Author Topic: HF linear amplifier - ALC  (Read 390 times)

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Offline TheMG

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HF linear amplifier - ALC
« on: March 24, 2020, 03:46:43 pm »
I'm building a 1kW HF linear amplifier based on the BLF188XR, and I've got a bit of a dilemma when it comes to how to handle ALC.

The traditional way for a lot of amateur radio HF amplifiers is to provide feedback in the way of an analog ALC signal (which is essentially just the detected output RF level), to the radio being used as an exciter. However, not all radios have an ALC input and some of the radios I intend to use with this amplifier, do not.

I don't really like the idea of having to manually adjust power levels all them, so would still like some form of ALC, within the amplifier itself.

The other feature this would enable the amplifier to have is foldback VSWR and over-temperature protection, instead of just turning off the amplifier completely as a lot of amplifiers do.

So that would mean some form of electronically variable attenuator at the input to the PA, but I'm not sure how to approach this. While the input power is relatively low (less than 4W), I think that's a little much for a PIN diode attenuator, at least using parts that aren't ridiculously expensive. So maybe knock the signal down to a few mW, run it through a PIN diode attenuator, then add some sort of pre-amplifier to bump it back up to 4W required to drive the PA?

Ideas?
 

Offline David Hess

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Re: HF linear amplifier - ALC
« Reply #1 on: March 26, 2020, 01:30:08 am »
I have built power PIN diode attenuators using relatively large high voltage standard recovery rectifiers which often use a PIN diode structure.  The power rating of rectifier diodes can be calculated from their specified maximum forward voltage drop at maximum current.
 

Offline awallin

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Re: HF linear amplifier - ALC
« Reply #2 on: March 26, 2020, 01:31:51 pm »
first attenuating and then amplifying is usually a bad idea (at least for SNR..).

something like RVA-3000 takes +26dBm of power, so can you try to put more gain in the PA and have the signal from the radios at max 1W or so?
https://www.minicircuits.com/pdfs/RVA-3000+.pdf
 

Offline David Hess

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Re: HF linear amplifier - ALC
« Reply #3 on: March 26, 2020, 05:53:53 pm »
first attenuating and then amplifying is usually a bad idea (at least for SNR..).

Usually that is the case but here, the signals are already high level so loss of SNR from added noise is insignificant.

Using a fixed attenuation, then a low level variable attenuator, and then an amplifier will certainly work and is the common solution but is more complex than if a variable power attenuator can be implemented.

My greatest concern would be added distortion.
 

Offline TheMG

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Re: HF linear amplifier - ALC
« Reply #4 on: March 27, 2020, 02:13:17 am »
I have built power PIN diode attenuators using relatively large high voltage standard recovery rectifiers which often use a PIN diode structure.  The power rating of rectifier diodes can be calculated from their specified maximum forward voltage drop at maximum current.

My Google-fu is failing me, got any links to info on how to build one?

Ideally should be able to handle 5W continuous in the maximum attenuation. Are there any concerns relating to introducing distortion when using standard rectifiers as opposed to purpose-made PIN diodes?

The attenuation range required according to my calculations would be about 0-17dB (amplifier tends to have a very high gain on the lower bands).
 

Offline tkamiya

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Re: HF linear amplifier - ALC
« Reply #5 on: March 28, 2020, 03:07:22 am »
ALC isn't that great of an idea either.  They tend to overshoot then settle down.

Can you make the amplifier gain variable?  Can you just make a few resistivite attenuator and manually or remotely select them?  If you are intending to use it with multiple radio, you'll have to create some kind of control box or interface box anyway.  An attenuator can be part of that, too.
 

Offline David Hess

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Re: HF linear amplifier - ALC
« Reply #6 on: March 28, 2020, 03:56:58 am »
I have built power PIN diode attenuators using relatively large high voltage standard recovery rectifiers which often use a PIN diode structure.  The power rating of rectifier diodes can be calculated from their specified maximum forward voltage drop at maximum current.

My Google-fu is failing me, got any links to info on how to build one?

I never found any data or application notes on making power PIN diode attenuators.  I just started with the theory and began experimenting.  5 watts will trash a 1N4007 pretty quickly; 1 amp rectifiers are really limited to a little less than a watt.

Quote
Ideally should be able to handle 5W continuous in the maximum attenuation. Are there any concerns relating to introducing distortion when using standard rectifiers as opposed to purpose-made PIN diodes?

The usual rules about making sure that the recovery time is much greater than the lower frequency limit applies of the RF signal will start modulating the PIN diode producing distortion.

As far as I know, all high voltage standard recovery silicon rectifiers use a PIN diode structure so that is where to start.  Buy some samples of different parts from different manufacturers and test them out.  They are not very expensive.
 

Offline TheMG

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Re: HF linear amplifier - ALC
« Reply #7 on: Yesterday at 01:22:34 am »
ALC isn't that great of an idea either.  They tend to overshoot then settle down.

It's done on a great number of commercial HF communications transmitters without issue.

The idea is for the ALC to have a very fast attack time, but a slow release time. To keep overshoots to an absolute minimum and minimize any distortion being introduced for example in certain digital modes.

Could also have the microcontroller "pre-set" the attenuation in the general ballpark depending on the selected band, which would help keep the initial overshoot to something sensible.

What I'm mainly looking to accomplish, is being able to select an output power level, say 250W, 500W, or 1000W on the amplifier itself, and have it maintain close to this power output regardless if it's getting, say 3W or 5W or whatever from the driving radio.

As far as I know, all high voltage standard recovery silicon rectifiers use a PIN diode structure so that is where to start.  Buy some samples of different parts from different manufacturers and test them out.  They are not very expensive.

I'll have to see what I have in my parts bins, to start experimenting with.

When you say "high voltage", about how much?
« Last Edit: Yesterday at 01:25:45 am by TheMG »
 


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