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| To: | <[email protected]> |
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| Subject: | Re: LF: TX converter for 137 kHz, help needed |
| From: | "James Moritz" <[email protected]> |
| Date: | Tue, 19 Jul 2011 22:11:52 +0100 |
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| References: | <[email protected]> <CAA8k23TQMhV_Ew-WMA8i1P5H94ty5bLaksWqhxsjwDxZKsEbEQ@mail.gmail.com> <[email protected]> <D81113E5E794498285F29E6EEED8C2DC@JimPC> <[email protected]> |
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Dear Stefan, LF Group, That was just an incidental remark - it is possible to generate high quality quadrature waveforms in software, which avoids the need for hardware phase shift networks and their limitations.Why? The design Andy shows has a mono audio input. This is what i am searching for :-)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. 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. Well, it just depends what you are trying to optimise - do you want a wide frequency range with moderate unwanted sideband supression (this is what Andy's initial values on the spreadsheet seem to be aimed at), or a higher supression over a narrow frequency range - you might like to try C1 1n, R1 5470, C2 4.7n, R2 6790. These are feasible values - but to get the response shown on the spreadsheet, you might need a screwdriver with reduction gears for adjusting the preset resistors ;-) BTW, the LM358 is primarily a low-frequency device - it is getting a bit short of gain and slew rate at 12kHz, it might be OK but you might want op-amps with a bit more bandwidth - e.g. TLC272 should work fine in Andy's converter circuit Cheers, Jim Moritz73 de M0BMU |
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