[UK-CONTEST] OT- Attenuation and IMD3

Wed Oct 28 05:47:10 PDT 2009

Thanks Stewart, Bob and Fred.

I suppose I knew a bit more than I was letting on- as you probably 
guessed-, and suspected that the IMD3 reduction was significantly 
greater than unity when applying attenuation. Actually I still can't get 
my head 'round the maths which obviously determines that the ratio is 3x 
for third order signals, but it matters not. I have always found that 
6dB of attenuation removes all IMD products, and assuming that my rx is 
in the 70dBm dynamic range class, this obviously puts would put it close 
to e.g. the Orion, the K3 or the latter day ICOMS which all achieve 
their performance through roofing filters of course.

Yes Bob, I did try to make the point about having sufficient signal 
input to tolerate at least 6dB of attenuation, and in my experience the 
noise figures of all modern rigs is satisfactory in this regard. I am 
talking about HF here of course which might not be valid at VHF or 
particularly UHF.

As far as SSB is concerned, a quick flick around the bands at the 
weekend was sufficient to convince me ( if that were needed!) that any 
rx deficiencies are blown away by the appalling quality of the average 
signal there. I didn't know about the issue with the K3 I must say, and 
my own experience  would indicate that -27dB third order would  
significantly degrade the  CW signal. This is based on  listening to 
many Yaesu Quadras  which  are probably in the low 30's and are very 
clicky beasts indeed.

We all have our favourite brands of rig of course, and we are all 
constrained to a  more or less extent by finance, or lack thereof! My 
own history of commercial rigs goes back to KW2000s in the late 60s, 
through the Sommerkamp ( aka Yaesu) separates from Bandit Bill in the 
early 70s, a QSK TenTec rig in the late 70s, through to the now classic 
( and still owned) TS930S in the mid 90's , through a TS870S( much 
underrated imo) in the late 90's and early 00's, finishing with a pair 
of IC756PROPIIIs which form the backbone of my extremely modest ( read 
antenna challenged) SO2R station of today.

The PROIIIs do a good job and have only two significant issues ( apart 
from a couple of tiny knobs!) being:

1. The QSK is not perfect in respect of shortened dots at higher speeds, 
possibly <5% shortening, but which can  readily be addressed with the so 
called key comp. parameter on the K1EL WinKey. I can live with that as 
in any case QSK is not relevant to SO2R operation.

2. Occasional IMD3 spurii in crowded contest conditions. As previously 
mentioned this is addressed by 6dB of front end attenuation when required.

I hanker after an IC7600 which addresses both issues at source ( caveat 
the QST review keying waveform is ghastly!) . However at a QRK of 
 >GBP3000 I will continue to use the aforementioned workarounds. The 
Inrad  roofing filter at USD200 looks promising but dealing with SMD 
devices is not currently in my portfolio of skills.

Thanks again for the input. The bottom line seems to be that attenuation 
rules!

73

Clive
GW3NJW

Bob Henderson wrote:
> 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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