[1000mp] cost of improved IMD performance

Wed, 17 Apr 2002 11:05:03 -0700

Hi Tom,

You are correct; I looked up ITU-R Recommendation M.1173, and it confirms
your figures.

If excessive transmitter IP3/IP5, and the practice of transmitting with
excessive occupied bandwidth,  give rise to a significant increase in
adjacent-channel interference complaints, we may yet see the ITU-T issuing
technical Recommendations for the Amateur Radio Service - especially if we,
the amateur user community, continue to remain silent. Some national
administrations already have technical amateur-service regulations on the
books which are much tighter than FCC Part 97 or the Canadian regs.

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:[email protected]

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Tom Rauch
Sent: Wednesday, April 17, 2002 05:36
To: [email protected]
Subject: RE: [1000mp] cost of improved IMD performance

> A typical marine HF-SSB radio, FCC type-accepted and fully compliant
> with ITU and CEPT requirements, has a transmitter IP3 spec. of -32 dB
> relative to PEP at 150W PEP output. This radio is designed for a
> nominal 13.6V DC supply, and uses 13.8V PA devices.

Amateur rules are different, but in neither case is is legal to cause
interference when it is the transmitters fault regardless of level.

For marine use IP3 is not restricted. The technical standard is now -
35dB at any frequency removed from the assigned channel by more than
150% and up to 250%, and at least -43dB plus 10log ten of MEAN power
in watts dB beyond 250% of assigned channel width.

If you were assigned 3800kHz, and had a 3kHz channel width on LSB,
3rd order IM distortion from a pair of tones near 3kHz would have to
be a minimum of -35dB. That is very easy and inexpensive to do.

The reason amateur radios are worse is because no one holds the
manufacturers to any standard. We accept crappy performance like -
25dB IM3, and keyclicks 3kHz wide despite the fact the cost
difference is nearly zero to build a clean rig.

> Can we as radio amateurs really expect much better than this from the
> manufacturers, when we are a much smaller market segment than maritime
> users, and our price point is often significantly lower than theirs?

Yes, we should expect the same or better. It is very easy to meet
that requirement. Collins made -50dB IP3 in transmitters in the
1960's.

As a cost example, my IC751A does much better than -35dB after
changing the driver transistors to different types. As a matter of
fact, with just that simple change it is 15dB better than stock form!

If I deduct the wholesale price of the original transistors from the
replacements I used, ICOM would have saved $0.56 US per radio to make
it cleaner. Contrary to popular misconception, it is NOT noticeably
more expense to build a radio that does not click or splatter
excessively.

It isn't a cost issue, the real problem is there is no representative
working for amateurs who demands for any sort of minimum
performance.73, Tom W8JI
[email protected]

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