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Author Topic: Wanted RM Italy amp for test  (Read 4757 times)
W8JI
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« Reply #15 on: March 17, 2010, 07:24:23 AM »

If your going to look at things on a system level, what about the antenna? If it's highly reactive won't that effect the IMD figures? How would properties of IMD in a FCC accepted exciter differ from radio to radio in such a wasy as to cause abnormal IMD in the amp?  just curious.....

Any deviation in load impedance affects IM in a pre-tuned or fixed tune amplifier. This is why even if a radio does not blow up or shut down, it is still a good idea to have a close impedance match.

If the power amplifier section has an adjustable network and the network can be adjusted to rated currents, load impedance does not matter.

As for exciters or transceivers, they are not required to be tested or approved for IMD performance. If they were tested, most of them would actually fail FCC specifications for spurious and harmonics. We really just depend on the manufacturer to have a reasonable design and ratings, and the operator to understand what to do and not do.
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N9MXY
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« Reply #16 on: March 18, 2010, 08:52:26 AM »

Quote
Any deviation in load impedance affects IM in a pre-tuned or fixed tune amplifier. This is why even if a radio does not blow up or shut down, it is still a good idea to have a close impedance match.

That's what I figured, so as Transistor amps become more common poor antenna matches will most likely result in higher IMD more often. 

Quote
As for exciters or transceivers, they are not required to be tested or approved for IMD performance. If they were tested, most of them would actually fail FCC specifications for spurious and harmonics. We really just depend on the manufacturer to have a reasonable design and ratings, and the operator to understand what to do and not do.


I thought all exciters had to be at least 35db down to be sold in the U.S.
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KM3F
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« Reply #17 on: March 18, 2010, 11:00:27 AM »

I believe that most transcievers in the last 15 years or longer are designed to run very conservitive in power output and to fold back above a 3 to 1 match, helping to limit the IMD to reasonable levels even if the match is not good, as well as offering final stage protection from excessive high RF voltages as a result of mismatch.
Many new hams today have a problem loading an older tube rig  as it is (I see the question being asked many tmes) so it's being 'taken care of' for them in the solid state rigs overall operating design.
Ameritron says in their operators manuel for the AL80B to 'over load' about 2 to 5% drop in power to lower IMD products.
With a solid state rig or amplifier, most cannot do this.
This is one reason to have an antenna match below about 2 to 1 for todays equipment and not just say don't worry about it if it is 3 to 1 or greater as well as using ALC back to the transciver.
Looking at it another way; the final amplifier is an equivalent resistance that 'swings' over a range with voice drive (SSB) and still must be impedence transformed to a 'fixed' output load impedence.
This also happens between a transciever and driving the input of an amplifier. That alone can generate IMD in 2 places that  may not be possible to eleminate so it is kept to a min by restricting overall input/output range of both the driver and the amplifier.
Out side of things, we must also do our part to run clean by offering the transmitter/driver etc the best conditions to work into.
Any equipment that shorts these considerations; may give you QRM the next time they are heard on the air.
Yes I know, you may be told it's your reciever but the station telling you cannot hear his own signal or care.
Any crap spread accross the band as spurs cannot  be filtered out no matter how good the reciever is.
Be sure you make that point to the offender since your reciever usually does not 'create interference on it's own from a 'single' signal, excluding mixing with another. So be carefull about internal reciever IMD such as from a noise blanker etc.
There is more to all this so be open minded to learn.
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W8JI
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« Reply #18 on: March 18, 2010, 04:59:48 PM »

I believe that most transcievers in the last 15 years or longer are designed to run very conservitive in power output and to fold back above a 3 to 1 match, helping to limit the IMD to reasonable levels even if the match is not good, as well as offering final stage protection from excessive high RF voltages as a result of mismatch.

I haven't tested many radios past their limit, but I have tested a few. I tested a few FT1000MP's because someone I know has the habit of turning up the internal power limit pot to get more power to drive his amps.

A Yaesu FT1000MP typically saturates at 150 watts, that's where very severe gain compression starts.  The MP is only in the low to mid -30's IMD range at 100 watts into 50 ohms. When it is bumped to 120 watts or so, even into 50 ohms, it is in the -25 dB third order range. On the bands I checked... at lower load impedances they got better on IMD, at higher load impedances they were worse.

My question is, how do we know the load mismatch is in a direction that makes things better or makes things worse?

This is why I say:

1.) Make sure SWR is low if you want maximum transmitter IM purity

2.) Do not crank the power pot inside the radio up. they really have no headroom for distortion.

This is why we use 1000MP MKV's, which can run 200 watts. We run them at 100-150 watts.
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VR2AX
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« Reply #19 on: March 18, 2010, 06:33:38 PM »

Tom,

According to Adam Farson, for a solid state amp "...IMD degrades quite rapidly if the design power rating is exceeded (IMD3 rises by 3 dB, and IMD5 by 5 dB, for every 1 dB increase in Po over rated output)..."

http://www.ab4oj.com/quadra/sshfamp.html

I don't know if that formula works backwards, but suspect it may not.

73,

Wyn
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W9PMZ
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« Reply #20 on: March 18, 2010, 09:12:06 PM »

"but IMD can add or subtract when multiple sources "

Reminds me of a past product that I implemented manufactuirng test for.

The product was a broad band RF system that carried multiple channels of cellular telephony.

In this case the test was a 16 tone test and the tone generator (all 16 tones) had the capability to move the phase independently.  So moving the tones (RF carriers) in phase you could obtain a really nice picture (e.g. low IMD products) on the spectrum analyzer, or obtain a really poor view (e.g. high IMD products).

73,

Carl - W9PMZ
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W8JI
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« Reply #21 on: March 19, 2010, 06:12:01 AM »

Tom,

According to Adam Farson, for a solid state amp "...IMD degrades quite rapidly if the design power rating is exceeded (IMD3 rises by 3 dB, and IMD5 by 5 dB, for every 1 dB increase in Po over rated output)..."

http://www.ab4oj.com/quadra/sshfamp.html

I don't know if that formula works backwards, but suspect it may not.

73,

Wyn

It does not work backwards because the transfer function shape is different as saturation is approached. It isn't always accurate moving forward either. It really is all about transfer function.
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N9MXY
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« Reply #22 on: March 19, 2010, 03:43:14 PM »

Quote
It does not work backwards because the transfer function shape is different as saturation is approached. It isn't always accurate moving forward either. It really is all about transfer function.


Sounds like a case for Bob Carver. Wink
 
 
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KF6QEX
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« Reply #23 on: March 19, 2010, 05:02:24 PM »

A Bob Carver HF amp....now that would be something!!!!
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W8JI
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« Reply #24 on: March 19, 2010, 07:34:02 PM »

I have results.

We do not have to guess.

http://www.w8ji.com/rm_hla-150_test.htm
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N9MXY
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« Reply #25 on: March 20, 2010, 04:32:32 PM »

Bob is on ebay now, selling some pretty cool stuff!

Tom, when you get a "real" power supply that can handle 8 2SC2879's you can test mine. :-D
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