High Power Amplifier
It is all in the control system!By Pete Juliano, N6QW
As electronics hobbyists, we are fortunate to have a large array of low-cost, cutting-edge technology available to us. Just a few short years ago, we could only dream of having the range of tools we have today.
I am talking about the Arduino microcontroller board, more specifically the Arduino Mega 2560 with over 50 digital inputs, 16 analog inputs and about 10X the program space of earlier versions. But enough of the technology, and let me tell you about what's on my workbench right now.
I am an amateur radio operator and unlike a lot of today's "hams", I like to build my own equipment, and the new technology makes this easier. Recently I decided to build a high power RF linear amplifier to boost my signal to about 1200 watts peak envelope power.
In the 1950s this would have required a large room with several racks of equipment. Fast forward to today, where with solid state devices it fits in a small box that will fit on the top of a desk. But alas, solid state amplifiers can be easily "smoked" should some aberrant condition occur. Issues such as over temperature or a problem with your antenna system could easily destroy your amplifier.
So in building a DIY amplifier (or as we hams call it, homebrew an amp) most of the emphasis is placed not on generating the RF but in the supervisory, control and protection circuits all which are easily handled with the Arduino Mega 2560.
Fortunately many have plowed the ground ahead of me and several pieces of hardware are available for use with the Mega. These include the use of a keypad as the main input device plus a 20 x 4 LCD to visually provide the status of various actions and the current state of the amplifier. Thankfully there are a wide range of "libraries" available to drive the keypad and LCD function with the Arduino Mega. Most microcontroller applications operate in a "loop" fashion, so there is a continuous polling of sensors and conditions. This is ideal as the Mega is fast enough to cycle through the loop quickly enough to catch any problems.
My next generation of this project will incorporate the use of a 320 x 240 Color TFT display. This provides graphics capability using bar graphs or flash colors to indicate the aberrant condition. There is no doubt the whole screen flashing red would get the operator's attention that there was a problem.
A kilowatt plus amplifier typically will use a 48VDC power source and to get 1200 watts PEP at around 65% efficiency means a very healthy power supply like something close to 40 Amps or roughly 2000 watts. So the control aspect of the Arduino is how to normally turn "on" or "off" that supply. Since that is a significant load, the use of 220VAC is highly recommend as the primary source for the supply. The "hockey puck" solid state switches used with the Arduino control easily accommodate the 220VAC source voltage – another bonus.
To make this all happen, I employed a standard simple motor starting circuit shown in Figure 1 as the main control element. In place of the motor, I have the power supply. The momentary NO or NC switches which form the heart of the circuit, are simulated using small 5VDC relays connected to the Mega digital pins and the code is written so there is a small time delay to mirror the momentary contacts. The latch relay keeps the circuit powered on until such time the circuit is broken with an opening of the NC switch. When the latch relay is energized, the 12VDC power is supplied to the solid state switches to "power on" the 48VDC supply. This 12VDC rail is also the source for the solid state switches that select the appropriate low pass filter banks.
For protection we need to be able to shut down that 48VDC supply quickly or to even bypass the amp. These two cases can be manually instigated by sending a signal to open the NC switch. In the same manner, there is the ability to automatically detect certain conditions that would harm the amplifier inclusive of an over temperature condition or a high standing wave ratio on the antenna and open that NC switch. With the 16 analog inputs on the Mega 2560, various sensors can be monitored and if such a pre-determined condition is detected, then the amplifier power supply is automatically shut down by sending a signal to open the NC switch. The "if", "else" functionality of the Arduino is superb for screening the conditions and causing an action to result.
Supervisory functionality ensures there is a proper timed sequencing of the interconnection of the amplifier to the antenna and the radio transceiver. You never want to "hot switch" a large RF amplifier! Thus when the amp is placed in line, the antenna is first connected to the amplifier, a bias is applied to turn on the amp and then the transmitter is connected to the amplifier. When I'm done transmitting, that same sequence is reversed. The LDMOS amp is controlled by a pair of transmit receive relays rated for RF. These are 12 volt relays, and the source supply for this 12 volts is a 48VDC to 12VDC DC to DC convertor connected to the main power supply output buss. So if the 48VDC supply is not "on", you cannot energize the relays. This was done to avoid the condition where you were pumping RF into the amplifier and it was not powered. Otherwise you would have a very expensive dummy load!
Supervisory control also ensures that the operator has selected an appropriate low pass filter for the intended band of operation. Basically the LDMOS RF amplifier is a 1MHz to 600MHz, 1200 Watt broadband amplifier. However, the FCC would get awful gnarly if some sort of adequate filtering were not provided to limit the output to specific amateur bands. Thus the amplifier is set up to cover six amateur bands in the 2 to 30MHz range. These filters are selected, via the keypad. Thus, logic must be incorporated into the code so if a low pass filter has not been selected then the amplifier cannot be put in line. Operating a 1200 watt RF amplifier into an open circuit will quickly smoke the RF device.
I am attempting to document the detail of this amplifier on my blog. http://n6qw.blogspot.com. This includes a YouTube video, https://www.youtube.com/watch?v=i0p9mve7kUI, which demonstrates the operation of the control circuit for turning on the 48 VDC supply as well as powering the low pass filter relays. Once completed, I will publish the code on my website. http://www.n6qw.com. Figure 2 is the test bed I am using to test the code that has been developed.
73's (as we ham's say ~ short hand for best wishes)
Pete Juliano, from Newbury Park, California, had a career in the aerospace industry and since retirement has been putting into practice what he learned while pursuing a degree in electrical engineering. Now he delights in not having to formally show up for work, and his current "day job" is building equipment for his amateur radio station, call sign N6QW.