This module is an analogue and digital SWR and power meter/monitor, designed to replace analogue SWR and power metering in an AM Transmitter project. Of equal importance was the ability to detect high SWR and raise an interlock (a control line) to inhibit transmitter power in the presence of unreasonably high SWR. I make no claim to either SWR or power accuracy; my version is an indicative tool and an interesting experiment, not an accurate test equipment, although it could be made into same with some skill and patience.
Lockdown has made 2020 a year unlike any other. Melburnians were dealt a long and painful period of isolation with a CoronaVirus second wave, from around July to September, still in force, and looking like continuing to (at least partially) keep us housebound for some months yet. Days merge together, work and leisure time is largely indistinguishable. People are rediscovering reading, knitting, and endlessly bingeing TV. Makers are melting solder and stringing wires in the air.
Some winter nights were passed dreaming up an AM transmitter, and this VFO module is the first piece of it. It is designed for use in a two-band AM transmitter capable of around 200 watts carrier power, but could easily be used for other projects, such as a transceiver, or a transmitter for CW or digital modes. This module provides a 5v square wave clock at 1.8MHz or 3.5MHz (or any frequency you desire up to 144MHz), and transmit control lines needed to sequence a transmitter. It also includes a few ‘nice to haves’ including a transmit timer, CW ident, over-beep (or any CW character such as di-da-dit or da-di-dah), and a sleep mode which dims the displays when idle.
This post wraps up a series describing the construction of a 200W Class D AM transmitter for 160m, built, tested and put on air over 6 months from August 2019. The transmitter consists of a digital PLL VFO and driver circuit, an Arduino controller, a microphone amplifier, a Pulse Width Modulator, two H-bridge Class D RF modules each capable of at least 100 watts of carrier, an RF power combiner, Low Pass Filter, a 300W power supply and a switching regulator. At the Australian legal limit of 120W carrier, all parts of the transmitter run cool due the use of switching designs. Previous posts describe the switching regulator, 300 watt DC power supply and dummy loads for this project.
Amplitude Modulation holds a fascination for me. It dates back to those hours spent as a teenager listening to the big broadcast-like amateur AM stations in the 1970s and 80s, on 160, 80 and 40 meters. Signals that seemed as wide and loud as medium wave commercial broadcast stations, bearing sonorous, paced voices that projected a wealth of wisdom and experience, and in many cases, a grandfatherly manner. Back then, AM was known as ‘The Gentleman’s Mode’.
My Pulse Width Modulated AM transmitter project required dummy loads to match the load impedance of the RF power stage (50 ohms) and the Pulse Width Modulator (12 to 16 ohms). Surely any self-respecting amateur radio station would have a decent 50 ohm dummy load, I hear you ask incredulously. I do have one at VK3HN … or I did.
This post describes construction of a 300 watt DC power supply for use in a 150 watt Class D AM transmitter. The transmitter design calls for about 150 volts DC at 2 amps continuous and up to 4 amps peak. This post describes the 240v mains DC supply. The switch mode regulator is described in another post. The other modules in this QRO transmitter project are to follow.
Laurie VK3SJ is an accomplished RF designer and homebrewer in Melbourne’s 160m scene. You can hear his excellent AM signal regularly on the 160m AM Coffee Break net most weekdays. Laurie has spent a great deal of time experimenting with Class D, E and Pulse Width Modulators. He has an interesting Class D AM transmitter design with a pulse width modulator. Laurie’s design has been built by Wayne VK3ALK, who I met online in the Class E Forum on AMFone. After lots of emails and a few calls with these homebrew AM experts, I decided to proceed with my build of a VK3SJ AM transmitter for 160m.
The FAT5 project is a 100 watt Class E AM transmitter from Dave GW4GTE and Eric GW8LJJ, dating back to 2011. The name derives from Dave’s desire to design a 100 watt AM workhorse using contemporary FETs to rival the ubiquitous AT5, a widely used war surplus boat anchor for low band AM.
The VK 160 and AM SOTA event (1st April 2017) provided the deadline pressure needed to complete this build. The rig is a separate transmitter and receiver, so seeing as there are no shared modules, I shall call it a trans-receiver rather than transceiver. The designs for both transmitter and receiver are by Drew Diamond VK3XU. See Drew’s Projects Volume 3 book for full details, available from the WIA or RSGB. I posted details of the 160m AM receiver when I first built it, a year ago. At that time I also built the transmitter and power supply. I got the transmitter going but blew up the PA FET (BUZ90). During the long wait for replacements, I shelved the build and moved onto something else. Now, thanks to SOTA, it is finished.
Standing on a crowded commuter train, going home at the end of the work day in Melbourne Australia, listening to a late night west coast AM QSO on 75 meters, 3870khz, using my Samsung smart phone, via W7RNA’s web SDR. This is just one of the miracles of SDR and the Internet — the ability to use someone else’s station anywhere in the world, from almost anywhere in the world. My old elmer from my school days in the 1970s, Moss VK5TU, would turn his tuning knob in his grave if he could see this. KB6 and WB6es chewing the rag, on big AM transmitters. You can almost smell the burning dust on the ultraviolet glass envelopes. Pressing the tiny audio buds into my ear to shield some of the ambient train noise I hear the tone of the transmitter, rounded low-mids and bass. It could be 1972. It’s a nostalgic and beautiful sound.
My journey of repairing and recycling anything I put my hands on that I believe is still useful. Not just hardware, but including software with relevant content and issues in the field of Cyber Security, Vulnerability Scanning and Penetration Testing.