[Elecraft] FCC Specs for marine SSB IMD compared to amateur specs

[RDStraw]

Comparing a marine SSB radio with an amateur radio for the transmitter
cleanliness can be misleading. 47 CFR (code of Federal Regulations by the
FCC) is for maritime radio stations. It says:


47 CFR Part 80, Maritime Stations FCC Rules and Regulations

§ 80.211 Emission limitations. 
The emissions must be attenuated according to the following schedule.
(a) The mean power when using emissions H3E, J3E and R3E: 
(1) On any frequency removed from the assigned frequency by more than 50
percent up to and including 150 percent of the authorized bandwidth: 
at least 25 dB for transmitters installed before February 1, 1992, at least
28 dB for transmitters installed on or after February 1, 1992;
(2) On any frequency removed from the assigned frequency by more than 150
percent up to and including 250 percent of the authorized bandwidth: At
least 35 dB; and
(3) On any frequency removed from the assigned frequency by more than 250
percent of the authorized bandwidth: At least 43 plus 10log10 (mean power in
watts) dB. 

Discussion:
The 50% to 150% of authorized bandwidth (which for an SSB transmitter is 2.8
kHz) specs mean the 3rd and 5th order IMD products must be suppressed by at
least 25 dB than the mean power output. For more than 150% up to 250% of
authorized bandwidth the 7th and 9th order IMD products must be suppressed
by at least 35 dB.

Now the killer spec: the 43 dB plus 10 log10 (mean power) dB spec. For a
1500 W mean power output level, this spec amounts to 43 + 10 log10 (1500) =
74.76 dB down for IMD products higher than 11th order. This commercial
specification is exceedingly difficult to meet, and few amateur amplifiers
can come close to meeting this spec. Amateur gear is rarely capable of
meeting the first half of the equation; i.e., only 43 dB down! 

Note that the stringent specs for higher-order IMD products mean in essence
that the off-channel “splatter” is much less for a commercial SSB
transmitter than for an amateur radio transmitter. 

The output amplifier output load line for commercial transmitters is
frequently “tweaked” to balance the close-in (3rd and 5th order IMD
products) with the greater than 11th higher order IMD products. Thus it is
rather misleading to fixate on the 3rd and 5th IMD, while ignoring the
higher-order products that are so annoying more than, say, 5 kHz away from
the carrier frequency.

To be really meaningful, listings for Transmitter IMD should be shown for
the 3rd, 5th, 7th, 9th and 11th order IMD products. You could make a case
that 13th order IMD products should be shown also to complete the IMD
profile. A systems approach would show the IMD for both the transceiver and
linear amplifier together.

73, Dean, N6BV

N6KR said:

> Date: Thu, 29 Oct 2015 13:39:23 -0700
> From: Wayne Burdick <n6kr at elecraft.com>
> To: jim <jbollit at outlook.com>
> Cc: <jim at audiosystemsgroup.com>, <elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] IMD and supply voltage
> Message-ID: <A2599806-3661-45E1-ADF4-ACB03A15FE53 at elecraft.com>
> Content-Type: text/plain;	charset=us-ascii
> 
> Jim,
> 
> This appears to *not* be true, at least for one radio lauded by those 
> concerned about TX IMD.
> 
> Earlier this year there was a long forum thread on Eham.net lamenting the
"fact"
> that 12-V ham rigs were not as clean as 12-V marine rigs. One radio in 
> particular, the Icom M802, was touted by forum participants as having 
> far better IMD specs than any 12-V ham transceiver.
> 
> We downloaded the manual for we this radio and found that Icom was 
> using the same devices and essentially the same amplifier circuit that 
> we were. Still, we gave them the benefit of the doubt and purchased a 
> new Icom M802 (from Amazon, $1813) to test in our own lab.
> 
> We very carefully measured transmit IMD at 100 watts on several 
> channels, covering the full range of the HF marine band. Here is an 
> example plot from about 12.28 MHz:
> 
>    http://www.elecraft.com/Icom%20M802%20100-W%20IMD.jpeg
> 
> This plot shows the two 3rd-order products being down by about 27 and 
> 30 dBc, respectively. A plot for the Elecraft K3 posted earlier on 
> this same forum showed these tones down 33 and 36 dBc--about a 6-dB 
> improvement over this particular marine radio.
> 
> A bit on our test setup: We used a very clean 14.0 V DC power supply 
> with short cables, a high-performance analog 2-tone generator, and a 
> very hefty 50-ohm nonreactive dummy load. We set the top of the 
> spectral plot at 100 W and equalized the amplitude of the tones at -6 
> dBc, consistent with the usual ARRL method. The tests were done by our 
> senior RF engineer, and I'm sure he was not overdriving the spectrum 
> analyzer :)
> 
> Chances are that virtually all marine HF SSB radios being made today 
> are using the same 100-W MOSFET PAs that we and other ham 
> manufacturers do. As far as we can tell, there is no magic in the marine
radios that is making them better.
> On the other hand, their marketing department may be better funded.
> 
> If you have another marine radio in mind that you think is using 
> different/better circuitry, please let me know so we can repeat this
exercise if necessary.
> 
> 73,
> Wayne
> N6KR
>