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Author Topic: Better IMD please  (Read 66730 times)
VK4KDD
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Posts: 20




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« on: November 21, 2011, 12:18:59 AM »

Hi all,

I opened this topic about band filters, but hams started to discuss about a better transmitter instead Angry,
It appears to me that hams are not satisfied with their transmitters.
They want a better transmitter, not just better IM3 Huh, but also a clean local oscillator (phase noise). Because what's the use of a good receiver if we don't have any good transmitters to match it. What's the use of a good final, if the exiter (transmitter) is poor.
The best amplifier does not clean up a poor quality TX. A poor signal in means a poor signal out.
 
So apparently we need to discuss this. So here it is the topic ... Better IMD please  Grin. You can discuss here why you want better IM3, why you want a better transmitter Grin.
(edited 29 November 2011) 66 replies in just one week! This has to mean something.

73 Ron VK4DD

QUOTED >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Hi Dan

Publish your design.  When you do I will build  it. I am sick and tired of expensive transceivers whose IM performance stinks.   It will be a eye opening exercise for the ham community to see what is possible using advanced design techniques that are affordable. For too long now hams have been apologists for the transceiver manufacturers who have been designing terrible transmitters. Hopefully  designs like yours will be the beginning of a new era in clean transmitters on the ham bands.


Quote from: 2E0CHE on November 19, 2011, 07:09:45 AM
All too true, given a reasonable loaded Q, ATC 100B porcelain MLCC are just fine to at least UK legal limit (500V rating at sufficient current).  
The bigger ATC parts walk all over any doorknob type cap for HF and VHF performance in a 50 ohm network.

I do not understand this fixation with 12V base station radios, almost everything gets easier at 24 or 48V, and it is not like building a base station rig with whatever power rails suit is exactly hard.

Incidentally it has been my experience that most RF power devices (even ones sold as linear) have a linear operating region that stops short of their rated power, designing to work within this typically requires a few more devices but makes for a much cleaner (and far more reliable) amp.  

A cool DIY build, even if it is not the way I would have done it.

ZENKI, I have a solid state -69dB ref PEP IMD3, 160-6 @250W on the bench at the moment, it can be done, eventual target is adding a second 250W module to get 500W TPO for 400W at the feedpoint. The downside is that the rig is not small, but will do that brick on key for as long as there is power.
Yes, I know I need to pass the advanced exam, not a big deal, just looking for a good opportunity that fits with my schedule.

Regards, Dan.

END QUOTED >>>>>>>>>>>>>>>>>>>>>>>>>>>
« Last Edit: December 13, 2011, 01:25:31 PM by VK4KDD » Logged
ZS5WC
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Posts: 412


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« Reply #1 on: November 21, 2011, 01:19:51 AM »

 Huh

I agree , give us a decent sized, reasoneably priced base station rig with built in PSU, use HV fets in the PA and run with dynamic bias in the device 'sweet spot'.
Some of the high end 12v rigs out there are just Shocking in the IMD department-especially the ones using 12v FET devices--(All the latest 100w rigs use the same..)

It gets so bad on 20m when one of these are coupled to a linear, that I cannot operate 50kc/s either side of the splattering station, even when he is over 1500km away locally in ZS!.

I also think a 400w well performing , self contained base set will be just what the doctor ordered--no need for a linear then.
(I know Yeasu makes a 400w FTDX9000mp model--most of us cannot afford it though..)
I personally would like to see a rig like the TS-590s with a 400w QRO output and inbuilt psu-who needs all the TV screens and bling anyway?..
(also, hallicrafters had a 2000watt INPUT TRX in the 60's already!)
I see new devices out there where a single push-pull device will deliver over a kilowatt, ie one package =1KW.(And some can run on 250v DC.

Seems all the development is in the RX and DSP-none in the PA department!.
73 de William, ZS4L
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ZENKI
Member

Posts: 989




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« Reply #2 on: November 21, 2011, 01:26:54 AM »

Well you want to see how bad radios are in terms of their IMD performance, look at the ARRL reviews of the Ten TEc Eagle and the Flexradio1500. The figures are dismal to say the least.

2 tone IMD measurements are not the ideal way to characterize the splatter performance of transmitters. Its actually better to use white noise, multi tones or your real voice to do the test. A dynamic voice test of a transmitter will tell you a lot more about how transmitters performs  in the real world in regards to their splatter suppression performance.

For the sake of simplicity  high 3rd   order figures and ensuring high levels of suppression of the high order products is a simple way of ensuring that most radios will not splatter. This assumes of course that there is no  ALC design issues.


