[UK-CONTEST] OT- Attenuation and IMD3

[Bob Henderson]

Clive

There's rather a lot could be written here but I won't.

Roughly speaking you can give your 70dB DR radio the equivalent strong
signal handling capability to that of a 100dB DR radio through the addition
of a 10dB attenuator.  At least in theory, 3rd order products diminish at 3
x the rate of 1st order.  So a 10dB attenuator should reduce these by 30dB
whilst diminishing real signals by only 10dB.

So why bother paying for a receiver with 100dB DR?  Well.... because strong
signal handling is not in itself the important criteria.  What you would be
paying for is the ability to copy a very weak signal in the presence of very
strong ones.  Adding an attenuator doesn't just improve strong signal
handling performance, it impacts minimum discernible signal performance
(MDS).  If I want to copy an S1 signal on a frequency which happens to
coincide with 2f1 - f2 of two other signals at S9+40, I had better have
rather more than 70dB DR because adding attenuation will sink my wanted
signal below MDS faster than it rids me of the IMD.

RX close in IMD performance is much less important so far as SSB is
concerned.  The expression 'linear amplifier' is something of an oxymoron
when applied to solid state p.a.  These typically output 3rd order products
only 30-35dB down.  Interestingly, the much vaunted Elecraft K3 is one of
the least impressive for TX 3rd order IMD.  It has been measured at only
27dB down.

2kHz 3rd order RX performance is essentially irrelevant to an SSB operator.
Rather more important to we CW types.

73

Bob, 5B4AGN


2009/10/27 Clive Whelan <clive.whelan at btinternet.com>

> Not directly contest related of course, but something we all have to
> deal with, so hopefully of interest.
>
>
> Let me declare that I am not  a techie, at least not in the engineering
> or radio field, just a hopeless appliance operator. We  all know I
> suspect, that in a less than perfectly  linear receiver that two
> interfering signals on F1 and F2 will generate spurious ( phantom)
> signals on 2xF1-F2 and/or 2xF2-F1, and if this product falls within our
> passband will generate QRM. The CW operator recognises this instantly as
> the headless chicken effect, i.e. a CW signal present only when both F1
> and F2 are simultaneously key down. Quite how it is manifest on SSB I
> confess I do not know, but suspect just as general " mush" which could
> equally be an overdrive artefact from an adjacent signal, so perhaps a
> little more insidious?.
>
> Now the question: Let's say we have a fairly average rx with a 3rd order
> dynamic range of 70dBm at 2kHz spacing. But if money were not an object
> we could go and buy a rx with a 90dBm range. We don't, so we have to
> deal with the issue in crowded band conditions. Typically modern
> receivers have step attenuators to deal with the matter, whereas in days
> of yore we would have had to turn the RF gain down, possibly losing some
> AGC performance in the process. Something in the dark recesses of my
> brain suggests that all things being equal,  attenuation is more
> effective than reduction of RF gain, and that x dB of attenuation will
> produce better than x dB improvement in the dynamic range, perhaps as
> much as 2 times x dB? If true I am not certain why this should be: is it
> perhaps because both interfering signals are being attenuated?
>
>  Put another way, how much attenuation do I need in my average rx to
> improve the IMD3 performance to the level of the super rig. Would this
> perhaps be (90-70)/2=10dB? Please feel free to disabuse me of this view
> if incorrect, but otherwise, having regard for the fact that virtually
> all modern rigs have this level of attenuation available, and also that
> IMD3 is likely to occur with generally high signal levels anyway, if I
> am prepared to flick a few attenuator buttons when the going gets tough,
> do I really need to spend mega bucks to obviate the issue in the first
> instance?
>
> Thanks for the bandwidth
>
> 73
>
>
> Clive
> GW3NJW
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