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Author Topic: The IMD Debate: Please Contribute :)  (Read 1387 times)
VK3BL
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« on: November 29, 2017, 12:20:31 AM »

IMD is such a loaded topic.  

I have personally measured my ANAN 100D at -55 IMD3 with the SDR Play (which is actually pretty decently calibrated), but had a friend down the road report my 'real' IMD3 as -30.

Why? Distortion was being generated in either his antenna system or my own.

I found evidence of intermittent arcing in an N Connector used outside, and promptly replaced it; the fact of the matter is I intend to move to 7/16 DIN for outdoor use when any length of coax feed line is subject to a vSWR above 5:1; it is very easy to overlook the fact that voice peaks in SSB can generate very high voltages on a transmission line, even if the loss is not worth worrying about due to the length.

In my case, I was feeding ladder line with a 10 ft section of LDF-450; I couldn't care less about the loss, but the vSWR was enough to intermittently cause arcing at the balun junction point N connector.  

Even at 400 Watts PEP on a 50 ohm line, a 1:1 vSWR will result in peaks of 141 volts.  Multiply this by 5 (vSWR 5:1), and you have 700 volt peaks; its easy to see how a dirty, broken, slightly wet or frankly cheap connector can cause rectification via arcing.

QRO and insufficient / poor choice of connectors should get equally as much attention in 'all knobs to the right' syndrome, IMHO.

On another occasion my 'monitoring receiver' was reporting an IMD3 of -20.  I tore my antenna system apart looking for a fault, only to find that the antenna attached to the SMA connector on the SDR Play had come loose... The second I tightened it back up, everything was rosy.  

There is a reason professional grade connector manufacturers offer specification sheets and torque calibrated tools.

We hams need to start paying attention to these things if we want to put out clean signals at high power; rather than just assuming 'all knobs are to the right' or 'tetrodes are always dirty'.  We can and should help each other.

The first thing we should start doing is STOP dropping the v from vSWR.
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J.D. Mitchell - VK3BL / XU7AGA - http://vk3bl.wordpress.com
N8FVJ
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« Reply #1 on: November 29, 2017, 06:20:28 AM »

You have to start out with a clean IMD transmitter. The new Kenwood TS-590SG is very clean at -42dB 3rd order. The Flex transceivers are also very good. Most Icom & Yaesu radios are poor in this regard. The IC-7300 is only -30dB third order.
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VK3BL
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« Reply #2 on: November 29, 2017, 12:24:32 PM »

You have to start out with a clean IMD transmitter. The new Kenwood TS-590SG is very clean at -42dB 3rd order. The Flex transceivers are also very good. Most Icom & Yaesu radios are poor in this regard. The IC-7300 is only -30dB third order.

Yes of course, such as one needs a fast car to win the Grand Prix.

My point really was you still need to learn how to drive fast.


Re: IC-7300.

I could get ~35dB down on peak voice tone in 16 sample FFT Averaged SSB use; thats pretty clean in my books.  The trick is, as Tom W8JI points out, don't boost your bass and treble.  On the other hand, no matter what I did, I couldn't get it worse than -30dB down IMD3. 

Bear in mind I am talking real world, peak voice tone vs out of band distortion product, not two tone, "lab testing".
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J.D. Mitchell - VK3BL / XU7AGA - http://vk3bl.wordpress.com
AC7CW
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« Reply #3 on: December 02, 2017, 10:22:15 AM »

I never seriously thought about antenna/feedline voltage until I investigated magloops. Voltages can run in the thousands. Vacuum variable capacitors get onto the parts lists and thank goodness for Russian surplus on eBay...

95% of all failed electronics things in general are due to electromechanical failures. That points to the idea that the aforementioned need for attention to connectors is indeed a good thing, not to mention switches and anything outdoors in wind and weather.

I guess that if I'm on the air with my club remote station I can use my SDR radio's bandscope to check the signal quality... should be fun, no end to the fun in this hobby.
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Novice 1958, 20WPM Extra now... (and get off my lawn)
DL8OV
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« Reply #4 on: December 07, 2017, 12:47:11 PM »

I think we may be getting lost in the abbreviation soup here.

IMD are the close in unwanted products, normally seen at your carrier frequency +/- your modulation frequency. If I want to check for IMD then my spectrum analyzer normally has a sweep width of 50 KHz. A signal with good IMD looks like a church steeple whilst a bad signal looks like an airplane wing.

If you want to built a transmitter with good IMD then you need a holistic approach and look at every step in the signal chain from the microphone to the low-pass filters, this takes time and it takes a lot of patience. After three years of work I have a transmit chain which I am happy with but I doubt that any of the big box manufacturers would bother.

Poor contacts on connectors or antenna switches will act like diodes and cause harmonics. This MIGHT affect your IMD but the results of a poor antenna connection will be seen over many MHz, I've personally seen 9th harmonics from a 14 MHz signal, that's nearly 130 MHz!

So, poor contacts, arcing, are we talking about harmonics or IMD here?

Peter DL8OV
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VK3BL
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« Reply #5 on: December 09, 2017, 04:09:05 PM »


So, poor contacts, arcing, are we talking about harmonics or IMD here?

Peter DL8OV

Both, if a poor antenna contact acts as a nonlinear system component, it by definition is capable of producing IMD.
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J.D. Mitchell - VK3BL / XU7AGA - http://vk3bl.wordpress.com
WB4SPT
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« Reply #6 on: December 15, 2017, 12:03:11 PM »

Even at 400 Watts PEP on a 50 ohm line, a 1:1 vSWR will result in peaks of 141 volts.  Multiply this by 5 (vSWR 5:1), and you have 700 volt peaks;

We hams need to start paying attention to these things...
The first thing we should start doing is STOP dropping the v from vSWR.

That's not how the math works on VSWR.  Or ISWR either.   Grin

And, "V" is capitalized...
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AF6LJ
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« Reply #7 on: December 15, 2017, 12:42:15 PM »

This is an interesting thread.
Measuring IMD isn't as straight forward as most would like to think. SDRs may not be the ideal choice for those measurements and one must be careful using an analogue spectrum analyzer making such measurements as they can be overloaded well before first mixer overload occurs.
I've done plenty of measurements with analogue analyzers, enough to know that how the test environment is set up is way too important to take casually. I would assume that the same holds true for the modern digital analyzers. As far as SDRs goes...
To put it bluntly; making any claims based on what a computer A to D, or a 10 or 12 bot run of the mill ADC presents is scary to say the least. The A-D subsystems built into modern analyzers are far more trustworthy.
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Take Care
Sue,
AF6LJ
G3RZP
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« Reply #8 on: Yesterday at 01:12:27 AM »

WB4SPT is correct.

Take a lossless 50 ohm line and leave the end open or put a short on it. Feed a 100 volt peak sine wave into it, and because the SWR is infinite, the 100 volts are reflected. If the reflected volts add in phase, you then get 200 volts peak. In other places along that transmission line where the forward and reflected volts are in anti-phase, you get zero volts.
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