Low-band AM Tx: Pulse Width Modulator and Preamp

This post describes the modulator section of a 200 watt dual-band (160 and 80m) AM transmitter, built during the COVID-19 first-wave lockdowns in Melbourne. The transmitter is fully solid state, runs cool to luke-warm at 85 to 90% efficiency, and produces high quality AM on the two lower amateur bands.

Pulse Width Modulation in simple terms

Amplitude Modulation in amateur radio transmitters was commonly done by using an audio modulated DC power source applied to the RF power amplifier stage to modulate the amplitude of the fixed RF carrier (sine wave) in sympathy with the modulating audio. In the days of valves, this was typically done using an audio power amplifier (not that different from a hi-fi amplifier) with the speaker replaced by a transformer arrangement, allowing the secondary to carry the DC high voltage supply to the RF power amplifier stage, modulated by the super-imposing audio voltage swings.

In more recent times, digital or class D techniques have become popular in which low level audio modulates the duty cycle of a square wave pulse train (at around three or four times the highest audio frequency). The low level pulse train (or Pulse Duration Modulation waveform) drives a FET power switch into a low impedance load to generate the required power level. The high-voltage PDM wave is passed to a low-pass filter with a cut-off at arounf 20kHz which effectively acts as a crude digital to analogue converter, delivering a DC voltage at around half the DC supply voltage to the switching FETs, and varying with the modulating audio waveform between close to zero volts and the maximum DC supply. This modulated HT supplies the RF power module(s). A TL071 audio amplifier provides the line level audio. A second TL071

Schematic

Pulse Width Modulator

The modulator is built on a single PCB, and uses a hand-made heatsink bracket allowing the power FETs and shunt diode to be mounted vertically. The main bracket is made from 3mm aluminium angle. Small T-shaped cross section aluminium pieces provide mounting brackets down to the PCB. The main bracket leaves space for the inductors and capacitors of the Low Pass Filter underneath. The PCB is hand-drawn with marker pens and etched in Ferric Chloride. Additional aluminium pieces provider rigid supports underneath the board. Fasteners are pop rivets, M2 and M2.5 nuts and bolts.

The modulator works as follows. A CD4060 clock divider runs an 8MHz crystal clock and divides it down by 2 to the power of 5 (32) to 250kHz. This clock drives a transistor Miller integrator for a 2Vpp sawtooth waveform. The sawtooth is compared to a line level audio signal on the inputs of a comparator (LM311) which modulates the 250kHz clock duty cycle with the audio (via a TL071 op amp), creating a pulse duration modulated waveform, with a 50% resting duty cycle. A small amount of negative feedback can be aplied for error correction. The resulting PWM waveform drives a high and low-side gate driver (IR2110), which directly drives the gates of the power switching FETs (IRFP260), which alternately switch the full DC supply across the Low Pass Filter which presents a 12 to 16 ohm load.

Complete PWM and LPF assembly. The aluminum angle bracket is designed to support a horizontal finned heatsink on top.

The Low Pass Filter is a conventional four pole filter with -3dB ppoint at around 25kHz. This filter can be modeled just as you would do for a transmitter LPF. The inductors are wound using reasonably heavy gauge enameled copper wire (since they carry the full modulated DC power to the transmitter) on RM10 gapped cores. The capacitors need to be good quality MKT 250 volt pieces, as they are handling high switching currents.

Low pass filter inductors wound on RM10 cores.
Rear of PWM assembly showing the LPF components.

Testing the modulator is done in steps. First the crystal oscillator and divider is checked for a square wave at the expected frequency on its outputs. As the IC dividers are all internally connected you can see the division as you move the scope probe down the outputs. Next, the Miller Integrator sawtooth wave is seen, then the resting duty cycle at the comparator output is set to 50% with no audio applied. Once drive on the FET gates looks good, a DC supply can be placed onto the FET switching stage, starting as low as 10 volts. If all is well, the switched supply can be seen at the input to the LPF and steady DC at V/2 on its output. Applying an audio signal modulates the DC output. A 10 to 16 ohm dummy load (I used eight 100 ohm 10 watt resistors in parallel) allows the HT to be wound up. With a DC supply of 40 to 50 volts the modulator easily delivered around 80 to 100 watts of power into the load, making it hot enough to alsmost scorch the pad it was sitting on.

Balanced microphone preamp, tone preamp

Balanced microphone preamp and tone preamp.

I wanted to have the option to use a balanced microphone with my modulator so I investigated Elliot Sound’s various microphone preamp circuits. Elliot Sound is an excellent collection of high quality build-able audio modules, mostly using discrete transistors and commonly available op amps, possibly a little dated now, but for amateur use more than adequate. The balanced microphone preamp is Project 122. The subsequent tone module is Project 97. These modules require a split supply, which necessitated building a completely separate mains powered += 12V regulated supply, just for this board. But that is normal. The preamps were connected up and provide more than enough signal for the PWM board’s analogue input.

Next

These modules complete the AM transmitter. Check out the other posts and videos in this project. Thanks for reading and please leave a comment.

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