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Re: [Amps] FCC Denies Expert Linears' Request for Waiver of 15 dB Rule

To: amps@contesting.com
Subject: Re: [Amps] FCC Denies Expert Linears' Request for Waiver of 15 dB Rule
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Wed, 28 Dec 2016 14:40:15 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Hi Jim, and all,

When you see an SDR receiver selling for a few hundred bucks, it's
pretty simple to add a low power transmitter.

So adding a high gain amplifier makes for a cheap SDR radio capable
of medium to high power a fairly cheap option.

Exactly!

At this moment I'm VERY tempted to brew a concoction from these ingredients:

- A Red Pitaya board, for around $300;

- A homemade board containing a low noise preamplifier, attenuator, and band pass filters for RX, along with a 10W or so driver amplifier for TX. The whole board should't cost more than $150 in parts;

- A high efficiency, high gain, deliberately non-linear, legal-limit final stage. This might take the shape of a simple overdriven broadband class AB stage, or perhaps a class E stage with tuned, band-switched output tanks. It might use a single BLF188XR for about $150, or several small switchmode MOSFETs for far less money.

To these hardware ingredients I would add some freely available software, such as Pavel Denim's software for the Red Pitaya, plus PowerSDR, using closed-loop, RF-feedback amplitude and phase predistortion to linearize the amplifier. The software has this already implemented.

The result would be a legal-limit, high efficiency, high performance, compact, black-box HF transceiver to be used with a PC, for under $1000 total cost, if you are good at ordering surplus power supplies on eBay, and such.

And those who prefer integrated transceivers, can of course add an embedded PC motherboard plus small monitor, for a few hundred dollars.

It would just take some tinkering with the final stage. The rest is pretty simple to do.

Some more development in the RF power device scenery would be welcome. The BLF188XR has the necessary capability at least for the saturated class AB version of this idea, in terms of gain, voltage, current and thermal characteristics), but it's quite hard (or more bluntly: close to impossible) to achieve the desirable 85% or higher efficiency on a broadband basis, due to the very low drain impedance and the ensuing trouble in properly coupling the drains together and to the output filters. Switchmode MOSFETs instead are available with voltage ratings that allow operation at convenient impedance levels, but they have less gain, higher capacitances, more variation between 1.8 and 30MHz, and require the use of several (or many) in parallel. It would be great to have true RF power devices that can operate from a few hundred volts, and have low enough output capacitance to allow broadband use to 30MHz. Along with good gain and convenient price, of course...

As usual, I'm asking too much. But I'm confident enough in this concept that I intend to buy a Red Pitaya board and start experimenting in that direction, using a class-E output stage based on ARF449 MOSFETs, which I happen to have in my "selection junk box". That would make a few hundred watts only, but serve as proof of concept. I want to test, hands-on, if it is really possible to achieve adequate spectral purity by linearizing a deliberately non-linear amp via SDR. Normally the predistortion system included in SDR software is intended to make a run-of-the-mill linear amplifier extra clean, and not to make a dirty-and-ugly but highly efficient amplifier clean enough!

Manfred


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