rf system tutorial
spbhu said:
Hi, thanks very much for your tutorial posted! It benefits us too much!
Just want to ask, for the differentail cs LNA,why we have no load for ac-simulation, why a 50ohms terminal is needed for s-parameter simulation? I am confused about this question for a long time! Hope for you answer... Thanks very much
Most measure equipmet is build for 50 Ohm reference (10 Ohm give very small distace between transmissions line and 900 - 1500 Ohm give very wide distance like High voltage air hanged power line and couple of meters between lines...)
Historic 600 Ohm using for air-hanged phone-line (around 400 mm distance between lines), 300 Ohm using for old TV and FM-broadcast depend of folded loop antenna give this impedances, and dipol give around ~75 Ohm and ground plane-antenna give ~50 Ohm. - and lastest used wide in mobile radio world and measure equipment build for this maket - and setting standard...
50 Ohm is only a reference - you can measure DUT in 50-Ohm NVA for making S-parameters and in simulators recalculate for example 300 Ohm port impedance or complex impedances typical situation on crystal-filters matching - or 2-5 Ohm typical on Power transistors in 1800 MHz etc.
I very fascinate first time long time ago I measure MF-filters separatly, feeding trasistors for this filter net, and complex load after this in separate S-parameter block.. (if not know feeding and load impedances first time - you need measure this as S-parameters)
MF-filters in VNA and 50 Ohm measure impedance seems like skit and crasy unrealistic value (if not know how this working - crystal filter seems possibly broken...).
But in simulator [1] - beginning feeding from S-parametes block transistor (I find working as current generator and high impedances) simulating as port in higher impedance value and filter going from very bad to good and after small adjust complex impedances (around 200-300 Ohm real part and small imaginary part), can make very good filters in simulators - and now understud why people mostly have tunable parallell resonace circurit each end of crystal-filters - just for make tunable very small amount of complex load in range of part of few pF to few nH or so... and very importent factor to give filter good shape.
...and back to phycial equipment now understud why how works and better to repair and tune this.
[1]
I try Vipec!! - open source linear simulator in first moment and working well for this experiment - and checking later on Ansoft for same result (but this is very expensive and need HW-keys/licenses... if using legal version... and give not so much more info compare to Vipec in this case)
/Xxargs