[TenTec] Filters, amps and IMD, etc

[Mike Hyder -N4NT-]

Hi Mike--

As I said, I was over my head.  Please help me understand something you said 
in your last paragraph, to wit: "If you drop the input signal tones levels 
by 10dB, the 3rd order products will drop 30dB for a net change in relative 
IMD level of 20dB (e.g. IMD changes from -20dBc to -40dBc as you drop the 
tone levels 10dB from the amplifiers 1dB gain compression point)."

Please compare that with what I said: "For this reason, a 10dB attenuation 
in the line will attenuate the desired signal by that much but will 
attenuate the intermodulation distortion products by much more"  and explain 
where my thinking differs from yours.

73, Mike N4NT

----- Original Message ----- 
From: "Michael Tope" <W4EF at dellroy.com>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>; 
<k4qo at earthlink.net>
Sent: Friday, July 29, 2005 10:34 AM
Subject: Re: [TenTec] Filters, amps and IMD, etc



----- Original Message ----- 
From: "Mike Hyder -N4NT-" <n4nt_m_o_hyder at charter.net>

> IMD is the product of two or more signals.  3rd order IMD is a signal's
> fundamental mixing with the second harmonic of another signal...
>
> For this reason, a 10dB attenuation in the line will attenuate the desired
> signal by that much but will attenuate the intermodulation distortion
> products by much more.

That isn't quite right, Mike. This only happens when you put the
attenuator in front of a non-linear amplifier. In this case, the attenuator
will appear to reduce IMD products more than the fundamental signal
is, but what is really happening is that you are reducing the drive to the
non-linear amplifier stage so that it doesn't produce as much IMD.
If you put a 10dB attenuator after the stage which produces the IMD
distortion, it will knock the signal down and the distortion products
by the same amount, 10dB. In other words, if the IMD products are
already present, the attenuator will knock them down by the same
amount as the desired signal (the attenuator can't tell the difference
between the desired signal and the distortion products).

>
> Amplification works in the opposite way.  It brings up one signal but
> brings
> up the IMD products more than it does the one signal.  In addition, if the
> amplifier is not perfectly linear, it serves to accentuate the harmonic
> products and worsen the problem (see paragraph 1 above).
>
A perfectly linear amplifier by definition will amplify distortion products
by the same amount as the fundamental signal (amplifiers can't tell the
difference between the signal we want and the signal we don't want). Most
amplifier, are somewhat non-linear so they typically add IMD products.
The amount of IMD that they add depends on how hard you drive the
amplifier. At the amplifiers 1dB gain compression point, an amplifier
will typically add 3rd order IMD products that are approximately 20dB
down from the fundamental tones. If you drop the input signal tones levels
by 10dB, the 3rd order products will drop 30dB for a net change in
relative IMD level of 20dB (e.g. IMD changes from -20dBc to -40dBc
as you drop the tone levels 10dB from the amplifiers 1dB gain compression
point).

73 de Mike, W4EF.......................

> I have just exceeded my knowledge.
>
> 73, Mike N4NT
>
> ----- Original Message ----- 
> From: "Randy K4QO" <k4qo at earthlink.net>
> To: <tentec at contesting.com>
> Sent: Thursday, July 28, 2005 11:50 PM
> Subject: [TenTec] Filters, amps and IMD, etc
>
>
> If the amp that kicks in at the tighter bandwidths is the problem, why
> not just adjust the gain of that amp down a bit.  Wouldn't that improve
> the IMD without resorting to 3rd party filters?  Or is it that ANY
> additional amp stages, regardless of gain causes the IMD to get worse?
>
> Iquiring minds wanna know!
>
> 73,
> Randy
> K4QO