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Design >> RF Design >> Insertion Loss or S21
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Message started by aaron_do on Apr 19^th, 2012, 8:36pm

Title: Insertion Loss or S21
Post by aaron_do on Apr 19^th, 2012, 8:36pm

Hi all,

in many datasheets for PAs, filters, switches etc, they specify "Insertion Loss". The input impedance of the devices is usually matched to 50 ohm, but the match is not perfect. So a typical VSWR might be on the order of 1.3.

So my question is, are they really specifying actual insertion loss, or do you think this is actually just Pout/Pavailable which is much easier to measure.

thanks,
Aaron

Title: Re: Insertion Loss or S21
Post by RFICDUDE on Apr 20^th, 2012, 3:25am

For passives I have always assumed that insertion loss is measured S21. I did a small simulation study (a long time ago) to see how the frequency response of a SAW filter changes with different VSWR mismatch at one port (pseudo load pull), and I generally found that if the mismatch was less than 2:1 the filter met the insertion loss and ripple/slope specifications. But if the VSWR was increased that it started to degrade the response at the band edges.

There are techniques for determining the extra loss due to mismatch, but I have not designed a system where I needed to do it.

For PAs, gain is available power gain I think (Pdelivered - Available Pin).

Title: Re: Insertion Loss or S21
Post by aaron_do on Apr 20^th, 2012, 7:31pm

Thanks for the reply. In some cases, it is the difference between an insertion loss of 0.4 dB and 0.5 dB. 0.4 dB looks better on the spec sheet. I understand that its more practical to simply measure S21, but insertion loss gives the better number, so I figured the datasheet is giving the true insertion loss...

thanks,
Aaron