Application
-----Original Message-----
From: j...@princeton.edu
To: wsjt-devel@lists.sourceforge.net
Sent: 2019-02-18 8:17:32 PM
Subject: Re: [wsjt-devel] DDS Generator
On 2/18/2019 17:53, N0UU wrote:
> A long time looker at this subject: Why do we need to take the AF
> signal and run it thru whatever steps to make an RF signal?
>
> That just seems to require way too much processing. Can anyone explain
> why the various signals can't be simply generated, especially at the
> lower bands, and put on the air directly?
>
> WSPRLite seems to do that.
>
> N0UU
Of course we don't "need" to generate FT8 (or MSK144, JT65, JT9, etc.)
signals at audio frequency and subsequently up-convert them to RF. One
can think of many possible approaches. You should feel free to design
and implement the scheme of your choice.
WSPRLite is a Tx-only rig. It generates a constant message, which is
fine for WSPRing but not useful for making QSOs.
Many factors lie behind the design decisions involved with WSJT-X and
its sister programs WSJT, MAP65, and WSPR. An important one is that
nearly every ham who wants to use the digital modes they offer already
has an SSB transceiver and a computer in the shack.
Special-purpose transmitters for modes implemented in WSJT-X are
certainly possible. It will not be trivial to do this with minimal
hardware, especially if you want a Tx signal as clean as those generated
as WSJT-X does it. And of course, you'll still need a computer for the
much more significant computational task of receiving, demodulating, and
decoding the signals, and for orchestrating the flow of standardized
minimal QSOs.
-- Joe, K1JT
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