[CQ-Contest] Re: 2-Radio Interference

[Tom Rauch]

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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