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Hi Wolf,
I think Stefan was only trying to solve the simpler
problem of generating a dual squarewave drive for his VLF PA, and was not aiming
for amplitude modulated output yet. So when he transmits Opera we'll just have
to live with ultra-strong keyclicks, devastating the 9 kHz band all over Europe
;-))
But I remember that in 2011 we discussed about
transmitting high-quality Hell piccies. As you say, the Kahn technique
with a separate AM modulator is certainly a good and efficient option. At
VLF, outphasing (aka Chireix or LINC technique) between two half-bridges
would be a valid alternative. There's some software for the purpose (including
Basic source code ;-) on
This works in conjunction with your SndOutpt
audio utility, generating the two outphasing carriers at the left and right
stereo output.
Best 73,
Markus (DF6NM)
Sent: Sunday, December 29, 2013 9:12 AM
Subject: Re: LF: VLF exciter
My suggestion:
Use two outputs from the stereo
soundcard:
An "amplitude" and a "phase" channel. The output from the phase
channel will be a square wave, coherent, constant amplitude, fed to the divider
chain. The "amplitude" channel delivers, well, the amplitude (as a waveform with
variable voltage) which, after rectification, drives the final voltage via an
additional mosfet. I think the software option is already there, and iirc it has
been used (by Jim ?) for an LF PA.
This way, you could even use amplitude
shaping if the bandwidth (from the hard-keyed PA) was a problem - which I don't
think it is, on VLF.
Cheers,
Wolf
(p.s. problem
with the "very long command lines" already fixed, new SL version will be
uploaded soon).
Am 29.12.2013 02:43, schrieb Stefan Schäfer:
Am
28.12.2013 23:33, schrieb Markus Vester:
[...]
PS Any progress on bypassing the divider in
your VLF transmit chain?
Aaah, no, i forgot that. So i cannot transmit OP32 on VLF
because it would be incoherent then. So i have to key the PA via a VOX and
OP32 tones, still :-/
In this H bridge PA config, an exact 50% duty
cycle isn't needed, so i may actually remove the divider. But then i need a
solid and defined input signal wave form! How do i get an inverted signal
then? A transformer isn't possible since i like to have a wide frequency range
of the PA. So far it works from 10 Hz to > 100 kHz. With the frequency
divider, the wave form can be quite distorted, no problem unless the peak
reaches the necessary voltage level. So maybe it is better to build the VOX
circuit instead? Also i want to leave the PA circuit as it is, for nostalgic
reasons :-)
Or do you have another idea? I remember that circuit with the
XOR gates...
73,
Stefan/DK7FC
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