[CQ-Contest] Re: 2-Radio Interference

[Michael Tope]

Hi Tom,

Yes, I agree wholeheartily that an S-meter shouldn't be relied
on for direct measurements in dB unless you have a Kenwood TS-870 
or something similar where the  meter is known to be accurate. I was 
going to throw in that caution, but I was in bit of a hurry the other 
day. Shame on me, I should know better. 

As far as the distortion behavior goes, again I oversimplified a bit
for the sake of brevity. I should have stated that non-linear distortion
products generally fall off at rates higher than 1dB/1dB as signal level 
applied to the stage producing the distortion is reduced. You are
quite correct that as one approaches the P1dB of an amplifier, the 
simple mathematical relationships I described fall apart rather quickly. 
At levels below P1dB, I have found in practice that the 3 for 1 relationship 
between 3 order products and the fundamental tones (sine waves)  is a 
pretty good approximation. This mathematical relationship between 3rd 
order IMD products and intercept point is described in just about any 
text on RF design (for instance see Hayward's book "Introduction to Radio 
Frequency Design" pp219-232). I have also seen these rules of thumb 
described in CATV technical tutorials (there is more emphasis on 2nd
order distortion in the CATV world), but unfortunately I got rid of some 
of that stuff before I acquired enough sense to know it was worth 
keeping. 

What I should have said to Martin was that if the interference goes 
down in level more than a known in-band signal of the same level (from
an outboard signal generator for instance) when the attenuator is inserted, 
then chances are good that the interference is caused by a distortion 
product occuring in the receiver.  If on the other hand the transmitter is 
generating the distortion (e.g. the products are already present ahead of the 
attenuator), then interference will drop in level the same amount as an in-band 
signal of the same level. 

I'll have to endeavor to be more thorough with my words in the future, 
as hastiness is one of my weaknesses. 

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


----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: <cq-contest at contesting.com>; "Marty Tippin" <martyt at pobox.com>; "Michael Tope" <W4EF at dellroy.com>
Sent: Saturday, December 23, 2000 8:36 AM
Subject: Re: [CQ-Contest] Re: 2-Radio Interference


> Hi All,
> 
> > Discerning fundamental overload in the receiver from harmonic/spurious
> > products generated in the transmitter is quite easy. Put a step attenuator
> > in front of your receiver and change the input attenuation from 0 to say 6
> > dB. If the interference heard in the receiver drops 6 dB then its
> > generated in the transmitter (or at least at a point ahead of the
> > receiver). If its generated in the receiver, then it will drop by a larger
> > amount than 6 dB. Second order products generally drop 2 dB for every 1 dB
> 
> A few catches to that:
> 
> 1.) Using a S meter is always unreliable, unless you calibrate the 
> meter by making a chart before making your measurements.
> 
> A fellow in Florida exchanging e-mails with me is convinced he has 
> receiver problems because someone advised him to do the 
> attenuator-pad trick, but failed to caution him that S-meters are 
> almost never 6dB /S unit at any area of the scale and typically 
> have horrid linearity.  
> 
> Most meters are not designed or calibrated to 6 dB, most actually 
> use  5dB standard. They also fall completely apart as the AGC 
> threshold or saturation  is approached.
> 
> On my FT1000D, the S meter is less than 1 dB/S unit around S-1 
> and 5 dB near S-9. My 751A's are worse.     
> 
> 2.) Correct me if I am wrong, but I believe those rules even-
> order=E^2 and odd-order= E^3 rules assume amplifying devices 
> causing distortion are reaching neither current nor voltage 
> saturation, the distortion is rooted in a single amplifying device, the 
> voltage is a pure sine-wave, and gain compression is not present in 
> the system. I don't know the origin, but it seems they are extracted 
> from analysis that takes pages to describe. If so, there is always 
> great danger in simplifying the results to one rule.
> 
> I'm not trying to pin you down Mike, I'm curious. Can you tell me 
> where I can find that "rule's" origin described? I have a nagging 
> doubt it would reliably apply to a problem like this.
> 
> 
> 73, Tom W8JI
> w8ji at contesting.com
> 


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