[Amps] Understanding S Parameters

[Carel, pc5m]

Hi Drax,

If S11 > 1 it means it is not unconditional stable, i.e. it might
oscillate...

Your formula for input impedance is only valid if s12 or s21 = 0, and I
think that is not the case.

Otherwise: Gamma(Inp)=S11+(S12*S21*Gamma(outp))/(1-S22*Gamma(outp))

73's Carel.

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Drax Felton
Sent: dinsdag 16 juni 2015 21:53
To: amps at contesting.com
Subject: [Amps] Understanding S Parameters

I am trying to understand S Parameters for a RD00HHS1 transistor.

The S11 parameters listed in the datasheet are as follows:
10 MHz		1.002 (mag)	-3.6(ang)
30 MHz		1.003		-9.9
50 MHz		1.005		-16.8
100		1.007		-33.5
150		0.989		-49.8

I'm trying to estimate the input impedance at 0.475MHz.  But I am uncertain
what it means when the mag is greater than 0.999.

Using the 10 MHz S11 value because it's available I get:

Gamma = 1.002 = (ZL-50)/(ZL+50) --> ZL= -50050 ohm impedance or a VSWR of
negative 1001:1.

What am I doing wrong?  What does this mean?  
My other attempts at calculating input Z also result in negative impedances
in linear form like -50.58-1589j My RF books do not seem to say anything
about this.


Kb3x






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