SM5BSZ has considered this topic and his web page makes excellent reading. He shows  the measured spectrum of a OLD radio like the TS520 and how it excels in producing a clean signal. He goes onto to say that modern radios should produce a signal like this old radio but they  dont.

http://www.sm5bsz.com/dynrange/dubus204/dubus204.htm

We wont get this matter resolved in the amateur service until  all transmitters have legislated transmitter IMD performance like all commercial HF equipment. It would be easy for most current amateur  equipment to pass this test if the manufacturers were made to do so. While they are not being made to comply with these standards nothing much will done about improving ham transmitter IMD.
Hi all, I. opened a topic about band filters,  but a experts like to discuss IM3 and how good or. bad their radios are.  So here is your oppertunity...  write here and delete your off. topic messages from the bandfilter. That makes it easier to find your discussions.

QUOTED >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Hi Dan

Publish your design.  When you do I will build  it. I am sick and tired of expensive transceivers whose IM performance stinks.   It will be a eye opening exercise for the ham community to see what is possible using advanced design techniques that are affordable. For too long now hams have been apologists for the transceiver manufacturers who have been designing terrible transmitters. Hopefully  designs like yours will be the beginning of a new era in clean transmitters on the ham bands.


Quote from: 2E0CHE on November 19, 2011, 07:09:45 AM
All too true, given a reasonable loaded Q, ATC 100B porcelain MLCC are just fine to at least UK legal limit (500V rating at sufficient current).  
The bigger ATC parts walk all over any doorknob type cap for HF and VHF performance in a 50 ohm network.

I do not understand this fixation with 12V base station radios, almost everything gets easier at 24 or 48V, and it is not like building a base station rig with whatever power rails suit is exactly hard.

Incidentally it has been my experience that most RF power devices (even ones sold as linear) have a linear operating region that stops short of their rated power, designing to work within this typically requires a few more devices but makes for a much cleaner (and far more reliable) amp.  

A cool DIY build, even if it is not the way I would have done it.

ZENKI, I have a solid state -69dB ref PEP IMD3, 160-6 @250W on the bench at the moment, it can be done, eventual target is adding a second 250W module to get 500W TPO for 400W at the feedpoint. The downside is that the rig is not small, but will do that brick on key for as long as there is power.
Yes, I know I need to pass the advanced exam, not a big deal, just looking for a good opportunity that fits with my schedule.

Regards, Dan.

END QUOTED >>>>>>>>>>>>>>>>>>>>>>>>>>>
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ZENKI
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Posts: 989




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« Reply #3 on: November 21, 2011, 01:33:39 AM »

Its very disturbing hearing splatter from many thousands of kilometers away. Some ESSB stations are so bad that even when the band is dead quiet  and their  signals are a low S1 or S2, I can hear their IMD rumble up the band. I can only imagine what damage they would cause to other stations on short skip within their own country.

Fortunately for most of us  most hams have reasonable clean signals. Its just the minority who are using a combination of poor equipment and operating practices that causes misery. Now that many new hams are being recruited from the CB band their past operating procedures are contributing to the misery. The excessive use  of mic gain and ALC and practices such as using CB amplifiers all  contribute to the mess. Its going to be impossible to clean this mess up once it becomes entrenched.

Huh

I agree , give us a decent sized, reasoneably priced base station rig with built in PSU, use HV fets in the PA and run with dynamic bias in the device 'sweet spot'.
Some of the high end 12v rigs out there are just Shocking in the IMD department-especially the ones using 12v FET devices--(All the latest 100w rigs use the same..)

It gets so bad on 20m when one of these are coupled to a linear, that I cannot operate 50kc/s either side of the splattering station, even when he is over 1500km away locally in ZS!.

I also think a 400w well performing , self contained base set will be just what the doctor ordered--no need for a linear then.
(I know Yeasu makes a 400w FTDX9000mp model--most of us cannot afford it though..)
I personally would like to see a rig like the TS-590s with a 400w QRO output and inbuilt psu-who needs all the TV screens and bling anyway?..
(also, hallicrafters had a 2000watt INPUT TRX in the 60's already!)
I see new devices out there where a single push-pull device will deliver over a kilowatt, ie one package =1KW.(And some can run on 250v DC.

Seems all the development is in the RX and DSP-none in the PA department!.
73 de William, ZS4L

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W8JI
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« Reply #4 on: November 21, 2011, 03:38:20 AM »

2 tone IMD measurements are not the ideal way to characterize the splatter performance of transmitters. Its actually better to use white noise, multi tones or your real voice to do the test. A dynamic voice test of a transmitter will tell you a lot more about how transmitters performs  in the real world in regards to their splatter suppression performance.

I strongly disagree with a white noise test, because it does not test varying dynamics associated with speech. It is a constant load, and will mask power supply, bias, and ALC issues.

I agree about ESSB. Such profound lack of concern for other people on the band is disconcerting.
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N2EY
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Posts: 3925




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« Reply #5 on: November 21, 2011, 04:24:25 AM »

give us a decent sized, reasoneably priced base station rig with built in PSU, use HV fets in the PA and run with dynamic bias in the device 'sweet spot'.

