40m AM Transmitter (120 watts, PWM/Class D)

This AM solid state Class D single band transmitter was assembled over a three year period. Started in 2018, it’s first configuration used a 100 watt push pull RF module published by Drew Diamond VK3XU in Amateur Radio magazine, modulated by a 200 watt linear power amplifier driving a reversed mains transformer, available as a kit from local supplier Jaycar. I built up the RF board, 50 volt power supply (using a stock 300VA toroids mains transformer, no regulator) and proceeded to destroy half a dozen power FETs (STW20NM50) in the RF power stage. Realising I didn’t really know what I was doing, I wisely put it aside.

Not long after I connected with Laurie VK3SJ and Wayne VK3ALK, who coached me along the twisty path of switching technologies for RF, power and modulation– class D H-bridge topologies, 300 watt buck regulators, and Pulse Width Modulators. I quickly learned that switching technology was dramatically smaller, lighter, and more efficient than old school linear approaches. Two homebrew 200 watt transmitters followed, as well as various built and tested AM transmitter modules. For most of this period the 7MHz band had been in the sunspot doldrums, but in 2021 a pulse returned, and so did VK AM stations on 7125kHz. The time had come to finish this project.

Homebrew solid state AM transmitters, 40m top, 160and 80m below. The 40m AM transmitter is in a recycled 19″ rack box, bought at a disposals — some of the original panel markings were left on (‘PTN’, ‘OUTPUTS’, SYSTEM MONITOR’) to preserve the unit’s provenance, and the labels sounded cool. and it avoided a re-paint.

The transmitter is comprised of the following modules/PCBs:

  • A 300 watt 0..100 V DC linear power supply, consisting of the original toroid mains transformer with an additional hand-threaded 50 VAC winding, a 50A rectifier block, 9,000uF 250v capacitor bank, and a buck regulator to provide continuously variable power from 0 to 100VDC
  • Two regulated linear 12V DC 1 amp supplies
  • A digital VFO comprised of an Arduino Nano, 16×2 Liquid Crystal Display, various transmit sequencing lines and si5351 triple multisynth PLL
  • A Pulse Width Modulator using a crystal clock divided down for 125kHz sampling, IR2110 gate driver, IRFP260s in push pull, followed by a four pole Low Pass Filter with hand-wound RM10 inductors, delivering the modulated DC supply to the RF module
  • A Class D H-Bridge PA using a single IXDD614 gate driver and four FETs delivering up to 120 watts carrier
  • A 7MHz Low Pass Filter using T106-2 toroids and 1kV glass mica capacitors
  • An unbalanced high input impedance microphone amplifier using an audio JFET and a TL071.

Cabinetry, socketry

A transmitter like this involves mains power, and many kilograms of metal and copper. Physical rigidity and having everything bolted down is paramount. I considered repurposing several surplus rack boxes and settled on my favourite, a nice aluminium 3U number, formerly some kind of video switch, that I picked up from Rockby disposals a few years ago. Most of these disposal rack boxes are steel which is difficult for an amateur metalworker like me to drill or file. So if you see an aluminium one like this… grab it. As a bonus, this box included a 240V IEC mains socket and two nice side mounted fans. It also had a front panel bevelled cutout that was cut for a 16×2 LCD, including welded-on mounting risers for the popular 1602 LCDs — perfect!

Stripped chassis, ready for a second life.

I left the original labels on the front panel, for provenance, and because they did not annoy me. I cut out the middle centre rectangular hole and backfilled it with 1.5mm aluminium plate, sprayed matt black and labelled. Coincidentally, The new white DecaDry labels I had on hand matched the original labeling nicely. White DecaDry label sheets are almost impossuble to get these days.

Pictured here with the 200W linear amplifier PCB and big heatsink rear right. This transmitter configuration never worked, and was later replaced by a Pulse Width Modulator.

Power supply

The power supply consists of a 300VA toroidal mains transformer with 40-0-40 secondary; I wound on another 20V AC winding to get a series total of 100VAC, as well as another 45 turns for 15VAC for 12 and 5VDC regulated supplies. So as not to load down one of these with the in-built fans I added a fourth winding (26 turns for 8VAC), rectified and regulated (via two 7805s), just for the fans. As it all worked out, these fans were not required, due to the 90% efficiency of the modulator and RF board!

