Intermodulation

distortion(IMD) &

PureSignal™BY NI0Z

Copyright Mark Abraham 2017

Purpose

Break down IMD distortion into high-level laymen's terms

Understand IMD correlation to the noisefloor

Understand what IMD looks like on a pandadpter and waterfall display

Review IMD significance to the bands welfare

Briefly review PureSignal™

Provide example showing PureSignal™ in action

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What is IMD Intermodulation distortion(IMD) is the amplitude modulation of signals containing

two or more different frequencies, caused by nonlinearities in a system.

Intermodulation is caused by non-linear behavior of the collective signal processing (physical equipment or even algorithms) being used.

Intermodulation is also rarely desirable in radio, as it creates unwanted spurious emissions, often in the form of sidebands. For radio transmissions this increases the occupied bandwidth, leading to adjacent channel interference, which can reduce audio clarity or increase spectrum usage.

IMD is only distinct from harmonic distortion in that the stimulus signal is different. The same nonlinear system will produce both THD (with a solitary sine wave input) and IMD (with more complex tones).

IMD is also distinct from intentional modulation (such as a frequency mixer in superheterodyne receivers) where signals to be modulated are presented to an intentional nonlinear element (multiplied).

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Source – excerpts from https://en.m.wikipedia.org/wiki/Intermodulation

Intermodulation distortion in audio is usually specified as the root mean square (RMS) value of the various sum-and-difference signals as

a percentage of the original signal's RMS voltage, although it may be specified in terms of individual component strengths, in decibels,

as is common with RF work.

Relevance of Noisefloors

A power level of 0 dBm corresponds

to a power of 1 milliwatt. A 10 dB

increase in level is equivalent to 10

times the power. A 3 dB increase in

level is approximately equivalent to

doubling the power, which means

that a level of 3 dBm corresponds

roughly to a power of 2 mW. For

each 3 dB decrease in level, the

power is reduced by about one half,

making −3 dBm correspond to a

power of about 0.5 mW.

More information on measurements

can be found here:

http://blog.prosig.com/2008/04/14/w

hat-is-db-noise-floor-dynamic-range/

All of it compounds into a base nosiefloor as seen to the left.

This example shows the noisefloor without an antenna applied.

This looks to be between -142dBm to -138dBm.

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In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored. In radio communication and

electronics, this may include thermal noise, black body, cosmic noise as well as atmospheric noise from distant thunderstorms and similar and any other unwanted man-made signals, sometimes referred to as incidental noise.

Relevance of Noisefloors & IMD

Signal-to-noise ratio (abbreviated SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise.

Once the antenna is applied, it brings in all the noise outside your shack as well as desired signals. When signals rise above the noisefloor, we can select and demodulate them into legible data or audio.

As you can see, this

raises our noisefloor

considerably.

The stronger a signal

the more it will rise

above a given hams

noisefloor.

Each hams noisefloor is

different.

Each radios noisefloor

differs.

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Improving NoisefloorsThere are several ways to improve ones

noise floor:

Isolate and mitigate unwanted

interference from appliances, bad power

sources, and other interference sources.

Use a better antenna system. (includes

Coax)

Use clean power sources, filter out local

AC noise if needed and or ground your

antenna and equipment:

http://kv5r.com/ham-radio/grounding-

and-receiver-noise/

Move to a quieter location ☺

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Locating unwanted signals on your panadapter and then chasing them down with a SW radio

Unwanted interference sources can be found and mititgated – Daughters LCD TV in RED

* Not all interference is manifested as noise or as an increase in the noisefloor, some appears as signals as demonstrated above.

IMD – Band Splatter

We select filter widths with which to receive and to transmit with. When someone's signal is poorly formed it will stray outside the set signal width.

On the left we see a filter size set with a red outline of an incoming signal displayed. Its top portion is narrowly displayed within the set filter size.

Its bottom portion near the noise floor is splattering outside the set filter width and demonstrates transmit distortion.

