LF,
Maybe someone can help me with an open question about IQ Mixers:
I want to build a mixer that generates a rectangular output at 137.7 kHz
(to steer my LF PA) by using SL's digimode terminal at 12.7 kHz and
adding 125 kHz that i get from a down divided 8 MHz xtal (divided by a
74HC4060 and locked into a PLL of my GPS 10 kHz Ref.).
There are several ways to do that of course. It should be a cheep
solution without special parts so i choosed a CD4052 analog switch /
multiplexer and built a mixer that switches the positive and negative
wave at the LO frequency to the output. I am using 2 AF transformers
(1:4 types, 1 on the soundcard side) at the input to achieve a galvanic
decoupling and a +/- voltage.
When looking to the output spectrum i can see a peak at 137.7 kHz at 45
dB above noise. The LO peak is about 20 dB below that. But the image
frequency at 112.3 kHz is just 10 dB below the 137.7 peak. So i need
filters of course. But the ditance between LO+RF and LO-RF is just 25.4
kHz and somehow it is difficult for me to achieve a suitable Q and i
have had problems with temp drifting Cs and do not like to make to much
effort on the filtering.
I do not know much about IQ mixers but i had in mind that the reject the
image frequency by the design and do not need much filtering at the
output. So i read a bit about the mixers in wiki and at some webpages. I
learned that i need 90 deg shifted drive signals for such a mixer and
found a suitable circuit. Since i down divide 8 MHz in a 4060 i am
easily getting a 500 kHz output and so i built that circuit that gives
me the shifted drive signals by using a CD4013 (2x D flipflop). Now i
already have the 125 kHz IQ drive signals and found a circuit how to
arrange a suitable mixer at http://www.elexs.de/iq1.htm and/or
http://www.yu1lm.qrpradio.com/DR2B%20HF%20SDR%20RECEIVER-YU1LM.pdf
But the question is where do i get my 137.7 kHz signal now? There are 4
"cables" that come out of this mixer type, 2 pairs that are fed to a
summer. Then i have the I and Q output that is fed to the soundcard in
these examples. Somehow it begins to dawn on me that i will not get the
wanted signal on the mixers output, right?`The signals can be calculated
in a suitable PC software, which is no solution for me here of course. I
would go on experimenting and will find the solution on the topic of
course but anyway i want to ask here before since i know that there is
someone who knows the answer.
Is there no chance to easily reject the image frequency or do i have to
spend the filtering effort? If there is a complete hardware solution (no
programmable devices, no software), please tell me :-)
The TX mixer should then allow me to remote steer my LF TX and even make
QSY, i.e. decide to call CQ at 137.71 kHz and if no one replies, make
QSY to beacon mode at 136.17x kHz for example. At the output i want to
add a OP device or comparator that forms a rectangular wave to drive the
PA. Later i want to add a AM circuit that filteres the 12.7 kHz and
provides a variable DC voltage that will steer the PA's SMPS step down
converter to allow QRO/QRP by varying the volume at the PC...
Tnx in advance.
73, Stefan/DK7FC
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