Re: LF: TX converter for 137 kHz, help needed

["James Moritz" <james.moritz@btopenworld.com>]

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.

> Which values
> do you recommend for R and C to come in the range where i can find the
> optimum by adjusting the potentiometer? Not so easy to find the best
> values by playing with R and C.
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.
> And why do i need 2 allpass filters? A
> 90 deg shift could be done by using just one, or not?
Yes, but only at a single frequency - you cannot make a network that 
produces both a fixed 90degrees phase shift and a fixed gain over a range of 
frequencies. But by using multiple phase shift networks, one can obtain a 
differential phase between the two channels quite close to 90degrees over a 
much wider range.
> BTW, is there an advantage using 74HC... devices instead usual CD40...
> CMOS devices regarding better image and LO rejection? Switching times at
> 137 khz should be no problem for the standard parts, isn't it?
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
> I cannot
> find the 74FST3253 at my usual distributors. Is there a problem using
> the CD4052 instead?
4000 series CMOS is a bit slow really - I have used a 74HC4052 in an I/Q 
receiver, which works quite well at 136k.
Cheers, Jim Moritz
73 de M0BMU