[TenTec] Filters, amps and IMD, etc

[Mike Hyder -N4NT-]

Compare the following two statements:

1. 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.
2.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...

Both those statements were in your first email chastising me.  In my opinion 
it is impossible for both of your statements to be true at the same time. 
First you say that the attenuator only reduces the drive so it doesn't 
produce as much IMD (but just "appears to"), then you say that a change in 
the input level is mathematically related to the change in IMD in the 
output.

Again I respectfully suggest that you make your own statements without 
regard to what others say.  Everyone is entitled to be wrong.  It is not 
your job to police everyone for accuracy.

I brought this back to the reflector so the folks here can get a better idea 
of what we think.

Mike N4NT

----- Original Message ----- 
From: "Michael Tope" <W4EF at dellroy.com>
To: <n4nt_m_o_hyder at charter.net>
Sent: Saturday, July 30, 2005 1:20 PM
Subject: Re: [TenTec] Filters, amps and IMD, etc


Mike,

I'll take this off the reflector. There is no need to make this
a public pissing contest (there is already too much of that
on this reflector). In any case, I assumed you said something
you did not, because your statements were NOT clear, and
in my opinion they could easily be misinterpreted. I have
reread your words in the "clear light of day" several times
and each time I do, I come to the same conclusion. That is:

a) This guy doesn't understand what he is talking about.

or

b) This guy does understand what he is talking about, but he
has explained things so poorly that I can't tell for sure.

Whatever the case, the only reason I chimed in was I
was worried that others might misinterpret what you said
and thus come away misinformed. Tell me that you
wouldn't do the same if you believed someone was being
provided misinformation? Of course you would, Mike.

As far as your statement about being "short-circuited by
fear" I guess you are accusing me in a roundabout way
of jumping to conclusions based on some neurotic fear
of new knowledge or some other personal insecurity.
Well, I suppose that I have my share of personal
insecurities, but I can assure you knowledge of basic
IMD theory isn't among them.

I do appreciate your advice, however, about guarding
against neurotic tendencies. Your are correct that when
left unchecked they can be an impediment to personal
growth and learning. I do try to stay vigilant in that regard.

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




----- Original Message ----- 
From: "Mike Hyder -N4NT-" <n4nt_m_o_hyder at charter.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Saturday, July 30, 2005 6:27 AM
Subject: Re: [TenTec] Filters, amps and IMD, etc


> In the clear light of day, reread all the words below.  What you've done
> is
> take my fairly clear and correct statements, assumed that I said something
> I
> did not or assume I did not know something I did know and then take issue
> with your assumptions.  I respectfully suggest that in the future you
> concentrate more on helping the original requestor instead of taking issue
> with someone else who is trying to help.
>
> Almost all of what we learn we learn by analogy.  In some people that
> learning is short-circuited by their fear -- I don't know if it's fear of
> being wrong or ignorant or what -- so that when they hear the first part
> of
> the analogy, their internal response is, "I know that!" and the new
> knowledge is lost.
>
> Guard against that.
>
> 73, Mike N4NT
>
> ----- Original Message ----- 
> From: "Michael Tope" <W4EF at dellroy.com>
> To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
> Sent: Saturday, July 30, 2005 2:07 AM
> Subject: Re: [TenTec] Filters, amps and IMD, etc
>
>
>
> ----- Original Message ----- 
> From: "Mike Hyder -N4NT-" <n4nt_m_o_hyder at charter.net>
>
>
>
>> 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
>>
>
> Perhaps it is just the way you worded your statement, Mike, but I
> took what you said to mean that an attenuator actually reduces IMD
> regardless of where it is in the RF chain. An attenuator can't reduce
> IMD products that are already present at its input any more than
> it can reduce the fundamental signal. It will reduce the IMD at its
> input by an amount equal to the amount that it reduces the
> fundamental. If the IMD is down 40dB relative to the fundamental
> signal at the input of an attenuator, it will still be down 40dB relative
> to the fundamental at the output of the attenuator. An attenuator can,
> however, reduce the fundamental signal level hitting a downstream
> non-linear stage thereby reducing the amount of IMD which that
> stage produces. In that case (non-linear stage downstream from the
> attenuator) the amount of IMD produced by the offending non-linear
> stage will be reduced in greater proportion than reduction in the
> level of the fundamental signal (~3 for 1 proportion in the case of
> the 3rd order IMD products).
>
> You also said the following:
>
> "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)."
>
> The wording of this statement implies that IMD products are
> created even in perfectly linear amplifiers (e.g. your wording
> implies that problem is always there, and just gets worse when
> the amplifier is non-linear). In point of fact a pefectly linear
> amplifier (if you could find one), doesn't generate any IMD
> products. Any IMD already present at the input of a linear
> amplifier will be amplified by the same amount as the fundamental
> signal. Of course, any real amplifier will have some non-linearity.
> If you increase the signal level to a typical amplifier that has some
> inherent non-linearity, the IMD products will grow by an amount
> greater than the increase in input signal level (~3 for 1 in the case
> of the 3rd order product). This is where the concept of intercept
> point comes from.
>
> If you keep increasing the signal level to a real amplifier (e.g. any
> real amplifier will have some inherent non-linearity), since the 3rd
> order IMD products grow faster than the increase in fundamental
> signal level, eventually the level of  the 3rd order products present
> at the amplifier output catch up or "intercept" the level of the
> fundamental at the output. Actually, however, a typical amplifier
> doesn't ever get there as its output will saturate just before it
> reaches the 3rd order intercept point, so generally the intercept
> point is a mathematical extrapolation from the slopes of
> fundamental signal level vs. input power and the 3rd order
> product level vs. input power curves.
>
> In any event, perhaps this is what you meant to say, but it
> just wasn't clear to me from your wording.
>
> 73 de Mike, W4EF................................
>
>> ----- 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
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