No amplifier is perfect, and every added amp will add to the IMD
problem, especially if it's on the input side of a filter, where a
broad band of interfering signals can get amplified and mixed
together. The schematic shows that when you use one of the standard
filter slots, you have (broad filter)->amp. When you use one of the
optional narrow slots, you have (broad filter)->amp->(narrow
filter)->amp. So there is an extra amp stage before the final narrow
filter. That seems to be the problem.
If you want ~ 100 dB IMD dynamic range, you need exquisite linearity
in your RF and IF system. Distortion from any unnecessary (pre final
filter) amplifier stage can easily mess up your overall IMD number.
73 Martin, AA6E, yet another non-rf-engineer with an opinion
On 7/29/05, Mike Hyder -N4NT- <n4nt_m_o_hyder@charter.net> wrote:
> 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.
>
> 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).
>
> I have just exceeded my knowledge.
>
> 73, Mike N4NT
>
> ----- Original Message -----
> From: "Randy K4QO" <k4qo@earthlink.net>
> To: <tentec@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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>
--
martin.ewing@gmail.com
http://blog.aa6e.net
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