Executivei Summary: Poor IMD makes for a wider signal at the receiver. For
crowed band conditions, receiver IMD is _not_ the issue.
All types of amplfiers, so both receiver and transmitter amplifiers generate
IMD. Receivers are designed to handle a worst case of two maximum strength
signals generating IMD that lands on a minimum strength signal and requires IMD
dynamic range performance in the 70 to 120 dB range. Transmitters are designed
to generate IMD 30-40 dB down. Modern receivers have far more dynamic range
that modern transmitters.
Receivers need high dynamic range for several different scenarios: 1) field day
with stations on _other_ bands; 2) scattered, widely spaced strong stations
with
scattered weak stations in between (where the splatter from the strong stations
is not the issue)
Surprisingly, receivers _do not_ need high dynamic range for crowded band
conditions. Imagine a band full of 40 over S9 signals. The spaces between all
those signals is filled with splatter at +10 over S9 from all those crummy
transceivers and poorly operated amplifiers. It doesn't matter if the the
radio
is Swan 250 or IC-7800, the band is full of crap coming from the transmitters.
When a strong signal wipes out the band for 20 KHz either side, you can almost
always blame the transmitter, not the receiver. How often has pressing the
Attenuator button on your radio actually made a difference?
On the subject of signal width... Think of an SSB signal with audio from 300
to
3000 Hz. The second harmonic has audio from 600 to 6000 Hz and mixes with the
audio from 300 to 3000. Each audio components from the 2nd adds and subtracts
form each audio component of the fundamental. This creates a splatter signal
with components from 600-300 = _300 Hz_ to 3000+6000 = _9000 Hz_. The 5th
order
products are 1500 to 15000 +/- 900 to 9000 = 600 to 24000 Hz.
The amplitude of the IMD signals at 600 to 24 KHz depends on the rated IMD
level
of the amplifier for the 3rd and 5th order products. The worse the IMD rating,
the higher the level of products. The products tend to have an amplitude
rolloff at high frequencies depending on voice pitch and harmonics and
depending
on the level of speech processing. Operators _perceive_ that a signal is wider
when the weaker high offset components become detectable.
This means that in a band fairly crowded with strong signals, each gap where a
weak station might fit, is clobbered with multiple overlapping IMD crap from
stations up to 24 KHz away, or more if the amp is seriously mistuned or
(operated by that famous 2M lid on Mt Equinox).
Note that there's a lot of IMD products in the 600 to 3000 Hz range that fall
within the transmit audio passband. This creates audible distortion on the
transmitted signal. If you want to sound good, keep the IMD low.
jeff, wa1hco
On 11-12-25 04:34 PM, Commander John wrote:
> I fail to see how having spurious sidebands at -20, -30, -40 or whatever
> makes the
> Signal wider. The sidebands are still there regardless of final suppression
> level. It is the receiving stations receiver having the rejection ability
> that reduces received interference. Having adjacent sidebands that are
> stronger than necessary causes a receiving station to hear these sidebands at
> a harmful signal strength further away from the transmitted frequency than if
> they had supression at a highter level. A legal limit station would have
> much stronger IMD because it is stronger not wider. It only is preceived as
> wider because they are stronger than from a 100 watt transmitter.
> john W9ZY
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