I've read some more articles and now my understanding is that:
-There are 4 drive signals, each 90 deg shifted to each other, switching
4 analog switches (e.g. CD4066).
-The 4 signals can be seen as 2 pairs with a 180 deg shifted phase (Q1
and /Q1 as the first pair. Q2 and /Q2 as the second Pair(Q is here the
output of a 4013 flipflop to steer the analog switches).
-Each pair switches the RF signal either positive or negative to the
output (just like i did before with the CD4052 device but here just for
a 90 deg period, not 180 deg).
-When feeding the two outputs of each pair to a summer one achives the I
and Q signal. Each signal, I and Q contains the LO+RF and the LO-RF
signal. But the I spectrum is 90 deg shifted and the Q spectrum is -90
deg shifted (?)
-If i substract I-Q in a OP circuit, do i then get (LO+RF)-(LO-RF)=2*RF?
:-)
This would be the solution. But i am not sure if i understand that
correctly. Can someone confirm or correct me?
So if i would be right, i would need the 4013 (drive signal generation)
and a 4066 (mixer) and e.g. a TL084 (4* OP amp for summers, substracter
and output gain) to achieve my wanted 137.7 kHz signal? :-)
73, Stefan/DK7FC
Am 18.07.2011 12:53, schrieb Stefan Schäfer:
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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