HF linear amplifier - ALC - Page 1
Forum section: electronics / rf-microwave

author: TheMG
author_rank: Frequent Contributor
timestamp: March 24, 2020, 03:46:43 pm
content: 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?

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2020, 01:30:08 am
content: 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.

author: awallin
author_rank: Frequent Contributor
timestamp: March 26, 2020, 01:31:51 pm
content: 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

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2020, 05:53:53 pm
content: Quote from: awallin on March 26, 2020, 01:31:51 pmUsually 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.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 27, 2020, 02:13:17 am
content: Quote from: David Hess on March 26, 2020, 01:30:08 amMy 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).

author: tkamiya
author_rank: Super Contributor
timestamp: March 28, 2020, 03:07:22 am
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: March 28, 2020, 03:56:58 am
content: Quote from: TheMG on March 27, 2020, 02:13:17 amI 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.QuoteThe 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.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 29, 2020, 01:22:34 am
content: Quote from: tkamiya on March 28, 2020, 03:07:22 amIt'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.Quote from: David Hess on March 28, 2020, 03:56:58 amI'll have to see what I have in my parts bins, to start experimenting with.When you say "high voltage", about how much?

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2020, 03:59:17 am
content: Quote from: TheMG on March 29, 2020, 01:22:34 amHigher is better but I would not use any diodes rated for less then 600 volts which seems to be the point where a PIN structure is used. I used 1000 volt 1N4007s and got excellent results at 2 meters but had to settle for working at an input of 0.5 to 1.5 watts; 5 watts would burn up the diodes. I did not need to go any higher in power for what I was doing.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 31, 2020, 11:47:17 pm
content: I went through my stash and found a handful of 1N5408 (3A, 1000V) standard recovery diodes.Not a whole lot of data to be found on reverse recovery time, so I'll have to just throw a circuit together and do some testing to determine if these would be suitable.

author: TheMG
author_rank: Frequent Contributor
timestamp: April 11, 2020, 06:39:55 pm
content: Well it's not looking very promising thus far. Seems the diodes are a bit too fast at 1.8MHz and that introduces considerable distortion.There are purpose-made PIN diodes available that would work and have sufficient power dissipation capability but the challenge with those is that they are only commonly available as surface mount devices. While working with surface mount components is generally not a problem for me, the issue with those comes from a thermal dissipation perspective, as the way these are designed, the heat has to be able to be dissipated in the PCB traces and/or substrate, which is not something that is easily feasible with DIY-etched PCBs.So I think I will go the other direction, and add a pre-amplifier stage. This will reduce the input power required to just a few mW, which is low enough to be easily controlled using an easily obtained and inexpensive voltage-variable RF attenuator IC.I'll have a switchable attenuator at the input (using plain old resistors), so it can be driven from a 5W or even a 100W radio, or directly with no attenuation using a low-level exciter (which I actually do have, it outputs approx 50mW, non-adjustable).

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2020, 09:47:49 am
content: I would get several different standard recovery rectifier types from different manufacturers. Some will be slower than others and they are not expensive.

author: M0HZH
author_rank: Regular Contributor
timestamp: April 15, 2020, 11:33:07 am
content: ALC is not a great system for SSB / linear operation, some amplifier manufacturers deliberately don't use it and some only use it for protection purposes rather than for power output leveling. The reponse curves are also quite different between transceiver manufacturers.I assume you have band data information from the transceiver, as you need to select output filters anyway. Why not use that to also set a fixed input attenuation per band ? A set of 4 attenuators in series (0.25dB / 0.5dB / 1dB / 2dB) would give you a 3.75dB adjustment range over the whole bandwidth, which should be enough for modern LDMOS amplifiers. If gain flatness is worse than that, you probably need a RC compensation network anyway in series with the FET gates. The 4 fixed attenuators can use small signal relays and 2512 SMD resistors and you use a microcontroller to control & calibrate the setting per band.You can use ALC as an "off" switch for drive, you need to output -10V to make sure any transceiver turns it off (older Kenwoods need around -9V, Yaesus -4 to -5V, some Icoms -3V etc). This way you don't switch the input attenuator relays with drive applied.Razvan M0HZH.

