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Date: Mon, 18 Jul 2011 14:17:55 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: TX converter for 137 kHz, help needed
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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=20
4 analog switches (e.g. CD4066).
-The 4 signals can be seen as 2 pairs with a 180 deg shifted phase (Q1=20
and /Q1 as the first pair. Q2 and /Q2 as the second Pair(Q is here the=20
output of a 4013 flipflop to steer the analog switches).
-Each pair switches the RF signal either positive or negative to the=20
output (just like i did before with the CD4052 device but here just for=20
a 90 deg period, not 180 deg).
-When feeding the two outputs of each pair to a summer one achives the I=20
and Q signal. Each signal, I and Q contains the LO+RF and the LO-RF=20
signal. But the I spectrum is 90 deg shifted and the Q spectrum is -90=20
deg shifted (?)
-If i substract I-Q in a OP circuit, do i then get (LO+RF)-(LO-RF)=3D2*RF=
?=20
:-)

This would be the solution. But i am not sure if i understand that=20
correctly. Can someone confirm or correct me?

So if i would be right, i would need the 4013 (drive signal generation)=20
and a 4066 (mixer) and e.g. a TL084 (4* OP amp for summers, substracter=20
and output gain) to achieve my wanted 137.7 kHz signal? :-)

73, Stefan/DK7FC


Am 18.07.2011 12:53, schrieb Stefan Sch=E4fer:
> 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=20
> kHz (to steer my LF PA) by using SL's digimode terminal at 12.7 kHz=20
> and adding 125 kHz that i get from a down divided 8 MHz xtal (divided=20
> 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=20
> solution without special parts so i choosed a CD4052 analog switch /=20
> multiplexer and built a mixer that switches the positive and negative=20
> wave at the LO frequency to the output. I am using 2 AF transformers=20
> (1:4 types, 1 on the soundcard side) at the input to achieve a=20
> galvanic decoupling and a +/- voltage.
> When looking to the output spectrum i can see a peak at 137.7 kHz at=20
> 45 dB above noise. The LO peak is about 20 dB below that. But the=20
> image frequency at 112.3 kHz is just 10 dB below the 137.7 peak. So i=20
> need filters of course. But the ditance between LO+RF and LO-RF is=20
> just 25.4 kHz and somehow it is difficult for me to achieve a suitable=20
> Q and i have had problems with temp drifting Cs and do not like to=20
> make to much effort on the filtering.
> I do not know much about IQ mixers but i had in mind that the reject=20
> the image frequency by the design and do not need much filtering at=20
> the output. So i read a bit about the mixers in wiki and at some=20
> webpages. I learned that i need 90 deg shifted drive signals for such=20
> a mixer and found a suitable circuit. Since i down divide 8 MHz in a=20
> 4060 i am easily getting a 500 kHz output and so i built that circuit=20
> that gives me the shifted drive signals by using a CD4013 (2x D=20
> flipflop). Now i already have the 125 kHz IQ drive signals and found a=20
> circuit how to arrange a suitable mixer at http://www.elexs.de/iq1.htm=20
> and/or http://www.yu1lm.qrpradio.com/DR2B%20HF%20SDR%20RECEIVER-YU1LM.p=
df
>
> But the question is where do i get my 137.7 kHz signal now? There are=20
> 4 "cables" that come out of this mixer type, 2 pairs that are fed to a=20
> summer. Then i have the I and Q output that is fed to the soundcard in=20
> these examples. Somehow it begins to dawn on me that i will not get=20
> the wanted signal on the mixers output, right?`The signals can be=20
> calculated in a suitable PC software, which is no solution for me here=20
> of course. I would go on experimenting and will find the solution on=20
> the topic of course but anyway i want to ask here before since i know=20
> that there is someone who knows the answer.
>
> Is there no chance to easily reject the image frequency or do i have=20
> to spend the filtering effort? If there is a complete hardware=20
> solution (no programmable devices, no software), please tell me :-)
>
> The TX mixer should then allow me to remote steer my LF TX and even=20
> make QSY, i.e. decide to call CQ at 137.71 kHz and if no one replies,=20
> make QSY to beacon mode at 136.17x kHz for example. At the output i=20
> want to add a OP device or comparator that forms a rectangular wave to=20
> drive the PA. Later i want to add a AM circuit that filteres the 12.7=20
> kHz and provides a variable DC voltage that will steer the PA's SMPS=20
> step down converter to allow QRO/QRP by varying the volume at the PC...
>
> Tnx in advance.
>
> 73, Stefan/DK7FC