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How to test a differential input LNA (200 Ohm)? (Read 3579 times)

plasma

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How to test a differential input LNA (200 Ohm)?
Jun 20^th, 2007, 12:24am

hello, all

I have a differential LNA fabricated, which works with a offchip matching network.
During the simulation phase, I simulated it with a 200-Ohm PORT, and found out the proper matching network.

So what topology should I use for test? How could I test it to find out if it is suitable for the circuit fabricated and packaged?

Thanks for your help~!

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ACWWong

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Re: How to test a differential input LNA (200 Ohm)
Reply #1 - Jun 20^th, 2007, 3:33am

If you would like to test your LNA using the matching components you used in the simulation, then you'll be wanting to drive from a 200 ohm differential souce. This can be achieved using a balun which will transform 50 ohm source to 200 ohm differential using a 1:4 ratio. The balun can be sourced as a single component from usual suppliers (murata, toko etc. do a google search) or can be made from an LC bridge circuit if you only need the transform over a narrow frequency band.

Otherwise, matchup your circuit to 50 ohms directly and do your test, and adjust your simulation to compare the results...

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mg777

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Re: How to test a differential input LNA (200 Ohm)
Reply #2 - Jun 21^st, 2007, 7:47pm

This isn't an easy problem. You need a good balun model including solder pads, and you also need a good DUT model including the packaging.

Start by making life simple - choose a balun that comes with a good S2P model (including pads), and try a first-order package parasitic model with your LNA. If this result is close to what you're expecting, then you can try a variance analysis. If you're sensitive, then you can go back & focus on accurate models - maybe use a VNA even.

What's the center freq and Q like? What's the target CMRR of the balun? That'll give some idea of the complexity.

M.G.Rajan

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