Transmitter composite noise

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Transmitter composite noise

Postby administrator » Tue May 04, 2021 9:56 am

Interesting read, thanks for sharing Warren.

https://www.dj0ip.de/transceivers/tx-noise/

Image

73,
Abhi
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Re: Transmitter composite noise

Postby w-u-2-o » Tue May 04, 2021 2:22 pm

Bragging rights for sure, and a technological tour de force, but what does this really mean?

Consider more typical conditions of an S9+20 CW signal, which is the largest one typically sees, not that there aren't larger signals out there, but say as a 90th percentile type of situation. That's -53dBm.

Similarly, consider an S5 noise floor. Again, very typical. This assumes a properly calibrated and linear S-meter and not the junk found on "big three" radios. That's -97dBm in a 500Hz bandwidth per the ITU standard. To compare this to the data provided above, we need to reference that to a 1Hz bandwidth by subtracting 10log(500), or 27dB, which gives an S5 condition as -124dBm/Hz.

This combination of circumstances represents a signal to noise difference of 71dB/Hz. Any transmitter with a "composite noise" performance better than -73dBc/Hz is simply not going to interfere with anyone under those conditions.

Let's look at this with an S2 noise floor, which is -142dBm/Hz. Then the signal to noise difference is 89dB/Hz. Composite noise is still not a factor for any reasonably designed transmitter..

Finally, let's postulate an S9+40 CW signal vs. an S2 noise floor. Now the signal to noise difference is 109dB/Hz. Now we are just barely starting to touch on the worst performer in the table.

The bottom line here is that unless you have a serious co-site issue, such as a nearby neighbor, this is not going to be a factor. And, when things get really serious, like at a field day or multi-multi contest station, you simply can't make a transmitter good enough, period, which is why each radio needs to be working a different band, not the same band.

Instead of "composite noise", the focus should be on linearization and proper ALC design, the way it's done in our radios. And only our radios (so far). By reducing gross intermodulation distortion (IMD) products, what one of the referenced papers loosely calls "splatter", that is how the industry and the hobby should be improving their radios. The amp manufacturers are doing their part, with more and more amp's including proper coupled feedback outputs. Now it's up to the radio manufacturers to add the necessary functionality for linearization.

73,

Scott
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Re: Transmitter composite noise

Postby K1LSB » Tue May 04, 2021 3:47 pm

Scott,

Can you please verify whether I am reading Section 5.3 in the article in the link below correctly, specifically that the IC-7610, FT2000 and FTdx101D all have rather poor SSB splatter at 2.5k offset when running 50 watts (see the table after the charts in that section)?

https://drive.google.com/file/d/1mrMCR2 ... sp=sharing

Thanks,

Mark
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Re: Transmitter composite noise

Postby w-u-2-o » Tue May 04, 2021 4:23 pm

Mark,

I can find no problem with your assessment. And there's no reason to single out those rigs. The best performing rig was listed at -40dBc. Based on the spectrum analyzer plots shown in the paper it would appear that this is measured in a 100Hz bandwidth, so -50dBc/Hz.

This is, of course, quite terrible, but also pretty much typical performance for any amateur HF transceiver. This includes ANAN-based openHPSDR radios like ours when PureSignal is not being used.

Now add an amplifier driven into significant compression, if not fully saturated most of the time, and that's when you become a candidate for the W1AEX Hall of Infamy.

As an aside, it is noteworthy somehow that even in an article that makes an attempt at scientific rigor, the authors still use amateur radio slang terminology, in this case "splatter", instead of the more proper terminology: IMD products.

73,

Scott
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Re: Transmitter composite noise

Postby K1LSB » Tue May 04, 2021 5:08 pm

Given that PureSignal is such a game-changer in terms of cleanness of signal, I'm at a complete loss to understand why none of the Big Three have yet incorporated the feature, especially given that the code is (per my understanding) open source.

Mark
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Re: Transmitter composite noise

Postby w-u-2-o » Tue May 04, 2021 6:24 pm

Mark,

Nobody has a hardware architecture that will support it, with the possible exception of Flex and perhaps such niche products as SunSDR.

Remember that you need a transceiver that provides full duplex transmit and receive, a path to that receiver when transmitting, and well controlled and understood path (delay) lengths on transmit and receive. Also, again other than the aforementioned SDRs, I think all of the radios rely wholly on firmware based processing which makes the development process much more complex and costly. Lastly, they would not be able to use the open source code without appropriate licensing, something which causes a great many intellectual property rights issues.

However, adaptive predistortion is no great secret. It is used in many applications, most notably cellular base station radios. It would not be difficult for a company that has the resources of, say, Icom, to develop their own implementation of the well known algorithms. This would avoid the IP rights issues.

What's more amazing is that neither Yaesu nor Kenwood have duplicated the Icom 7300. A little competition in that area would go a long way to improving the breed. But I suppose this is unsurprising. Many, perhaps most, hams are sticks in the mud. Heck, even in our community we see a small group who desperately wants the look and feel of a conventional radio, right down to a tiny display with low resolution. They could have done a fancy control surface with softkeys instead, and ergonomically that would have been superior, but it just didn't scratch the itch to have it all in one box.

73,

Scott

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