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Hello Markus,
Ok .. i had assumed the problem was the flipflop (or binary
frequency divider) which would be in an unpredictable state when
not fed with a phase-coherent signal "in the gaps" (zeroes in the
Opera- or whatever amplitude keyed signal).
This could be solved by providing the auxiliary output from the
soundcard (at 2 * f_tx), to keep the divider chain running.
Depending on the MOSFET gate drivers, it may also be possible to
feed the 'keying signal' to an output-enable-signal, but I'm not
sure if that would put the driver output in high-impedance state
(which would result in a catastrophic 'bang' if both FET gates
were left floating).
Anyway your suggestion with the xor gate may be the simpler
variant than using two different output signals from the
soundcard.
73 and HNY,
Wolf .
(p.s. just managed to build a transformer for a low-voltage MF PA,
which transforms 0.6 Ohms drain load impedance to 12.5 Ohm with
very low stray inductance.. using copper foil instead of wire, and
thin adhesive tape ("Tesafilm") as insulator. But that's another
story..)
Am 29.12.2013 12:00, schrieb Markus Vester:
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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