CN0566 Phased Array Tx Chain Design Choises

Question

Hello ADI, 
 
 I have a couple of questions about the CN0566 Phased Array Tx Chain. 
 
 Why is the mixer LTC5548 used instead of LTC5549? Is there a benefit to use LTC5548 that LTC5549 can't provide? 
 Why is the LO frequency set between 12.2 - 12.7 GHz instead of around 8 GHz for upmixing? What advantages does a LO frequency of 12 GHz have over a LO frequency of 8 GHz?

Best regards

Jon.Kraft · Accepted Answer

Yes, those are good questions: 
 1. We could have selected the LTC5549, instead of the 5548, and that would have saved us a balun. The difference between those 2 mixers is the low IF freq that each mixer can do. But with the external balun we choose for the LTC5548, we could go down to as low at 5MHz. This puts the Rx1 and Rx2 ports in range of many oscilloscopes (or ADALM-2000). At the time, we just wanted to open up the range of data converters to more than Pluto. However, in retrospect, pairing with Pluto or AD9081/2 works so nicely that I don't think we need the lower freq Rx1/2 operation. So if I had to do it over, I probably would have just used the LTC5549. 
 2. Placing the LO 2 GHz above the RF is called "high side LO", and it produces fewer spurs than "low side LO" (2 GHz below RF). And the spurs on high side LO are also easier to filter -- because I can use a LPF instead of a bandpass filter. There's more info here: 
 www.analog.com/.../cn0566.pdf)

Jon.Kraft · Answer

Oh, ok. It's the same reason--to enable lower frequency IFs.

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CN0566 Phased Array Tx Chain Design Choises