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Author Topic: Envelope feedforward for amplifier efficiency?  (Read 2479 times)
M0HCN
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« on: June 11, 2011, 02:24:38 PM »

Hi all, I have been hacking about with a cartesian loop transmitter design (Not anything like as hard as it used to be!), and the following thought occurred....

If a rig has a DSP modulation processor as many do these days then it should be almost trivial to delay the I and Q modulation outputs from the DSP by a millisecond or so and to output an undelayed voltage representing sqrt (I^2 + Q^2), which will represent the envelope that will then arrive at the power stage a millisecond or so later due to the delay in the I & Q path. This envelope signal would then be far enough in advance of the modulated RF to allow the DC rail to be ramped up in time to follow the modulation.

Obviously the gain of the power stage will also tend to change with the supply voltage as both the drain capacitance and the output match change, but caresian feedback should be able to take care of that. 

The idea is that by dropping the supply voltage under low or no modulation conditions the standing heat load will be reduced and th mean efficiency (Which is utterly pathetic in most amateur service gear) increased, while letting the cartesian feedback take care of the linearity.

Obviously this will only work for an amplifier that is closely coupled to both the modulation processor and a suitable switchmode supply, but the win is huge, lower 2 tone IMD (My experimental TX manages -60dBc from a standard push pull '150 mosfet pair) combined with much better efficiency under SSB modulation).

Thoughts?

Regards, Dan.
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G3RZP
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« Reply #1 on: June 12, 2011, 02:12:08 AM »

As I recall, this was done in a simple fashion in the early 1970s in a Collins tx, although it may have been purely experimental. The envelope amplitude was used to modulate a switching regulator, but I can't remember the details.

It would probably be easier to use the envelope to modulate the bias source, since the energy involved in the switching is less. One point worth bearing in mind is that Cartesian feedback can lead to enhanced Out of Band emissions, which is why the ERC Rec 74-01 and ITU-R REC SM.329-11 have relaxed requirements for 'digitally modulated systems' - if we go back far enough, the company that was doing a lot of Cartesian feedback tx was having trouble meeting the original ERC/REC74-01 limits and got an easement: the main problem as I recall was the noise added in the loop by the down converting mixers, together with the phase noise of the downconverting oscillator. There was a similar but less troublesome problem with the original polar loop system, too.
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M0HCN
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« Reply #2 on: June 12, 2011, 09:58:04 AM »

Modulating the bias does not however change the mismatched collector load impedance when the amplifier is not producing full output.

Consider a PA designed for say 100W with a 50V rail (and ignore device saturation), the design drain load will be Rl=50^2/200 = 12.5 ohms.
Now consider the same amplifier running at say 10W, with the same load impedance we get a drain voltage swing of V^2 = 2RP=sqrt 250 = 16V, with the other 34V going purely as heat in the transistors.

With SSB modulation having a peak/average ratio of say 4 on a good day, that amounts to a massive heat reduction in any mode except FM, RTTY, some data and CW.

While designing a suitably fast power supply used to be a major issue there are a new generation of switchmode audio amp parts available that offer some possibilities.

My cartesian loop scheme does work and does not appear to increase out of band emissions (careful consideration of loop bandwidth and very careful noise analysis of the error amplifiers and master oscillator, two carefully placed AD797s also help with the noise), at least according to my old Marconi spectrum analyzer.

I cannot of course put this on the air until I upgrade my license, waiting for the (almost trivial) exam now.   
 
Regards, Dan.
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G3RZP
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« Reply #3 on: June 12, 2011, 10:51:07 PM »

If you don't modulate the bias, then (assuming the amplifier is Class AB) the duty cycle will vary. For a given standing current, the ratio of DC to RF currents will increase as the  power drops, which suggests that efficiency will suffer.

Any signal more than 7.5 kHz away from the centre of the emission should be at least 50dB down, and the average noise over any 10kHz band starting 7.5 kHz or more away should be -90dBc/Hz.

There are no OoB limits for amateurs, fortunately......or maybe not, listening to some the splatter on 20m!
 
