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Author Topic: High level modulation  (Read 1937 times)
KB1GTX
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Posts: 462




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« on: June 13, 2012, 10:55:52 PM »

I'm going to be playing with high level modulating a http://www.rfparts.com/pdf_docs/SAV/SAV17.pdf.

The question is I've seen some circuits that modulate the driver stage along with the last stage?
Any reason for doing that?

Dave
« Last Edit: June 13, 2012, 10:58:08 PM by KB1GTX » Logged
WX7G
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« Reply #1 on: June 14, 2012, 12:56:15 AM »

High level modulation of AM is done to increase efficiency. High level modulation of FM is not done.
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TANAKASAN
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« Reply #2 on: June 14, 2012, 03:36:34 AM »

Interesting.

If you look at the third graph you will see that the increase in power is quite linear when Vcon is varied between about 3V and 6.5V. So, use an op amp to provide some DC bias to (say) 4.5V then modulate this. I would be interesting to see the results on a spectrum analyzer.

Note: DO NOT overdrive a module like this or it will self destruct.

Tanakasan
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G8JNJ
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« Reply #3 on: June 14, 2012, 06:16:30 AM »

The last graph shows a much more linear curve near full output power when Vcon = Vcc.

If you only wish to run upto about 50W then just use Vcon (which is easy - bias at around 4.5v as stated before). If you want full output power then you would need a modulated high'ish' current regulator circuit (which is much more complex).

Personally I'd stick with 50w output and Vcon control. You are less likely to 'let out the magic smoke' this way too, as the device isn't really designed to be used in this way.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com

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WB2WIK
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« Reply #4 on: June 14, 2012, 08:55:30 AM »

Looks to me like with Vcc = 12.5V, it's only linear to about 300-400 mW drive and then goes into compression.  I'd limit carrier power to about 12W output to allow for full modulation without clipping.

Never tried one of these on AM, let us know how it works out.
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KB1GTX
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« Reply #5 on: June 14, 2012, 09:11:09 AM »

High level modulation of AM is done to increase efficiency. High level modulation of FM is not done.

It's for AM, modulated with a transformer and an amp.
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K4FMX
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Posts: 17




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« Reply #6 on: June 14, 2012, 10:11:50 AM »

I'm going to be playing with high level modulating a http://www.rfparts.com/pdf_docs/SAV/SAV17.pdf.

The question is I've seen some circuits that modulate the driver stage along with the last stage?
Any reason for doing that?

Dave

This may not work well trying to  modulate the collector as in a class C amplifier. I believe that is what you are talking about doing. The conduction angle may be too great (not far enough into class C) to modulate properly.
The modulated amplifier must be operating non linear in order to modulate it. If it is operating in a linear mode it will not modulate.

Sometimes in tube transmitters the driver stage is modulated a small amount along with the output stage. Some tubes do not plate modulate as well as others and adding modulation to the driver stage helps achieve full modulation.

This chip looks as though it is intended as an amplifier for FM signals and you may not be able to control the drive properly in order to properly modulate it.

73
Gary  K4FMX
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AA4PB
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« Reply #7 on: June 14, 2012, 10:20:56 AM »

Normally, a power amp used for FM would be non-linear because there are no amplitude changes in FM. I doubt that they would use a linear PA for FM because it would be lower efficiency and provide no benefit.
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WA3SKN
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« Reply #8 on: June 14, 2012, 11:29:08 AM »

Modulating the final and driver was done when using old class C finals.  If you are building or working an old tube rig consider it.  If you are using a newer rig in AM mode, do not.
73s.

-Mike.
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