author: geggi1
author_rank: Frequent Contributor
timestamp: June 11, 2020, 03:51:48 pm
content: I would build a stepped resistive attenuator with relays.Depending on your exiters I would have the attenuator to be able to run your amp on half power for your highest power exiter and then just make your attenuator levels 3dB down form there.For instance if required drive is 1W and your exiter is 100W the attenuators would be something like this 3dB, 3dB, 3dB, 6dB, 9db. It would have to be an upscaled version of thishttps://www.qsl.net/z33t/rf_step_attenuator_eng.htmlbut with diffrent attenuation.The differnet steps in attunation should be designed in a way that the first attunator is capable of handliing at least 50% of your most highpower exiter and then you can step down the power handeling with 50% per level of attenuation (NB do the math for each step of attenuation)Depending on power level the regular Omron relays like thishttps://www.w6pql.com/using_inexpensive_relays.htmIf you want to automate this a bit it is possible to use a power sensor and a MCU( PIC, Arduino, STM32 or similar) to give you some higher level of protection by controlling the attenuation.Regarding PIN diode design have a look herehttps://www.ieee.li/pdf/essay/pin_diode_handbook.pdfAs far as i have seen PIN diodes are only used for low level attenuation and switching. But you might be able to do the attenuator switching by PIN diodes if you read the PIN diode hand book.73LA9XNA

author: TheMG
author_rank: Frequent Contributor
timestamp: June 27, 2020, 01:10:25 am
content: I'm beginning to rethink the whole idea. I got a variety of relatively inexpensive PIN diodes to play with, but I'm quickly realizing that the carrier lifetime needs to be much longer (best I could find, within reason, is 5uS), especially at frequencies below 10MHz, in order to avoid undesirable distortion when the PIN diodes are used as variable RF resistors in a continuously variable attenuator circuit. Especially as the power level is pushed up to a few tens of milliwatts, it becomes a big problem especially at 1.8MHz.Unfortunately, while PIN diodes with carrier lifetime of 10+uS do exist, they are either unreasonably difficult to obtain (minimum order quantities, lead times) or astronomically priced.So I think a continuously variable PIN attenuator is looking like it's not going to be practical for this project.Quote from: geggi1 on June 11, 2020, 03:51:48 pmThat's what I'm leaning towards, but with diodes instead of relays, which would allow for much faster switching. I still want to use it in such a way that the amplifier control circuit will automatically adjust the attenuation to achieve the desired peak output power level despite changes in drive power. This to eliminate the need to adjust the power output on the driving radio or have to manually configure/adjust the attenuation. Also it will be used for the various protection features, such as high reflect (VSWR) protection, so it can very quickly reduce the drive level but not have to latch the amplifier off entirely, as most protection schemes seem to do.The other advantage is if I build it with large enough resistors to handle 10W, the driver amplifier becomes optional. This means less extra harmonics and IMD introduced by an additional amplification stage when it is not required.I've thrown together a quick and dirty test circuit using plain old 1N4007 diodes and this seems to work, no measurable distortion with the attenuator "in" or "out". Insertion loss with the attenuator "out" is about 0.5dB but I suspect this should get better when the circuit is built onto a proper PCB (it's on a breadboard at the moment).

author: geggi1
author_rank: Frequent Contributor
timestamp: June 30, 2020, 09:24:22 pm
content: I would probably put some capacitors at the bias A & B to kill the last RF going through the inductors.

author: TheMG
author_rank: Frequent Contributor
timestamp: July 18, 2020, 06:32:47 pm
content: I've been doing some more experimenting and optimization and so far this is what I've come up with.Turns out the biasing arrangement is becoming the most complex aspect of the whole thing. The trick is, when the diodes are forward biased, the current needs to be high enough for the diode to have a low impedance, thus minimizing losses. However, in reverse bias the voltage needs to be high enough to prevent the RF signal from turning on the diode at all and decrease the reverse capacitance, which otherwise results in losses and distortion.To avoid consuming too much power and dissipating lots of heat in the biasing resistors, I had to use different voltages for the forward and reverse biasing. This necessitated splitting up the biasing circuitry for the direct and the attenuated paths, as well as using a dual half-bridge circuit and a logic inverter so that the whole thing can be switched in and out from a single 5V logic from a microcontroller. Note that the half-bridge setup has not yet been tested, I've been doing tests with two bench supplies, flipping the polarities around manually.At 1.8MHz (the lowest this thing will be used at), insertion loss is approx 0.1dB with the attenuator "out", 10.1dB with attenuator "in". This is with the whole thing on a breadboard, so I expect better results on a proper PCB. Loss increases noticeably above 10Mhz, but again, breadboard.If anyone has any ideas on how this could be simplified or improved, I'm all ears.Next thing to do will be to put this on a PCB for testing over the full range from 1.8-54MHz, as the parasitic inductances and capacitances of the breadboard are definitely a limiting factor at higher frequencies, but my first objective was to make sure the circuit would work adequately at the lowest frequency, since that is where it's hardest to keep the diodes from being biased by the RF signal and causing distortion, and why my previous attempts to use inexpensive, commonly available PIN diodes failed (carrier lifetime not long enough for the low frequencies).