On 5/19/22 7:51 AM, Gerard wrote:
re,
I¡¯m back on track. I wonder, should we not focus too much on the gain curve, but rather improve the loss curve?
see attachement
cdt
The S11 (return loss)?
Most filters have a terrible return loss outside their passband.
What you need to ask is "what's driving this" - will the reflected power out of band cause a problem? (i.e. make an amplifier unstable) - generally not.
And, because S22 is likely similar, will the next stage care about the input having a poor match out of band (i.e. become unstable).
If you're running into/out of a mixer, the usual thing is to put some resistive attenuation (or a circulator if you're at microwaves) - a 3 dB pad means that no matter how bad the filter is, the mixer only sees -6dB at worst.
The fact that the match varies "in band" is already taken into account in the S21 measurement - it's normalized to a ideal 50 ohm source, so the fact that the filter reflects 10% of the power (-10dB) (transmitting 90% into the filter) at some frequencies and 1% at others (-20dB, transmitting 99%) is accounted for in the S21 measurement.
10 dB reflected corresponds to -0.5 dB transmitted.
A device that reflected -10dB at some points and -20 dB at other points, when looked at in the transmitted sense (leaving aside any absorption internally) would look like varying between -0.5 and -0.004 dB gain.? Real filters have some loss inside the filter, so the gain is typically a bit flatter than you'd expect just from the S11 and S22.