Hold that thought....

Quote

I also think a 400w well performing , self contained base set will be just what the doctor ordered--no need for a linear then.

That one too....

Quote
(I know Yeasu makes a 400w FTDX9000mp model--most of us cannot afford it though..)
I personally would like to see a rig like the TS-590s with a 400w QRO output and inbuilt psu-who needs all the TV screens and bling anyway?..

The problem is that the PA and PSU things cost big money. Display screens and processor-based features don't.

Look at how much transceivers for commercial services cost, and think about how many hams could afford them.

Quote
(also, hallicrafters had a 2000watt INPUT TRX in the 60's already!)

The SR-2000 Hurricane, basically an SR-400 with a pair of 8122s in the final. It cost $1095 in 1965, which inflates to about $7500 today. That's without the power supply. My guess is the power supply would bring the price to at least $10,000 in today's money.

There was also the NCX-1000...

The reason amateur transceivers are built the way they are when it comes to IMD is that the alternatives just cost too much. The reason for the 12 volt devices is so they can operate off a 12 volt battery - even though most of them never will. (And the top-end ones don't use 12 volt finals).


73 de Jim, N2EY
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M0HCN
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Posts: 473




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« Reply #6 on: November 21, 2011, 06:39:31 AM »

Sounds like a modest boost converter in there would still allow 12V operation but would allow the use of sane finals (Not that I understand the 12V thing, as you say these radios are not often mobile).
 
My lashup is not yet at a publishable state (Lots of dead bug construction on bits of PCB), and I am still working on improving PA efficiency (Mostly DSP software tweaks), but eventually I might hawk it around Radcom/QEX/Dubus or the like. I want to see if I can move the IF up to say 70Mhz first as that would reduce the number of mixers and filters required and would generally simplify things.

At the moment the PA is actually a very pedestrian BLW50F/BLW96 as two push pull pairs, a real blast from the past, but that could easily be replaced with something more modern. The secret is in a couple of extra mixers and a pair of AD9951 DDS chips at the first IF, together with a power sampler at the output and some very low noise opamps.

Regards, Dan.
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K4EJQ
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« Reply #7 on: November 21, 2011, 09:07:09 AM »

Ron, Dan, et al;

I applaud your desire to attempt to "clean up" our bands by improving the IMD specs. of our transceivers/transmitters.  As you know, taking the time to design, build and tweak the equipment is going to push the costs up even more. As someone pointed out, and rightly so, manufacturers would rather just add the "glitz", i.e. monitor screens ,etc as they are cheaper .

What with our current economic state, worldwide, I can't see that many hams willing to fork out even bigger bucks for "cleaner" operating equipment. I know it's sad, but the name of the game is "Bang for the Buck" which equates to" Dollars for Watts". Add to this the current lack of knowledge required to operate said equipment legally and properly and you can see there is more to this than better equipment design. In the long run this will require changes on several fronts. , not the least-education!!

My two cents ..

73, Bunky, K4EJQ
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M0HCN
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« Reply #8 on: November 21, 2011, 09:26:44 AM »

Well, most folks are still running 100W class radios, and I don't really see that changing much.

It may be possible to trade a modest amount of DSP and small signal electronics for improved IMD AND better PA efficiency which means that a smaller heatsink could be used. If this can be pushed far enough it may be that the reduction in thermal management cost can pay for the improved signal processing.

Consider that many modern radios have extensive DSP on board that is mostly used on receive, there should be spare cycles available key down to implement the loop control stuff.

Doing the I/Q generation in DSP allows for neat tricks like feed forward control of the PA supply voltage (and bias voltage) to improve efficiency without significantly compromising IMD in the closed loop case.  Given that the mean power for SSB is usually something like 10dB down on PEP, that is a large efficiency gain.

A large part of the trick to designing anything is to minimize (and ideally eliminate) the need to tweak things, tweaking costs money!

Of course, TV screens sell more radios then minor unimportant things like good quality mixers and linear amplifiers that are actually linear and that turn the majority of the DC into RF instead of heat.....

Regards, Dan.
« Last Edit: November 21, 2011, 09:30:18 AM by 2E0CHE » Logged
W9PMZ
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« Reply #9 on: November 21, 2011, 10:29:30 AM »

"Doing the I/Q generation in DSP allows for neat tricks like feed forward control of the PA supply voltage (and bias voltage) to improve efficiency without significantly compromising IMD in the closed loop case.  Given that the mean power for SSB is usually something like 10dB down on PEP, that is a large efficiency gain. "

My company does this for products that we sell to cellular operators.

Running open loop without correction for CDMA TPAs 4MHz away from the main signal the power is typically 30dB below the main signal.  Turning on correction results in an improvement in 20dB to 30dB.