HT (120V DC) power supply.

I’d never threaded enameled copper wire through a power toroid before. The trick is to use a bobbin as per traditional hand weaving.

Winding on an additional secondary using a bobbin loaded with 1mm enameled copper wire.

Switching regulator

A switchmode voltage regulator (buck converter) regulates the 120V DC HT down to 0 to 100V DC, continuously variable, also performing current limiting and a high SWR cut-out control. The PWM heart of the module is a TL598C PWM controller, with a variable duty cycle pulse train at 120kHz. This drives an IR2110 low side gate driver to a switching FET, that swings the HT across a 120uH inductor and 470uF low-ESR capacitor. A low-value series shunt resistor is monitored by a transistor that turns on at a threshold voltage drop, backing off the PWM controller’s duty cycle. This regulator is identical to that used in my 200W AM transmitter project. For a schematic and PCB (designed later) see Module #2 on this page.

VFO/Controller

I opted for my Arduino Nano/si5351 VFO/Controller. Happily, the original rack chassis had sported a 16×2 LCD and so a perfectly cut and beveled slot and mounting posts were there for the taking. I built a Nano/si5351 and 16×2 LCD to the Raduino circuit on a custom board to fit the front panel cut out and posts. Being that this rig was not a superhet transmitter, I adjusted my script to output a VFO at the signal frequency (7MHz) in transmit (not with the usual IF offset). My script is here. #define SS_VK3SJ_40AM_TX at line 51 to pull in the right code for this project.

Transmitter VFO and controller, Nano and si5351 breakout.

The Arduino Nano controls:

  • LCD control and data lines
  • PTT sensing
  • T/R relay control
  • Transmitter enable line, which enables the modulator to place DC HT onto the H-bridge PA
  • Receiver muting.

I decided to omit any software and hardware for reading RF power (as the PA voltage and current are displayed on front panel meters) and SWR, given the base station antennas always have these inline, and I did not want to over-complicate this build when it had streatched out so long.

Mic amp

The microphone preamp, a 2N5484 FET and TL071, was made up on a small etched board and mounted in its own screened box, including the microphone gain potentiometer, all fitting snugly onto the front panel. This one-off assembly avoided the need for long screened audio cables between the board and front panel. There are no tone controls, this module may be replaced with a more sophisticated mic amp paired with a preferred microphone type.

Microphone amplifier assembly.

Pulse Width Modulator

This module takes an HT supply in the range 0..100 volts DC, and line level audio, generates a modulated pulse stream at the chosen Pulse Duration Modulation frequency, and performs power switching into a low pass filter. The result is a modulated DC voltage, suitable for powering an H-Bridge module to generate high quality Amplitude Modulation.

Pulse Width Modulator board, Low Pass Filter hidden behind the heatsink.

The clock is a 4060 clock/divider that divides an 8MHz crystal down to a 125kHz clock. This clock pulse is transformed into a ramp wave by a Miller Integrator, and fed to one input of an LM311 linear comparator, with line level audio on the other input. The result is an audio modulated pulse stream at 125kHz. This drives both high and low sides of an IR2110 gate driver, then a pair of IRFP260s in push pull, followed by a four pole Low Pass Filter with hand-wound RM10 inductors to effectively convert the PWM into a varying DC voltage (the modulated DC supply) to the RF module.

After assembly and initial testing of the 12v circuitry, the LPF output was connected to a 10 to 16 ohm dummy load. The modulator’s low pass filter has been designed for a load impedance that matches that of the H-Bridge module. As well as following the original designer’s values, I modeled the PWM LPF using SVC Filter Designer from Tonne Software to check the cutoff frequency (28kHz), and input and output impedances.

This module is sized to power and fully modulate up to two of the H-Bridge modules (module #5). This modulator is identical to that used in my 200W AM transmitter project.

