a confusion in LT5560 0.01MHz to 4GHz Mixer datasheet

[yyan71]

Hi there, I'm confused in understanding about how to determine the source impedance of the input port of LT5560, which is on page 16 of its datasheet.

It says the input transformer has a 1:1 impedance ratio, so Rs = 50ohms. However, according to Figure 5, the RL = 28.1ohms. If impedance ratio is 1:1, I think Rs is 50ohms. Confused....

The screen cut of the relative page has been attached. Key words have been highlighted.

The whole datasheet can be found here:
https://www.farnell.com/datasheets/72197.pdf

Please help me... [/img]

 

[Mazz]

The answer is not so difficult: the 28 load impedance is matched to 50 Ohm source impedance through the LC matching network.
L is splitted into an internal part (you cannot act on it) and an external tunable part, depending on frequency usage.
Once you have transformed internal impedance at a certan freq to 50 Ohm differential, then you use an 1:1 transformer to convert it into single ended 50 Ohm.

I hope it can help.

Mazz