[AMPS] IM, Harmonics & Spurs ( was: Quiet)

Jon Ogden jono@webspun.com
Fri, 1 May 98 10:32:38 -0500


>>>...if the amp is being driven non-linearly, you will produce TONS of 
>crap where you don't want it.  Intermod is the technical name.<<
>
>No, Jon, intermod is not the technical name.  He says he's seeing stuff 
>up to 350 MHz.  That's not an intermod problem, it's a harmonic problem.  
>Intermod is when you mix two signals together.

We are both right on this one...let me explain:

If you are driving an amp non-linearly with a CW signal, then no, you 
won't produce intermod distortion.  This is the reason that a class C or 
a class E amp can be used on CW.  It's a constant envelope form of 
modulation.  The distortion products seen are harmonically related as you 
point out.

However, IF the amplifier is being driven by a single SSB signal, driving 
the amp non-linearly WILL result in intermodulation distortion.  This is 
because an SSB signal is not at a single frequency but is actually spread 
out over a portion of bandwidth.  Due to the variety of frequencies in 
your voice, there are plenty of products to mix. Tha't why IM is 
specified for transceivers or amplifiers.  An SSB signal WILL produce IM.

Also, it is not a constant envelope form of modulation, which is why you 
can't use a class C or E or whatever amplifier with it.


I thought I pointed out in my message this fact.  Perhaps I didn't make 
it clear.  I did not know what signal he is driving the amplifier with so 
I generalized.  I said that if he was driving it with just one CW tone, 
then what he is seeing is NOT intermod.  If you doubt, please go back and 
read my original post.

>  If I stuff 30 MHz and 
>30.1 MHz into the amp and I see 30.2 and 29.9, it's intermod.  If I 
>stuff 30 and 30.1 in and I get 60 and 60.2, it's harmonics.

What you are specifiying here are 3rd order intermod products.  However, 
intermod products are produced all across the band.  Second order 
products are the largest and will be out of band.  I forget the exact 
formula, but it is fairly far out of band.  It's not usually specified as 
they can be filtered out quite easily.  Even order products basically 
occur out of band, odd product in band.  3rd order IM products go by the 
formula of 2*F1-F2 and 2*F2-F1.  There are products (not sure if they are 
second order or which order) that go by the formula of 2*F1+F2 and 
2*F2+F1.  In this case those are 90.1 and 90.2 MHz.  Definitely out of 
band.  Definitely IM.  With each succeeding order, the intermod products 
get weaker and weaker.

However, if the low order products are really large and significant, 
higher order products can be significant as well.

Also, since every signal has harmonics, the harmonics of the intermod 
products could also show up on the output as well.

So....you could get lots and lots of crap on the output signal of the 
tube that are either harmonically related or IM related.  The original 
poster did say that the signals were all across the band.

However, I would agree that at 350 MHz, any IM products that far away 
from the original carriers would be quite small.  But for that matter, so 
should be the harmonics!
>
>He did not say where he put the probe.  If he put it near the anode, 
>then he's not getting the benefit of the lowpass action of the output 
>network, and I would definitely expect high order harmonics.  The only 
>way to use a "sniffer" and get decent results is to somehow closely 
>couple it to the final output of the amp, and nowhere near the anode.  
>The anode will always be worse than what you put on the air, and by a 
>wide margin.

Oh, I agree here as well.  No question about it.  Unfortunately, if you 
are looking for oscillations, you won't find them after the output 
circuit as they will be filtered out as well.

Quoting from "Solid State Radio Engineering" by Krause, Bostinian and 
Raab: "Other unwanted signals include parasitic and subharmonic 
oscillations and mixer products;  they are called spurious products or 
simply 'spurs.'  In an RF PA, the harmonics and some of the spurious 
products may be removed by filters..." page 349

So...in order to catch higher order spurs, one would need to look at the 
anode.......

73,

Jon
KE9NA


--------------------------------------------------------------------------
Jon Ogden

jono@webspun.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm