136kHz 1-2 kW PA? - Page 1
Forum section: electronics / rf-microwave

author: awallin
author_rank: Frequent Contributor
timestamp: July 13, 2019, 10:13:22 am
content: Previously I used a BK Electronics 1kW module, with a slightly hacked pre-amp (caps exchanged for smaller values) and was able to get ~few 100s of W output at 136 kHz. This was used with a transformer so the PA sees a low-ish (2-5 Ohm) load while feeding a 50-ohm cable+load through the transformer.But the site now says it's discontinuedhttp://www.bkelec.com/MODULES/mf1000.htmHas anyone tried modifying class-D audio amps for LF?there are ebay listings like "IRAUD2000" for IRS2092S based boards (PWM frequency of 800 kHz)Could it be possible to modify the filtering and get decent power-output above 100kHz?Or does anyone know of a current and available (and cheap) 1-3 kW A/B-class amp. that can be modified for 100kHz:ish??thanks!

author: OwO
author_rank: Super Contributor
timestamp: July 13, 2019, 10:52:26 am
content: Do you need linear operation? If not then class E/F is the way to go. You can get close to 98% efficiency at that frequency.

author: awallin
author_rank: Frequent Contributor
timestamp: July 13, 2019, 11:15:02 am
content: Quote from: OwO on July 13, 2019, 10:52:26 amI want to use an SDR to generate AM/PM modulated carrier at LF -> then feed that to the PA.In any case the PA needs to have a bandwidth of just ~10 kHz around the carrier (e.g. 136 kHz) - so perhaps class E/F would work?then the question is where to find an E/F-PA or how to build one..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2019, 02:04:45 pm
content: Yes, this bloke comes to mind:https://www.eevblog.com/forum/beginners/class-d-fet-driver-questions-(long-)/msg1390409/#msg1390409Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 13, 2019, 03:29:26 pm
content: 136kHz is very easily done with common switching power supply designs. In fact, a power stage running on rectified mains is pretty much right at the voltage range you're looking for to reach your target power level.

author: ogden
author_rank: Super Contributor
timestamp: July 13, 2019, 10:35:23 pm
content: Quote from: NiHaoMike on July 13, 2019, 03:29:26 pmAre you saying that common switching supply designs can be used as power amplifier for 10KHz BW @ 136KHz RF transmissions? Would be nice if you provide more details of what the (idea) you are talking about.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2019, 11:52:53 pm
content: Fundamentally yes, but you are unlikely to find an off-the-shelf SMPS that could be used with little modification.The most obvious problems are a poor stability oscillator (typically an RC multivibrator), poor control (usually done by PWM, frequency, or blanking), poor spectral purity, and the complete lack of an output coupling network suitable for driving whatever kind of antenna.A high efficiency "amp" (more of a converter, really) looks very much like an SMPS, but differs in the particulars. Most important for radio is the spectral purity, which demands a crystal controlled reference, and at least makes PWM less desirable (though not impossible to utilize).One particular, for example: consider the half bridge forward converter, typical of ATX power supplies. The inverter goes open circuit when not delivering power; the output network tends to force the inverter voltage towards zero (load current flowing in the filter inductor, acts to forward-bias both rectifier diodes, thus also forcing the transformer voltage toward zero), so the inverter waveform looks like ordinary alternating-pulse PWM. But with a reactive output coupling/matching network typical of an RF system, this condition (diodes forcing the inverter voltage towards zero) will not apply, and the inverter voltage when open-circuit can be anywhere inbetween. In short, the inverter output voltage no longer follows the idealized PWM waveform, and therefore the delivered power will not, in general, vary proportionally with duty cycle. (Worse still, that curve varies with load condition, so if your output SWR changes slightly, so does your distortion.)To address this, a 3-level inverter can be used (a bidirectional switch is added, to yank the output to definitely-zero volts while the high and low side switches are off), or a full bridge with phase shift PWM.More often, the inverter is simply left at "100%" duty, and the supply voltage is varied (say with a buck converter, to preserve efficiency) -- good old fashioned "plate" modulation. This allows for keyed CW, FM, ASK and AM, but makes DSB or SSB difficult.Probably the closest commercial candidate would be a bridge based power supply, of which you'll possibly be able to reuse the input (mains?) circuit, driver, inverter and transformer, and for which you'll have to add your own controller and output coupling network. If ASK or AM is needed, well, you'll have a lot more work to do.On the upside, there may be protection features available (current sense shunt or transformer?), which you should not discard. A voltage-source inverter handles loads of resistive to inductive phase, and nominal to infinite impedance, but can not handle capacitive or low-impedance loads -- in other words, only a half a Smith chart's range of possible SWR. Current limiting or protection allows your amp to at least survive such loads, perhaps even operate into them as well.Tim

author: ogden
author_rank: Super Contributor
timestamp: July 14, 2019, 12:32:08 am
content: Quote from: T3sl4co1l on July 13, 2019, 11:52:53 pmRight. If you are unlikely to find (solution) then what's the point of suggestion?QuoteCould not ask for better enumeration of possible problems, thanx. Yet I am waiting for eye-opener "switching power supply" solution from @NiHaoMike.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 14, 2019, 05:01:08 am
content: I suggest starting with the drive circuit for a solid state Tesla coil. You can do AM by varying the phase shift (for a full bridge, probably the best topology at that power level) or adjusting the duty cycle. FM is done simply by varying the oscillator frequency, but keep in mind that for a resonant converter, the operating frequency range is pretty limited.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2019, 08:21:36 am
content: Quote from: ogden on July 14, 2019, 12:32:08 amYou're taking it too literally, it was a conceptual comment. It's a relevant statement to a designer, because the designer is concerned about, say, using a "conventional" switching topology, versus a linear amplifier topology, or an RF amp topology. For the builder, it doesn't much matter either way because a small design change can be a huge physical change, and a proto build is usually from scratch each time anyway, so there isn't usually much to salvage.Likewise, solving backwards from a given  point, to a new  point, requires designer knowledge. That is, figuring out how much of a thing youcouldsalvage, even if it requires heroic effort, and how the existing hardware affects the final design. So either way, it's really a design statement.Tim

author: ogden
author_rank: Super Contributor
timestamp: July 14, 2019, 01:01:41 pm
content: Quote from: T3sl4co1l on July 14, 2019, 08:21:36 amYes you are right, I was challenging to produce more than just concept. Let's put aside literal qualities and talk about technical ones. Idea of using switcher is worth to consider actually. Frequency stability issue could be easily solved by using switcher IC/controller with freq.sync input. Connect modulator there, add output network/filter and you are more or less done building AGC-enabled power amp for FM transmissions.BTW look into page 25 of LTC3623 switcher datasheet (rev.A) and you will find... class-D audio amplifier circuithttps://www.elektormagazine.com/news/switching-regulator-is-also-class-d-audio-amplifier 2KW into 50ohms is 316V RMS. Serious stuffTalking about switching IC sounds like offtopic in result.