Saturday, July 16, 2016

Back to the LDMOS Linear RF Amplifier

7/21 The Role of Analog Pin A8

One action that always has a bit of mystery is how to actually put the amp on line? This amp will be used principally for SSB (voice) but the principles would apply to CW or digital modes.

Basically I hear a station or I want other stations to call me and the 1st thing I do is grab the microphone and squeeze the PTT bar. A lot of things have to happen once you key the microphone. Essentially on the amplifier side there has to be a detection that the PTT was closed and then the TR relays are sequenced and the bias applied to the amp.

Part of the TR changeover is hardware and another key piece is software. The software essentially runs some tests such as asking are a set of Low Pass Filters in line? Another is the 48 VDC to 12 VDC DC to DC convertor energized?

Assuming the requirements are met the important pin is Analog Pin A8. If a low (ground connection) is observed on that pin then the amp is in line. A circuit schematic for A8 is shown below.
Not a lot to it but really important.

Pete N6QW

7/19 First Test of the Arduino Control System!

See the Arduino going through its paces.

Pete N6QW


Back to the Amplifier Project ...

I must sound like a broken record when I mention that at times I only have 15 or 20 minutes to work on a project then it is off to my caregiver duties. Well eventually those short periods of working on the project result in something substantive. Starting with wiring up the several power supplies and then moving on to the basic motor starting circuit, I am happy to report that the first tests were very successful.
Using the Keypad and engaging Key #1, I am able to deliver control signals to the "hockey pucks" (Solid State Switches) which  turn on the amplifier power supply (48VDC). Depressing Key #2 (normal Off) drops the control signal. The same applies for Key #3 which is Amplifier By Pass and Key #0 which is Emergency Off. This was a huge proof of concept and reflects the suitability of the Arduino to control the LDMOS Amplifier.

By way of review below is the "Motor Start" circuit I learned about when I worked at Steven Engineering (South San Francisco). This has been around a long time and is used even today. All we have done is to use the Arduino operating via a keypad to simulate the NO and NC momentary pushbutton switches. A new day has dawned!

Then I transitioned to actually controlling a load in this case a large floor mounted pedestal fan. Punch #1 and it goes on and if you punch 2, 3 or 0 it goes off. This is like the Karate Kid Movie ! I can see Mr. Miyagi standing in a corner of my garage chanting "Fan On" "Fan Off". What a great day. Now I will move on to adding the other circuits such as the keypad  selection of the Low Pass Filters and then the supervisory controls such as over temperature and high SWR.

I would like to plug a highly reliable parts supplier and that is Jameco Electronics. Many of the components seen on the above bread board were supplied from Jameco Electronics located in Belmont California and available on the web. I will highlight the Jameco items as we tour the board with the letters JE.

Starting at the upper left corner is a 9.0 VDC power supply with connectors (both JE) This supply is used to power the Arduino Mega 2560. Oh the main power switch is also JE (lower left hand corner). To visually see that 9.0 VDC is available for the Arduino a simple LED with a dropping resistor is connected to the output pins of the 9.0 VDC supply. Next to the 9.0 VDC supply is a 12 VDC supply (JE) and that is used in the Motor Start circuit. Next to the Arduino Mega is the 4X4 Keypad (JE) and the 20X4 from SainSmart.  The board next to the Keypad/LCD will contain many of the key items of electronic hardware . Currently on that board are two small orange relays that simulate the NO and NC momentary switches  and the larger black relay is the latch (American Zettler). The green terminal block (JE) facilitate the interconnections to the Mega 2560 and other parts of the circuit. The two multi-pole barrier type terminal strips on the right hand side are also from Jameco Electronics. They will be used to interface with the Low Pass Filter Relays and the TR Relays. Not currently on this board is the really small 48 VDC to 12 VDC DC to DC convertor (JE) that will solely power the TR relays with the Arduino supplying the sequencing logic.

Stay Tuned,
Pete N6QW


  1. Hi Pete
    You may want to consider adding a hardware watchdog just in case excess RF fries the Arduino.

    73 Gareth, M5KVK

    1. Hi Gareth,

      Thanks for your post and suggestion. Certainly a valid consideration and concern. The Mega 2560 is around $15 USD but the RF device cost $200 USD --so protection of the RF device is critical. Not sure how to do what you suggest but that is another item on the list. Thanks again.


    2. Pete
      A 555 timer that is reset in the Arduino loop and connected to a crowbar of some sort would work.



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