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Re: [Amps] Linearization problem

To: "'Manfred Mornhinweg'" <>, <>
Subject: Re: [Amps] Linearization problem
From: "Peter Voelpel" <>
Date: Fri, 5 Dec 2014 16:22:31 +0100
List-post: <">>
Yes it is just a sampler and very useful to provide the feed signal for pre
distortion with the Hermes board or ANAN transceiver.
But you might prefer to build up a DC6NY board.


-----Original Message-----
From: Amps [] On Behalf Of Manfred
Sent: Freitag, 5. Dezember 2014 15:18
Subject: Re: [Amps] Linearization problem


> I use a bird sampler

> es_15-35-MHz-Variable-RF-Signal-Samplers.aspx

That one is non-directional. It's a plain simple sampler, that will produce
output attenuated a certain amount from whatever is on the line. It will
absolutely no rejection of the distortion introduced by the amplifier stage
the drive signal!

You can do exactly as well by connecting a resistive or capacitive divider
the drive line.


> Maybe a band specific, power compliant isolator? 

That would be great, but the only idea I have about how to build isolators
HF results in something very impractical!

> A directional coupler could be used to detect the output of the
> transceiver before the isolator.

If an isolator is used, no directional coupler is needed. A simple divider 
connected to the line, between the driving transceiver and the isolator,
will do 

The problem is just how to make that isolator!


> Simple; Dougherty amplifier.  Two tubes one class "C" (positively
> the other class "A" (negatively modulated) many variations exist.

That has nothing to do at all with what I'm trying to do! A Dougherty
is relatively complex, and it's hard to splice the two amplifiers together
in a 
way that will result in low distortion. And the resulting efficiency isn't
great as to warrant all that complication. The approach I'm following is far


About amplifier classes, okay, let's then agree that some people call an 
amplifier "class B" when it is in fact class AB, but very close to B. In any

case, when I talk about a solid state class AB amplifier, I mean one having
a small idling current. Just enough to get reasonable linearity in the 
cross-over region. Unlike tetrodes, which are often run with a huge idling 
current to linearize the mid-level amplitude region too, in solid state amps
mid region is more linear to start with, and the final linearization is done

with negative feedback.


 > In the microwave world one would follow the coupler with a circulator and
 > the energy reflected from the amplifier would dump into a dummy load.

Yes, that would be the right thing to do at HF too, if there just was a 
circulator for HF!

 > Is there a circulator design made for HF?

  At HF it's surely simpler and cheaper to use several active buffer stages,

than a circulator.I have never seen one, at least not one having a size and
that makes sense. But I would love to get by without those buffer stages!


 > Instead of driving the amp with RF, I would suggest doing it digitally.

In my opinion, that's certainly the way to go, for any RF power amplifier
into a modern, DSP-based radio. I think that the HF radios we should be
are basically a DSP box, with a full-spectrum, wide dynamic range A/D
at the antenna input, and a 1.5kW, switching RF output stage, directly
driven by 
properly predistorted digital phase and amplitude signals. The conventional 
concept of a big, bulky, heavy add-on amplifier, to provide the final 12dB
gain, looks so lame in comparison!

But the fact is that there are millions of conventional 100W HF transceivers
there, and most hams want to keep using them. So we still need those ugly,
bulky, heavy, expensive add-on legal limit boxes. And what I'm trying to do
coming up with a way to make them significantly smaller, lighter, cheaper,
efficient, while at least maintaining decent IMD performance, or ideally 
improving it.

And for an add-on amplifier, digital driving just isn't very practical,
it means burning up the original analog 100W drive signal in a dummy load, 
dissecting a sample of it into its phase and amplitude components, then turn

them into digital signals, put them into a DSP, and then built the whole 
digitally driven amplifier! It's hugely more complex than most homebrewers


 > > And a bonus question: Do you think that the phase distortion in
 > > such a class-A-AB-C-E amplifier will be bad enough to still
 > > cause poor IMD performance, even while the envelope linearity
 > > is excellent?
 > >

 > Yes. Around 1980 I prototyped an amplifier aimed at improving
 > efficiency in a military hf manpack. The output stage was a
 > wideband push pull affair which behaved like class B at low drive
 > levels, moving towards class D at full output. A 'polar loop'
 > (Petrovic and Gosling, Bath University) applied amplitude feedback
 > via the PA bias supply and phase feedback via the synth PLL. With
 > either not connected, the output spectrum was very wide -

Thanks for that info, even if it's bad news to me! But I suspected it.

Several years ago, while experimenting with pure EER and with combined class

AB/F EER amplifiers, I ran into severe phase distortion problems. I finally
up that line of development.

My present hope was that by keeping the supply voltage constant, and
the gain just through the bias, it might be possible to get a phase
low enough to live with it. But your experience tells otherwise.

So, it looks like I should better forget this whole stuff.

 > as best
 > I remember, both loop bandwidths needed to be some 10's kHz to get
 > acceptable results.

Correction loops will always correct the distortion products that fall
the bandwidth of the correction loop, and they will _worsen_ and __create_ 
distortion products outside their bandwidth! So, the correct way to use 
correction loops is to make the correction loop's bandwidth larger than that
the amplifier's output bandpass filter. In ham practice, this forces the use
tuned amplifiers, and with pretty high Q networks.

Now I have to think if I can come up with a reasonably simply phase
modulator in 
the drive signal path, that would allow implementing a phase correction loop
addition to the bias-controlling amplitude correction loop.

It all is starting to look unreasonably complex. I mean, just to get 12dB
TX signal...

Last item in this post, for all those who suggested directional couplers to
my envelope signal: I did the theoretical analysis this morning. The result
that a directional coupler could indeed be used to provide a correct, 
undistorted envelope signal from the drive signal, even while the amplifier 
stage introduces a varying load and thus some distortion, but _only_ if the 
driving radio behaves like a perfect voltage source with a fixed series 
resistance! This is how signal sources are normally modeled, but
HF radios don't behave like that, at all! In practice, when the amplifier
puts a 
  non-constant load on the drive signal, the radio's output will vary in a
that is not how a perfect signal source with fixed internal resistance would
As a result, the drive signal would get distorted, at the output, input, and

side ports of the directional coupler!

Sure, an attenuator would reduce the distortion, at the cost of some drive 
power, but only by a few dB.

Do you want to see the effect yourself? Take any good SWR meter. They are 
basically directional couplers with built-in detectors. Connect the SWR
between your radio and an antenna tuner. Load the tuner with a dummy load. 
Adjust for 1:1 SWR. Then set the radio for a safe output power, say 10W, and
detune the tuner while watching the forward power indication. It will vary 
strongly! This indicates that the radio is not performing as a true 50 ohm 
signal source.

Putting any kind of directional coupler in place of that SWR meter, and my 
non-linear amp stage in place of that tuner and dummy load, would be the
situation. As my amp varies its input impedance depending on drive and bias,
sampled envelope would become untrue.

Okay. Project shelved. It was a nice dream.


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