I am trying to confirm on a Smith chart the matching network for 169MHz frequency using an external PA and and 437Mhz with an external LNA.

I am not able to end at the origin of the Smith chart using the reference design provided.

The transmitting and receiving paths have different frequencies:

For 169MHz, I am trying to verify the conversion from 80 ohms to 50 ohms

For 437MHz, I am trying to verify the conversion from (50+j30) ohms to 50 ohms

In the figure outlined below, could you let me know which lumped elements are used for the matching and which ones are used for decoupling or serve other purposes.

Thank you.

reference design


1 Answer 1


Not having tried to verify this myself, I'd say it wouldn't be very surprising if matching wasn't exact.

Basically, on small device TX, engineers nowadays tend to just waste a bit of PA power (they've potentially got plenty, compared to what they're allowed to transmit, typically) on a non-optimal matching (which implies worse SWVR) in favor of easier design, and better filtering.

Look at it this way: a 80Ω vs 50Ω mismatch would imply a reflection coefficient of enter image description here

So, for your lower frequency, you'd be losing but 23% of energy if you didn't do matching at all. That's not very much, considering the amplifier in that chip won't work constantly efficient at all frequencies; other effects just outshine that mismatch, and if you're not into building the most energy-efficient system possible, you simply wouldn't care.

At the 433MHz ISM band, you get frequency-dependent contributions, too, so that's what I'd try to optimize.


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