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IMD – Band Splatter

We are looking at 14,265 and the incoming

signal should typical stay within 14,265 and

14,268.

As seen in the yellow circle up closer, we can

see that the splatter bleeds over about 2K-3K hertz on each side, this this transmitted signal

is actually occupying 10K hertz.

If other hams were using 14, 261 or 14273 they

could be impacted by the bad distortion and

bleed over.

Some splatter can occupy several Khz of band space impacting multiple hams.

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Band Pollution 9

3K

Noisefloor A

14265Mhz 14265Mhz14260Mhz

Filter

Noisefloor B

Noisefloor C

SSB example

Amount of distortion

proportional to signal strength

and noisefloor level

Example of splatter and signal strength as it relates to different level

noisefloors, the better the noisefloor the more splatter may be observed.

IMD – Band Splatter

We can also see transmit distortion

on the waterfall.

In this example we could see that

it primarily occurred when the

transmission started and then

improved. That’s no uncommon.

Until the Warren came up with

PureSignal™ this sort of distortion

was and still is very common.

This is a very mild example.

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IMD – Band Splatter

In this example we can see bleedover as far as 10K wide! Again, this is a mild example,

however, width 1000’s of hams on the air at the same time, this can compile into a lot of

band pollution.

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IMD – Band Splatter

Today there are not many radio manufacturers that address transmit distortion and this sadly impacts the overall band welfare and state of band pollution.

Figuratively speaking, it impacts ever ham out there by making it more difficult to copy desirable signals.

It would be very beneficial to the past time for manufactures to start putting more effort into producing cleaner signals for both the transceivers and the amps.

These changes need to take place in both the hardware and software of the radios to effectively start cleaning them up.

In an ideal world, it would be awesome if all electronics manufactures had to pay attention to the noise and interference their products produced, of course this is very unlikely.

In astronomy though, they have been effective in getting local government's to implement some levels of light pollution control. Hawaii’s Big Island for example values lowering light pollution to better enable the telescopes at Mauna Kea Observatories to have better viewing conditions.

http://www2.ifa.hawaii.edu/newsletters/article.cfm?a=301&n=1

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What can we/you do?

We can continue our awareness and work to reduce band

pollution.

We can lobby radio and amp manufactures to care and understand we value improved distortion control.

We can put our money where our beliefs are at and buy radios that

have improved IMD Distortion control.

It should be noted that a poorly calibrated mic and or audio chain can also contribute to transmit distortion. If you do not have

PureSignal™ you can still make some impact by cleaning up your

transmitted audio.

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LDMOS RF Amplifier Linearization

The HPSDR team has worked to create and

LDMOS RF Amplifier based off a 50V power

supply.

This design change enables the transceivers

using it to produce cleaner signals.

This design allows for 200 Watts of output.

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In order to reduce the levels of noise and thereby improve the sensitivity of the radio receiver, the main element of the receiver that requires its performance to be optimized is the RF amplifier. The use of a low noise amplifier at the front end of the receiver will ensure that its performance will be maximized. Wither for use at microwaves or lower

frequencies, this RF amplifier is the chief element in determining the performance of the whole receiver. The next most important element is the first mixer.

In mathematics and physical sciences, a nonlinear system is a system in which the

change of the output is not proportional to the change of the input.[1]

PureSignal™

You can learn more about the details for PureSignal™ using the

following LINK: http://flarc.net/eme-info/PDF/N1JEZ-2.pdf

By deliberately creating distortion on the amplifier input signal we

can compare it in the output in near real-time and correct for it. By

correcting for the known distortion inserted in the input we also

correct for the unknow distortion in the output.

A feedback loop must be created for this where part of the output

signal is fed back into the input.

Apache-labs has added this capability and loop in its newest

models and it can also be added to its older models as well manually after market.

