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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