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Direct downconversion from 125MHz?

Hello, for my application I need to downconvert a 125MHz carrier to quadrature baseband signals (and I really would rather it be analog, please don't roll your eyes and tell me to throw down an FPGA), and I was hoping I'd find an active mixer IC which has an IF range down to baseband.  However all the parts I've seen either don't go all the way down to DC such as the AD8343 (or at least don't specify any operation there), or have a limited RF/LO frequency range where my frequency doesn't fit, such as the AD8347. I don't absolutely need a quadrature package, just having a suitable mixer core would be enough right now.

From reading the AD8343 datasheet, I'm wondering if I could terminate the open collector outputs with low value resistors (10-100ohms) instead of chokes to get DC outputs, but I have a feeling that if this worked well, then the datasheet would mention it.

So are there any options for me?

Thanks in advance,

-Mike

  • The AD8348 IQ Demodulator (www.analog.com/ad8348) should do the job nicely. Or you could torture yourself and try to build a discrete IQ Demodulator using two AD8342 mixers and a quadrature phase splitter from Minicircuits. The quadrature accuracy of the integrated IQ Demod will be much better.  The only potential downside of AD8348 is that the LO needs to be twice the RF.  

  • Hi enash, thanks for the reply,

    I've seen many AD products with the divide by 2 quadrature splitter, but unfortunately there's no way for me to have a doubled LO frequency (my LO ultimately will be a phase modulated carrier input, so I can't synthesize an equivalent version, or use a frequency doubler).

    But are you confirming that the AD8342 or AD8343 can work  at baseband for direct downconversion? Is this done using resistive output terminations like I suggested?

  • Hi, if you want to have your output work for low-frequency applications, you could simply increase the capacitance of the capacitor at the output. If you want to dc-couple the output, you need to be careful about the output bias, especially when it serves to drive another device, e.g an amplifier, in the next stage. For either case, you may need to do some experiment to validate your modifications to the original circuit.

    Peng

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