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Re: [Amps] Understanding S Parameters

 To: amps@contesting.com Re: [Amps] Understanding S Parameters Steve Thompson Wed, 17 Jun 2015 22:17:51 +0100 mailto:amps@contesting.com>
 As it's a 1W transistor, the input capacitance is likely to be less than 1-5nF. ``` ```Don't get too hung up about three decimal places of accuracy, reflection coefficients anywhere near 1 are prone to huge errors in measurement. As Alex says, the input looks like a capacitance in parallel with a very high resistance (or in series with a very small resistance if you prefer). ``` ```Up to 100MHz the angle of reflection is pretty much linearly related to frequency which allows you to calculate the equivalent capacitance - somewhere around 10pF I think. ``` Steve ``````Quite so. However, at low frequencies the input impedance of a fet is about 1 to 5 nF in parallel with a quite high resistance. You might say a pure reactance, more or less. On the other hand, terminations affect the stability of a device wuite strongly and they can cause a state of negative resistance at the input- particularly some feedback capacitance from source to gate dou tp imperfect source grounding, bue to package parasitic inductance. Alex 4Z5KS On Jun 17, 2015 10:19 AM, "Markku Oksanen" wrote: ``````If my memory serves me correctly: Is it not so that S11 refers to the input port "input impedance" of a two port network?So, in this case the input impedance is just simply about 1 ohm with small reactance (phase angle -3.6).SWR at the input should be around 50. Without S12 or S21 you should not be able to know stability? Markku > From: pc5m@xs4all.nl > To: amps@contesting.com > Date: Tue, 16 Jun 2015 22:45:08 +0200 > Subject: Re: [Amps] Understanding S Parameters > > 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@contesting.com] On Behalf Of Drax Felton > Sent: dinsdag 16 juni 2015 21:53 > To: amps@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. `````` _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
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