[1000mp] ARRL Lab transmit IMD test-result reporting

[Tom Rauch]

> Those who want to learn more about how ARRL does its tests can see the
> following:

One comment about the IMD specs Ed and all.

I don't think it is correct to base transmitter IM testing on "dB- 
below-PEP" when most of the rest of the testing world in on the 
"dB below one-tone of two-equal tones" standard.

First, the FCC does NOT specify an IM level for modulation 
byproducts, nor do they require that parameter in TA applications. 
The dB below PEP they look at is for harmonics and spurious 
emissions **not** related to modulation.

97.307 (a) and (b) define modulation bandwidth requirements, and 
neither gives a dB value. Those sections clearly say bandwidth 
must be **no wider than necessary** for the mode and data rate, 
that modulation byproducts **can not** cause QRM to adjacent 
frequencies, and that modulation products **must** be confined to 
the legally authorized band for the class of license and the mode 
being used.

The requirements in sections (a) and (b) (and other sections) are 
independent limits, we can violate any one or "pick the one we like" 
and use it as the standard. It is similar to traffic laws, we can not 
go 45 MPH through a red light because the speed limit is 45.

The most restrictive rule applies, not the least restrictive!

While I am not allowed to have spurious emissions over 50mw and 
at least 40db below the **mean power** of my transmitter, the rules 
CLEARLY do NOT allow me to have those levels for modulation 
products unless I never bother anyone else....and confine those 
emissions to the band segment for that mode. The rules require 
that any modulation products (or harmonics or spurious signals) 
NOT cause problems to others no matter what the level of those 
products are, and NEVER be outside the authorized band.

The bottom line is we are not allowed to have harmonics and 
spurious signals that cause harmful interference even when such 
emissions are suppressed below the 50mw and -40dB standard! 
Keyclicks and splatter clearly fall under section 97.307(a) and (b), 
rather than the looser 97.307 (d) requirement, which simply says 
modulation products can NOT cause interference on adjacent 
frequencies.     

Even if we used the 97.307(d) rule, most rigs miserably fail even 
those slack requirements when it comes to SSB and CW spurious 
emissions. Following the FCC's actual rules, MOST of the radios 
we now use are questionable (if not totally illegal) to use. 

Using 10kHz to transmit what should be 2.5kHz wide (a SSB 
signal), or a 3kHz signal that should be a few hundred Hz wide  (a 
CW signal) is not "occupying necessary bandwidth". It is actually 
"occupying UNnecessary bandwidth".

There is no justification for dB below PEP. Paragraph (d) and (e) of 
part 97.307 deal with levels of spurious emissions. In all cases, the 
reference is against the **mean** power of the transmitter...not the 
peak power. Even if the reference was against peak power and did 
deal with modulation products rather than spurious emissions and 
harmonics, sections 97.307(a) (b) and (c) simply state we can not 
have emissions that cause QRM outside the normal necessary 
bandwidth....and also clearly state such emissions can NEVER 
cause harmful QRM.

I believe the "dB-below-PEP" thing is just something to make us  
feel good about having rigs that do not even have 1950's transmitter 
bandwidth performance. Not only that, it allows some marketing 
people to whitewash dirty products and make unfair comparisons.

There is an obvious problem with the ARRL using a special IM 
reference that inflates performance by 6dB compared to 
conventional commercial standards. The inflated performance 
confuses the typical amateur who is tying to be a smart consumer, 
and opens the door to deceptive advertising.  

For example, an importer marketed tetrodes by comparing dB-
below-PEP measurements of their tetrode tubes under the best 
conditions to Eimac triodes measured using the conventional dB-
below-PEP under worse-case conditions! 

A person looking for a state-of-the-art amplifier might wrongly take 
the dB-below-one-tone **worse case** figure Eimac uses and 
compare it to the ARRL's dB-below-PEP tests of another tube-type 
and conclude a mediocre or poor linear amplifier has acceptable 
performance.

With less and less FCC enforcement, we depend more and more 
on the ARRL keeping manufacturers in line. Heaven help us all if 
the marketing departments and bean counters start setting 
emission standards!

I hope the ARRL takes an active role in cleaning up the 
transmitters that have been getting worse, or certainly no better,  
over time.
73, Tom W8JI
W8JI@contesting.com