RF output matching using baluns: LTC6946

Question

The datasheet that I refer to below can be found here: https://www.analog.com/media/en/technical-documentation/data-sheets/6946fb.pdf

I have a question about baluns. In the datasheet for LTC6946 integer-N synthesizer, the manufacturer recommends that the RF+/RF- output port be driven differentially. However, they also include the option to drive it single ended. They then include a table (on page 26) titled "single-ended RF output impedance." Why do they specifically say single-ended? If I wanted to transform the RF+/RF- differential ports to single ended using a balun, would I still need to do the impedance matching? If so, how would I do that?

As a side note, I would like to get RF frequencies of approx 400 MHz and 900 MHz out of the device, but it only mentions output impedance for 500 and 1000 ohms. If I was to design a matching circuit, would I just use the matching for 500 and 1000 and say "close enough, good enough"?

Thanks for any help in advance.

Answer 1

Why do they specifically say single-ended?

They are referring to each of the two RF outputs individually. In other words, at 500 MHz, each RF output has an impedance of 102.8 Ω - j49.7 Ω.

If I wanted to transform the RF+/RF- differential ports to single ended using a balun, would I still need to do the impedance matching? If so, how would I do that?

Matching the impedances is all part of designing a balun circuit. I can't tell you how to do that without knowledge of what impedance you require. However, the data sheet does give recommendations: -

These recommendations will be for a 50 Ω output.

As a side note, I would like to get RF frequencies of approx 400 MHz and 900 MHz out of the device

Looks like you should choose the first balun mentioned in the list. If you want to dig deeper you should get a data sheet for the balun specified and look at what its performance is.