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Date: Tue, 19 Jul 2011 12:21:15 +0200
From: =?ISO-8859-1?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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Tnx Roelof and Jim for help and comments!

Jim, some questions:

Am 18.07.2011 19:17, schrieb James Moritz:
> Dear Stefan, LF Group,
>
>> So what is missed in my circuit is the allpass network that makes a 90
>> deg delay of the input signal.
>
> ...Unless the software generates I/Q outputs from the sound card L/R 
> stereo outputs.
Why? The design Andy shows has a mono audio input. This is what i am 
searching for :-)

> 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.
A spectrum analyzer is available at work so i can report about the 
results, hopefully this week.

> The propagation delay adds an error to the phase shift, causing 
> imperfect cancellation of the unwanted sideband, so it helps if the 
> propagation delay is small
>
> 4000 series CMOS is a bit slow really - I have used a 74HC4052 in an 
> I/Q receiver, which works quite well at 136k.
OK, understood. So i will use a 74HC4052, a 74HC74 and LM358 which all 
are available at a local store here, just 1km away (VLF near field ;-) )...


Tnx agn es 73, Stefan/DK7FC