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I am working on a mixer which will be used for 4QAM, QPSK. The local oscillator is 9.6 kHz and the IF would be + or - 1 volt DC in order to have the RF port output either cosine or -cosine. Which type of mixer architecture would satisfy this? Any design ideas are welcome.

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  • \$\begingroup\$ Who downvoted my question and why? The specs are included, and the question is useful. \$\endgroup\$ Mar 13 at 14:39

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Use two opamps to generate the positive and negative versions of the carrier. Alternatively a center-tapped transformer will do. Then switch between the two phases via an analog switch controlled by the baseband. There are dedicated switch ICs available, but it may be as simple as two complementary transistors (BJT or MOSFET).

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  • \$\begingroup\$ Thank you for your input! I was thinking of the MAX319 IC (bipolar analog switch), and ordered that a few days ago (as a backup design if the mixer option won't work). I was hoping to use a mixer design though if possible. I was experimenting with diode ring mixers, which satifty the DC IF requirement, but the low speed 9.6 kHz LO freq was giving me problems. Specifically, the toriod core mix, I could not find a mix that would perform below 100 kHz. I am thinking of trying audio isolation transformers now instead of toroids in the diode ring mixer. \$\endgroup\$ Mar 13 at 14:46
  • \$\begingroup\$ Well, the diodes in a mixer really are acting as switches. If the diode performance is sufficient all the better. About the switch IC you suggested: It is SPST only. You'd need two of them. Also, the input signal is logic level, ie not compatible with a +/-1 V signal. \$\endgroup\$
    – polwel
    Mar 14 at 9:11
  • \$\begingroup\$ Yes, SPDT. For the 2 DT ports, that is the +/- cosine, the 1 v signal will be amplified to logic level to control the SP port. I am liking the the analog switch idea right now, since it does not have the impedance matching and power requirements of a mixer. I guess I'll save the diode mixer for another project. Thank you for your collaboration. \$\endgroup\$ Mar 14 at 13:59
  • \$\begingroup\$ Ah yes, the '319 is SPDT. I took 'NC' to mean 'no connection' rather than 'normal closed' when I saw the diagram. *facepalms \$\endgroup\$
    – polwel
    Mar 15 at 16:30
  • \$\begingroup\$ If you amplify your IF signal anyway, do consider using 2 discrete MOSFETs as switches. Might not work for you, but much more cost effective than these Maxim ICs. \$\endgroup\$
    – polwel
    Mar 15 at 16:33

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