differential matching impedance

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pareman

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differential matching

dear all, I am design matching impedance for differential input mixer. I've had the recommend design, but I have problem how to calculate and prove the value of each component. the differential input is 10+j5.2 for 456MHz frequency and want to match with 50 ohm impedance...

you can see the file I've attached...

I hope you can help me and give me some theory to calculate all components value, thank you for your advice...
 

differential matching

Smith chart is an appropiate tool to design or verify the matching network. Strictly spoken, the TL (differential) impedance and electrical length rather than physical length has to be considered. The 70 ohm impedance given in the data sheet is single ended, I think.
 

Re: differential matching


thank you for your advice, but I can't use smith chart to calculate the matching impedance because usually smith chart use for single-ended signal, but actually the case is differential input... 70 ohm is microstrip impedance for differential input....
 

but I can't use smith chart to calculate the matching impedance
Click to expand...
The smith chart itself doesn't care if the impedances are single-ended or differential. But obviously you have to transform all single ended component values (C1, L1, R1 etc.) to equivalent differential ones.
70 ohm is microstrip impedance for differential input
Click to expand...
May be. You will know from the actual geometry. It isn't mentioned explicitely in the datasheet, usually I would expect the term "differential microstrip" for clarity. Also I don't know the substrate height of the EvalKit layout to estimate the impedance.

Generally, there are more parasitic component parameters, that have to be considered for the exact matching calculation. But the final design can be easily tuned by connectecting a VNA to the 50 ohm input port.
 

 

Perhaps it's more intuitive for you to transform differential to single ended by cutting it into two identical halves. Then the transformer output changes from 12.5 ohm to 6.25 ohm, C3 and C9 are doubled, the differential 70 ohm TL is replaced by singled ended 35 ohm lines and the chip input impedance is also divided by two.

Regarding 70 ohm differential TL impedance, it would involve about 55 mil trace width at 8 mil separation, rather extreme dimensions.
 



thank you, but how do we know the output impedance is 12.5 ohm????any way to calculate it???
 

but how do we know the output impedance is 12.5 ohm????
Click to expand...
Because the transformer does a 4:1 impedance transformation.
 

Because the transformer does a 4:1 impedance transformation.
Click to expand...

I know it, I mean how do we know that impedance if we calculate from IF input port...
 

You don't know it, it's a target value for impedance matching.
You'll adjust the component values in smith chart design to meet it.
 

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