My guess it is a matter of cost to implement this...

73,

Carl - W9PMZ
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W9PMZ
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« Reply #10 on: November 21, 2011, 10:31:45 AM »

google "digital predistortion".

73,

Carl - W9PMZ
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M0HCN
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« Reply #11 on: November 21, 2011, 11:08:59 AM »

I am doing the cartesian loop in the analogue domain, with the PA bias and power supply feedforward  being open loop and relying on some 40db of cartesian loop error amplifier gain to linearize everything, so really a sort of cartesian/EER hybrid. The DDS pair used for the variable phase shift LO are of course digital and the processor handles measuring the phase shifts and tuning the loop, but the basic loop itself is all analogue.
Given the better then one decade tuning range inherent to a amateur HF set, the LO phase does tend to swing quite as you tune, but such things are eeproms made for.

The modulation feedforward stuff is really because the closed loop IMD with a conventional PA is actually far better then it needs to be, so I can afford to trade 10dB IMD for better efficiency and still embarrass the commercial ham stuff.

The tetra crowd seem to like cartesian loops in their handsets, and it really is not that expensive to do, my major sticking point is that to get the out of channel noise floor down takes really quiet opamps, I am starting to wonder about coming out of the DSP at maybe a megacycle just to get away from the low frequency 1/f crap. 

Regards, Dan.
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VK4DD
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« Reply #12 on: November 21, 2011, 11:56:22 AM »

Ron, Dan, et al;

I applaud your desire to attempt to "clean up" our bands by improving the IMD specs. of our transceivers/transmitters.  As you know, taking the time to design, build and tweak the equipment is going to push the costs up even more. As someone pointed out, and rightly so, manufacturers would rather just add the "glitz", i.e. monitor screens ,etc as they are cheaper .

What with our current economic state, worldwide, I can't see that many hams willing to fork out even bigger bucks for "cleaner" operating equipment. I know it's sad, but the name of the game is "Bang for the Buck" which equates to" Dollars for Watts". Add to this the current lack of knowledge required to operate said equipment legally and properly and you can see there is more to this than better equipment design. In the long run this will require changes on several fronts. , not the least-education!!

My two cents ..

73, Bunky, K4EJQ

Yes that's realistic, for the majority of the radios... unless offcourse we have a break through in technology and these things made cheap in mass.

There is a group who has got absolutely no idea what IMD is. I like to think that it is a small group,  but that might be too optimistic.
The different back grounds, I mean the diffeerent technical backgrounds between hams are huge these days. On one hand we have the operator whlch got his ham radio licence in the supermarket..and on the other hand the people who design new technology.

I solder connectors on coax cables for folks, that is difficult enough for truck loads of hams.
If you tell them that their rig which they bought for 1200 bugs stinks... than you got a real problem. So I wont. Lets face it we are a group of idealists, who know that rigs were better 30/40 years ago. We cant just expect that everyone has the $$ to buy the top of the line Amplifier and top of the line Radio.

Look its nice to dream about better IMD, I would build it if I could. But that's all we can do... dream about it. There is nothing wrong with that? We are allowed to discuss this?

The predistortion and all these DSP techniques sound really cool.
Technology is intresting.. isn't it? That why I think I am a ham.

73 Ron VK4DD






« Last Edit: November 21, 2011, 12:03:56 PM by VK4DD » Logged
K1ZJH
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« Reply #13 on: November 21, 2011, 12:06:27 PM »

   
If you tell them that their rig which they bought for 1200 bugs stinks... than you got a real problem. So I wont. Lets face it we are a group of idealists, who know that rigs were better 30/40 years ago.  73 Ron VK4DD


You didn't see a lot of discussion about IMD in the olden days. And many
of the early SSB rigs had some horrid IMD specs. Especially ones
with overdriven sweep tubes in the PA.

The only way things will improve is if magazines publish honest reviews,
and if hams are educated enough, and willing enough, to walk away from
a product that isn't quite what it should be.

Legislating new regulations wouldn't do it, since no enforces the laws and
only the honest vendors and manufacturers will suffer at the expense of the
illegally imported garbage, which is apparently above the law and below
the radar of the FCC.   

Pete
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W8JI
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« Reply #14 on: November 21, 2011, 12:16:13 PM »



The only way things will improve is if magazines publish honest reviews,
and if hams are educated enough, and willing enough, to walk away from
a product that isn't quite what it should be.

Pete
[/quote]



Mhh thats like pointing the gun at their own head.
They publish about their advertisers.

Ron.


Actually that is not true.

Most of the problem is magazines are just technically not up to speed. QST, for example, just started to do a better review process. CQ, as far as I know, has no real review process at all.

I can assure you 100%, based on direct experience, QST does not give manufacturers any special slack. They will not even correct mistakes properly, they bury them!



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