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LTC5569 IF output options

Question asked by epos2 on Jul 9, 2018
Latest reply on Jul 9, 2018 by epos2

The LTC5569 has a differential IF output while I need a 50 Ohm unbalanced line. I can work with IF frequencies in the 400-800 MHz range and don't really have a problem with anything in there. My IF bandwidth is 10 MHz and I was considering using a SAW filter at 782 MHz, but this is not a hard requirement.

 

I would like to understand what my options are with respect to the output of the mixer.

 

Options I see:

 

1) Use the TC8-1X transformer from the reference design - this is easy but bulky and expensive

2) Follow the suggestions on page 15 of the datasheet to change the required transformer from 8:1 to 4:1, then use a 200:50 Ohm balun in a chip like the Murata DXP18BN5014TL, replacing the discrete RF transformer with it.
3) Use a discrete SMD RLC network.

 

Questions!

 

Is 2) even possible? I have never done it before. It does look very appealing. Is there any reason why the DXP18BN5014TL shouldn't work as a replacement for the bigger RF transformer? It is lacking a DC feed, but maybe DC blocking the inputs of the transformer might work?

 Alternatively the 1720BL15B0200 seems to have a DC-feed port! I didn't find much to go on about this topic online unfortunately...

 

About 3), this seems like a pain mostly due to tolerances and IF bandwidth. The IF bandwidth might be fine as it should double and be around 20-30 MHz based on my pixel counting on the datasheet, but I believe that with 2% tolerances I would have very bad yield (0.02 * 800 MHz = +- 16 MHz, that seems to way too big to be usable with such narrow IF bandwidths).

 

Still about 3, I couldn't get the formula in the datasheet to give results matching the table. From page 16, L1 = (1/(2 * Cif * Wif * Wif)). Cif should be given on page 14, that is 1.3e-12 for 380 MHz, Wif is 380e6. Plugging those numbers in gives (1/(2 * 1.3e-12 * 380e6 * 380e6)) or 2.66 uH, a very long way off from the suggested 68 nH.

 

Thank you and have a nice day.

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