LF,
I going on with my monologise ;-) ...about my LF TX converter (maybe
someone silent is interested) that directly convertes 12.7 kHz
(generated by SpecLab) to 137.7 kHz in an IQ mixer. It is a modification
of G4JNT's design...
The converter is quasi finished and works well!
The last steps have been:
I measured the output impedance of the transformer, it is 100 Ohm. Then
i added a PI low pass filter that transforms to 10 kOhm. This filtered
signal looks almost like a perfect sine wave, just very small peaks are
modulating the signal. I checked the narrow band spectrum when applying
a single tone at 12 kHz and when applying a white noise, limited to
11...12.8 kHz.
Single tone:
http://dl.dropbox.com/u/19882028/LF/Sidebands%20with%20single%20tone%20at%2012%20kHz%20analog%20output.jpg
White noise:
http://dl.dropbox.com/u/19882028/LF/Sidebands%20with%20white%20noise%2C%20analog%20output.jpg
Harmonics on the analog output (12 kHz tone):
http://dl.dropbox.com/u/19882028/LF/Harmonics%20on%20the%20analog%20output.jpg
After filtering the analog signal is fed to a LM393 comparator running
at 12VDC. First i used a LT1028 but the comparator has a much higher
voltage gain. Rise time is about 50 ns while it took 800 ns on the
LT1028. Furthermore the obtained duty cycle is 50+-1 % when using the
comparator but it was totally apart fraom that when using the LT1028.
After the LM393 i added a ICL7667 to invert the signal and reduce the
output impedance of the mixer.
Actually it is possible to vary the soundcards audio level without
losses in performance of the digital output! I.e. i can vary the
soundcard level from 0 dB (full range) to -20 dB without any changes on
the output. If i reduce the level even more, strong jitter occurs on the
output. Maybe this could be even improved by further filtering the
analog signal. But a range of 20 dB is sufficient. It allows to QRO/QRP
say from 10W...1000W remote controlled :-) I hope i will get some kind
of linear mode PA by this "AM stage" (future project) that steers the
SMPS of the PA...
The digital output of the mixer has strong harmonics of course. There
are many peaks in the spectrum that made my worry if this is a suitable
signal! But the spectrum of my DDS VFO didn't look much better. So i got
the idea to directly measure the spectrum of the TX antennas signal. I
put the scope probe out of the window, about 3m distant to the TX
antenna while running about 100W by the DDS and the class E PA. The
spectrum looks impressing good to me so i lost all my worries about
making QRM. See the harmonics at
http://dl.dropbox.com/u/19882028/LF/LF%20TX%20antenna%20spectrum%20with%20120W%20class%20E%20mode%20PA%20driven%20by%20DDS%20VFO.jpg
and a narrow band view at
http://dl.dropbox.com/u/19882028/LF/narrow%20band%20spectrum%20of%20my%20TX%20signal%20with%20class%20E%20PA.jpg
These curves can be compared to the new H bridge PA that i will run soon.
Some smaller works have to be done until i can replace the DDS by the
new converter and then transmitting with SpecLab :-) Apart from QSY this
will finally allow me to use a higher DFCW shift than 1 Hz when
operating at DFCW-3...
I thought about trying CW to UK. Probably this can only be done when
leaving the city. A few km apart i could steer the TX by a 70cm FM link
and got the idea that i can even QSY when feeding the 70cm TX with a
tone in the passband (e.g. 1...3 kHz). The AF signal on the RX could be
put to the soundcard (a 2nd one) and is then converted by SpecLab to
11...13 kHz.... LF seems to be a never ending story regarding upcoming
ideas and projects :-)
73s Stefan/DK7FC
Am 21.07.2011 20:38, schrieb Stefan Schäfer:
Hello Jim, LF,
Am 19.07.2011 23:11, schrieb James Moritz:
Scale the values of either R or C proportional to the inverse of
the centre frequency (or both - it is 1/RC time constant you need
to scale). Yo might find faster op-amps neccessary at high audio
frequencies.
OK, i did that for a center frequency of 12 kHz but the result was
poor. Then is systematically played with the values and found two
pairs of 1n/5k8 and 15n/2k27 which showed at least 47 dB image
rejection in the LF band, peaking at -76 dB at 12.55 kHz, which will
be a theoretical value of course. I'm going to try these values in a
practical circuit and see what comes out.
Well, it just depends what you are trying to optimise - do you want a
wide frequency range with moderate unwanted sideband supression (this
is what Andy's initial values on the spreadsheet seem to be aimed
at), or a higher supression over a narrow frequency range - you might
like to try C1 1n, R1 5470, C2 4.7n, R2 6790. These are feasible
values - but to get the response shown on the spreadsheet, you might
need a screwdriver with reduction gears for adjusting the preset
resistors ;-)
I tried 1n / 5800 Ohm (5k1 and a 1k Pot) and 14.7n / 2300 Ohm (1k8 and
a 1k Pot) and the 74HCT4052. After struggling a bit due a hardware
mistake i found that the output waveform looked like nonsene. But this
was due to massive overloading the input of the OPA which runs at 5
VDC (i tool the LM358 which seem to work reasonable). When setting
SpecLab to -5dB of the full audio level, everything seems to work well
for the first attempt.
This is the board:
http://dl.dropbox.com/u/19882028/LF/TX%20converter%20board.JPG
I recorded the output voltage of the 2 busses. It looks not really
like a sine wave ;-). But i think this is normal since the RF has a
much lower frequency than the LO and a 90 deg shift. So the curve will
look a bit like a staircase. Or am i wrong? This output of the busses:
http://dl.dropbox.com/u/19882028/LF/Pin3and13of74HC4052.BMP
When feeding this differential signal to a transformer ( i took a
FT50-77 and 12 turns primary, 10 secondary, so far) and adding a 100
Ohm Pot, the output spectrum looks OK for me, for the first attempt,
see:
http://dl.dropbox.com/u/19882028/LF/Output%20spectrum%20without%20LPF%20and%20without%20optimising.BMP
This is the spectrum without optimising the two Pots. It shows above
30 dB image rejection and the LO (=125 kHz) is not visible at all :-)
Next week i will add a white noise to the RF input and will optimise
in the region of 11...12.8 kHz. Then i will a simple and uncritical
LPF and will take a look at the spectrum. Then another amp stage will
follow. Then i'll come on air on 137.7000000 kHz (both, the LO and SL
is GPS "locked")...
Any comments please? Do the waveforms look OK for you or is there a
big mistake?
Vy 73, Stefan/DK7FC
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