A collection of modules for building medium to high power Amplitude Modulation (AM) transmitters. Each module includes a schematic PDF, essential construction details, and many include links to Gerbers for the PCB.

Module AM#1: Triple DC buck regulator

A PCB design for three variable voltage 0..35 volt, 3 amp buck regulators, using the LM2576 switching regulator, and designed for use in a homebrew AM transmitter, but generally useful. Unwanted sections of the board may be hacksawed away if only one or two regulators are needed in a project.

Gerbers, BoM and other supporting files are available in this Github repository.

Module AM#2: 100VDC (500 watt) buck regulator

A schematic and PCB design for a DC power buck regulator capable of delivering 0..100 volts DC from an input of up to 120 volts DC at 3 amps continuous, and at peaks of 5 amps. Designed as the main power supply regulator for a 250 watt AM transmitter.

Gerbers, BoM and other supporting files are available in this Github repository.

Module AM#3: Digital SWR and power meter

A schematic and PCB design for a simple digital RF power and SWR meter. The following video demonstrates the meter:

Gerbers, BoM and other supporting files: email me.

Module AM#4: Balanced microphone amplifier with phantom power and equalisation

A schematic and PCB design for a balanced microphone amplifier with phantom power and equalisation, and using all discrete transistors. The module runs on a single 12 to 18 volt DC supply.

[ To Be Added ]

Gerbers, BoM and other supporting files: email me.

Module AM#5: H-Bridge Class D RF Power Amplifier

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.

The FETs are Infineon devices (IRFP4019) aimed at classs D audio amplifiers, and cost a few dollars each (if a supply can be found, as of Apr 2022 due to the global semiconductor shortage). Better performance is delivered by another Infineon FET, the IPP530N15 (out of all known stock globally!). 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. This selection of driver and FETs has worked with an efficiency of around 90% from LF to 7MHz.

A subsequent set of photos will illustrate the angle aluminium heatsink piece using 50x25x3mm stock angle. Two of these PCBs fit side by side under a 150x60x25mm finned aluminium heatsink, commonly available on aliExpress, eBay and other sources. These H-Bridge modules may be combined for higher power levels, either by using an RF power combiner (W8JI has an excellent one), or by arranging the secondaries in series whilst adjusting their impedance so as to sum to 50 ohms.

Gerbers, BoM and other supporting files: email me.

Module AM#6: Universal VFO Drive

This module takes as input a 3 to 5 volt square wave clock (such as a VFO generated by a crystal oscillator and buffer, si5351 multisynth PLL or similar source), and delivers one, two or four equivalent drive signals (I/Q or I/Q/I-bar/Q-bar). Outputs may be configured to be all in phase, or in some combination of 90 degree shifted phases. The input frequency may be at the desired frequency or at twice the desired frequency. An TTL-level enable line is provided for (on/off) drive control. A schmitt trigger may be added to square up an imperfect drive clock. It is intended to be used in a multiphase AM transmitter, to drive one, two or four H-Bridge modules, but is universally useful for any projects that require VFO/clock conditioning, with or without frequency division, or up to four phase-differentiated outputs.

Gerbers, BoM and other supporting files: email me.

Module AM#7: PDM Driver

This module takes an HT supply in the range 0..100 volts DC, and 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. This module is sized to power and fully modulate up to two of the H-Bridge modules (module #5).

Switching is done with a pair of Infineon IRFP260 power FETs, driven by a high and low side IR2110 gate driver. A modulated PDM (PWM) signal at 5V is required at the input. A TTL-level enable control turns the switching stage on or off, for transmit/receive switching, or to drop the (modulated) DC output to subsequent H-Bridge(s). An Over-sample line provides a degree of over-sample error correction back to the multi-phase modulator (to be described).

The Low Pass Filter is designed for a PWM clock (sampling) frequency of 100 to 125kHz). It has a -3dB cutoff at around 28kHz with the components as specified. Inductors are wound with 1mm ECW on RM10 cores. Capacitors must be metallised polyester (MKT) and rated at 200V for a DC supply of 100VDC. Electrolytic capacitors are Low ESR.

The PCB is identical in side, mounting holes and heatsinking arrangement to the H-Bridge module. One of these modules will power one or two H-Bridge modules, for a conservative 150 to 200 watts of fully modulated AM carrier power. With two H-Bridges running 200 watts carrier power, passive cooling using a 150x60x25 heatsink on each of the H-Bridges and the PDM Driver module without a fan will suffice. Two of these modules will power four H-Bridge modules, for up to 400 watts of fully modulated AM carrier power.

If using two modulators, the PDM drive may be supplied to each modulator in phase, or at 180 degrees for a multiphase configuration (using module #8). It is possible to use four of these modulators and four H-Bridges, and drive each with 90 degree phase shifted PDM drive, for a true quad-phase modulation configuration. This is probably unnecessary but would make an interesting test!

Gerbers, BoM and other supporting files: email me.

Module AM#8: Multiphase modulator

To be described.

Module AM#9: RF Power Combiner

To be described.

Module AM#10: Triple Low Pass Filters

To be described.

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