[HCARC] Fw: Linearity - What Does It Mean

[Kerry Sandstrom]

Sorry Gary.  I had some trouble getting that e-mail out.  You should have it 
now.

Kerry
----- Original Message ----- 
From: "Kerry Sandstrom" <kerryk5ks at hughes.net>
To: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>; 
<hcarc at mailman.qth.net>
Sent: Sunday, August 26, 2012 3:52 PM
Subject: Re: [HCARC] Linearity - What Does It Mean


> Linearity is an easy term to describe.  In electronics linearity means 
> that the output is a direct function of the input.  It is described by the 
> equation V(out) =  k times V(in) where k is a constant.  The equation 
> describing the device is a linear equation.  resistor, capacitors and air 
> core inductors are generally considered linear components.
>
> I don't know of a linear amplifier.  I'm serious, ham linear amps are not 
> linear.  They are only relatively linear if operated within their ratings. 
> 3rd order distortion is the result of non linearity as are harmonics. 
> Every amplifier generates both.  Harmonics can be filtered out but because 
> 3rd and other odd order products are in-band to the signal, it is 
> difficult to filter them out.  a linear amp will have a spec such as for 
> 100 W PEP output the 3rd order products will be 30 dB below either tone of 
> a two tone signal. Class C amplifiers are much worse.  If you overdrive a 
> linear or have it improperly tuned, it can also generate much stronger 
> distortion.
>
> An example of a non linear equation is V(out) = V(in) squared.  If you 
> take V(in) = (a cos 2 PI fa t + b cos 2 PI fb t), square it and use a few 
> trig identities you will find there are more frequencies coming out than 
> you put in.  Thats the result of nonlinearities.  For higher power the non 
> linear products increas even faster.  For 3rd order products they increas 
> approximately 3 time as fast as the input power.  Some devices such as 
> tubes run as frequency multipliers, the operating conditions are selected 
> to maximize the output of a given harmonic.  Incidently, some devices are 
> described as square law devices, that should give you a clue to their 
> linearity.  A real device may have an equation like V(out) = kV(in) + 
> lV(in) squared + mV(in) cubed + nV(in) to the forth power and so on.  not 
> a pretty equation!
>
> Transistor, FET's, diodes, iron core inductors, ferrites, etc are all non 
> linear.
>
> Receivers also generate intermodulation products.  I'm sure you've seen 
> numerous articles talking about 3rd and 2nd order IMD in receivers.  It's 
> all the same.
>
> By the way, there are no linear ferrites.  Some can just handle higher 
> powers than others before they cause a problem, just like so-called linear 
> amplifiers.
>
> Kerry