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M0HCN
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« Reply #4 on: June 13, 2011, 07:38:32 AM »

True, but I am already running much lower bias current then is normal for those transistors because I don't need the amp to have spectacular uncorrected linearity. The logic that shuts the power stage down on receive could I suppose be modified to provide for variable bias fairly easily, I might experiment with it.

In fact there is another trick that variable bias would enable: For CW the PA can sit in AB at the start and end of each pulse but once the power has stabilized the bias can ramp down and the drive power up so as to give class C operation. The keying envelope should have a few ms rise and fall time during which the amp would need to be linear but the bulk of the time such a stage would run with zero bias.

I guess I just find the notion of going to class A PA strips as some of the big manufacturers have done, at least to moderate power levels, fundamentally inelegant.

That 7.5KHz wide band limit is not met by a lot of the commercial radios under modulation, especially when hooked up to an external amplifier and somewhat over driven, but is actually not hard to meet this way.

What does go pearshaped is overmodulation as the PA runs out of stick and the loop tries to compensate, splatter for days.... A fast limiter monitoring I,Q, and the required envelope helps solve this.

Interestingly my cartesian loop board is almost all analogue with the exception of two DDS chips used for the forward and reverse path LO and the micro that programs them and monitors a couple of phase comparators. The trick is that both DDS are clocked by the same master clock and so run at exactly the same frequency, however the relative phase can be easily adjusted to control the feedback phase and compensate the delay inherent in the amplifier and output filter.

I am thinking of using this as my required intermediate license project (But probably just the cartesian loop part).
 
Regards, Dan.
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G3RZP
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« Reply #5 on: June 13, 2011, 09:53:58 AM »

Dan,

I don't want to be rude, but how many of the examiners for the amateur exams understand Cartesian loops (and their derivation from Polar Loop) or even what they are? There are quite a lot of professional RF designers who don't!

The idea for CW is good.  Good luck with it. I have wondered about a Polar loop tx, but have never had the time. Maybe now that I've been laid off ('let go' is the US term!), it can become one of the projects on the list....but after the steam locomotive. Unless some of suggested consulting work comes along...
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M0HCN
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« Reply #6 on: June 13, 2011, 11:28:08 AM »

It seems to be money but no time or time but no money, I too am in the time but no cash phase at the moment, software work is a bit thin on the ground at present and the fact that I never bothered to get a degree does not help.

You do live steam as well (He says meaningfully looking at a well used colchester and tom senior amid the usual pile of swarf)!
Out with it, what loco?

I have to admit to tending to build 'scaleish' things that never actually existed rather then slavishly copying real engines, but it is all good. 

The examiner does not really have to understand it, the UK practical is basically to make sure you can do a workmanlike job of whatever you build, and given that one of the tutors is an ex agilent design engineer I am not to worried.
Sure, I could just build a morse practice oscillator or direct conversion RX set or something of the sort but frankly, boring!

Regards, Dan.
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G3RZP
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« Reply #7 on: June 13, 2011, 11:17:21 PM »

Plan is for a Sweet Pea - after finishing the rebuild of 30L1, building the auto tune 4CX1000 amp, the 'new' HRO, a 50MHz 100 watt transceiver, a 2m amplifier, an investigation into tube mixers, a high performance solid state HF rx and the other odd radio projects, all of which are so much easier with a lathe, a mill, a guillotine (shear in US terminology) small bender (brake), small brazing hearth (not big enough for a Sweet Pea boiler) grinder and pillar drill.

No matter how much room you start with, equipment always manages to overflow it at some stage!
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M0HCN
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« Reply #8 on: June 14, 2011, 01:00:27 PM »

So many projects, so little time.!
Going to use the blackgates stuff or build from scratch?

I have a (very) part built simplex in 7.5, but ask me for anything but time!

Regards, Dan.

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G3RZP
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« Reply #9 on: June 16, 2011, 09:14:34 AM »

Probably from scratch, except perhaps the boiler. But time is the problem....It appears that I'll have at least 60 days consulting work from September to next July - at £500/day plus expenses, so not to be sniffed at.
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