RF Power

This module is an H-Bridge class D switch, not a power amplifier in the pure sense as it is non-linear, rather a switching module capable of delivering over 100 watts of power into a 50 ohm load, or other loads with a different output transformer turns/impedance ratios.

For the four FETs you could try Infineon IRFP4019, aimed at class D audio amplifiers, available and priced a few dollars each. I used a better device, the IPP530N15 (was out of stock globally for many months, check supply!). The IPP530N15s have a lower gate capacitance and also a lower drain source on-resistance (Rds) which improves efficiency. The module includes a gate driver (IXDD614, still available), which can be driven with a 5V TTL square wave from a crystal oscilator, synthesiser or PLL (followed by a 74HC-series TTL buffer or equivalent). I have had excellent results wit this driver and FET pairing, with efficiencies of around 90% from 1.8 to 7MHz.

This H-Bridge is identical to that used in my 200W AM transmitter project (which used a pair driving a W8JI RF power combiner). For a schematic and PCB (designed later) see Module #5 on this page.

RF driver and H-bridge board, custom made to fit against a nice hefty heatsink.

7MHz Low Pass Filter

The LPF is a conventional W3NQN design. I used T106-6 toroids and 1.2mm enameled copper wire, probably capable of a kilowatt. The capacitors are beautiful 1kV glass mica pieces, quite pricey but essential. I tried what I thought were decent quality 1kV ceramics at one point, and they got worryingly hot! My local supplier for these glass mica pieces is PKLoops, you will need to email them for a current stock list, check out their other products as well.

7MHz Low Pass Filter.

Comments

Testing and final alignment was done one module at a time. The safest approach with the power switching modules (Regulator, PWM, H-Bridge) is to bring up the 12V section and validate correct operation, then apply HT at around 10V with an appropriate load attached, apply drive, and carefully monitor the gate and drain waveforms.

In general, these circuits have mostly been easy to get going, and stable. A few FETs were blown up in the H-Bridge when operating at around 100 watts or more, mostly due to ragged looking drive waveforms across the gates. It is fairly much essential that you use a current limited DC HT supply — to test and put these H-Bridge modules on air withour current limiting is tempting fate. The Regulator module includes this feature.

With switching circuitry, most of my problems seem to have traced back to improper gate drive waveforms. When the drive looks good, you can turn up the HT to the switching FETs and there should be an almost linear increase in the output waveform amplitude. This is particularly impressive on the H-Bridge, where you see the board deliver 5 watts on a 8 to 10V HT, then up to 120 watts as the DC rail approaches 80 to 90 volts (depending on the load impedance presented by the output transformer’s primary at the frequency of interest).

The H-Bridge’s IXDD614 low side gate driver can draw 600 to 800mA on a 12V DC supply. It has been worth reducing the supply down to 10V, 9V and even 8V whilst monitoring the H-bridge’s output waveform. Most times, the output square-ish wave maintains its shape when the driver’s supply is reduced, which allows the IXDD device to run a lot cooler.

Another protection mechanism for modules permanently installed in an AM transmitter is SWR protection. An SWR bridge and detection unit can be used to detect high SWR and kill the PWM board (thereby dropping the HT to the H-Bridge) in the presence of high SWR. The one I used in another transmitter is module 3 here.

I have not included a complete schematic for this project as is my usual practice, as all of modules are described in other posts and pages. As noted, the H-Bridge, Pulse Width Modulator and 100VDC Regulator are each described on the AM Modules page, with schematic diagrams and some build notes.

Leave a comment below if you want to scratch build any of these. I can share prototype PCB Gerbers if you wish, but these are my own first version prototypes, and I cannot guarantee these are not without minor issues. If you try any of these modules yourself, let me know how it goes.

Acknowledgements

Thanks to Laurie VK3SJ and Wayne VK3ALK for guiding me in understanding and reproducing these modules over several years.

Build notes

Secondary winding 2: 68 turns gave 24VAC (0.353 volts AC/turn); therefore 127 turns should give 45VAC.
Secondary winding 3: 15VAC == 43 turns
Secondary winding 4: 8VAC == 23 turns.

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