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Very High-level SSB Symbolic Example 16

AMPVoice Audio

Unwanted

Distortion

Injected Controlled

Distortion

Voice

AudioVoice

Audio

Reduced

Distortion

Feedback loop

Feedback loop

Corrects with

Feedback

Note: digital signals could also be used. Signals themselves can be a source of distortion. A complete transceiver

design in itself beyond the amplifier can also be a source of distortion and distortion is by no means limited to signal

processing.

This is an extremely over-

simplified example and loosely described how distortion is reduced.

Known distortion is injected in the front side of the amp and its modified result is analyzed on

the output, the differences noted and then the proper correction model is formed for all subsequent processing.

I encourage you to get a complete understanding of PureSignal™ .

Distortion Reduction Example 17SSB PS™example

3K

Noisefloor A

14265Mhz 14265Mhz14260Mhz

Filter

Noisefloor B

Noisefloor C

PS™ reduces the distortion

to signal strength

and its relationship

to noisefloor level

Filter3K

PS™

BEFORE AFTER

Note steeper signal skirts

Unwanted

Distortion

Reduced

Distortion

Over Simplfied Example of distortion reduction using PureSignal™. Note that the total signal width and strength have a relationship to the three noisefloor levels A, B & C that represent 3 different hams operating different locations and equipment. The bester a hams received signal strength and better noise floor the more they will be impacted by distortion.

PureSignal™

PureSignal™ is trademarked by Warren Pratt NR0V

You can learn more about the details for PureSignal™ using the following LINK: http://flarc.net/eme-info/PDF/N1JEZ-2.pdf

By deliberately creating distortion on the amplifier input signal we can compare it in the output in near real-time and correct for it. By correcting for the known distortion inserted in the input we also correct for the unknown distortion in the output.

A feedback loop must be created for this where part of the output signal is fed back into the input.

Apache-labs has added this capability and loop in its newest models and it can also be added to its older models as well manually after market.

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PureSignal™ In Action

PureSignal™ example using an Anan 8000 DLE.

To conduct the experiment I needed a distant receiver with

panadapter and waterfall view which would work for my test case.

K1RA has a receiver in Virginia online via sdr.hu that seemed perfect

for the test.

The results were rather interesting. I used the 8000DLE in and out of

PureSignal™ mode and then the EE MB1 as a control monitor to

have another reference SDR to compare results with.

The next few slides present the screenshots and results. The transmit audio on the Anan was recorded so that the same transmission

could be used for both on/off modes.

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PureSignal™ In Action

Slope

Note on the right the orange line showing the slope of the envelope as it enters the noise floor. Keep in mind that the noise floor here is about -130dBm. If the Noisefloor were better, say -150dBm or the signal were stronger and its knees/skirt (the signals wider part) started higher up then it easily could extend the set 3K filter width.

* dBm is used with regard to the tool and its method and reported result

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PureSignal™ In Action

SLOPE

Note on the right the orange line showing the slope of the envelope as it enters the noise floor. You can see the skirt slope is much deeper into the noisefloor and much more vertical meaning its continued trajectory for a user with a better noisefloor would still be far cleaner than without PureSignal™

* dBm is used with regard to the tool and its method and reported result

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PureSignal™ In Action

OFF ON Note where the PS

improved signal sits in

the noisefloor

Note the

increased

concentration of the

received signal at the

top of the signal

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Wrap-up

With the use of improved hardware in the amplifier state of the transceiver and NR0V’s PureSignal™ we can realize significant improvement in Transmit IMD.

Hams using cleaner transmitters are thus being better band citizens and reducing their footprint on the bands. Improved signals are received better on the distant end with grater clarity and power.

You can learn more about the details for PureSignal™ using the following LINK: http://flarc.net/eme-info/PDF/N1JEZ-2.pdf

If you own an Anan, this video can show you how to set it up on older models. LINK: https://youtu.be/0adHZOTqTlQ

Excellent video showing how PureSignal™ makes a difference. LINK: https://youtu.be/ztTHCb4MUv8

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