Low-band AM Tx: Digital VFO/Controller

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.



The code for Arduino is here.


The video narration covers the functional features of this VFO and controller unit. It will become the heart of a 200 watt AM transmitter for 160 and 80 meters. This is a slower-paced Corona-lockdown project, so there’s no big rush to finish this project! I’d picked up some of the 0.56 inch 7-segment LED displays, very cheaply, and was looking for an excuse to try them out. They are large and bright, and it is an interesting challenge to use the 7-segment character segments and tyhe brightness and blink controls to convey the kinds of information that a transmitter needs to communicate.

If you want to reproduce something like this, you will no doubt want to use your own variations, and you should have no problems tweaking the code to do what you want. Drop a comment in below if you want to discuss any aspects of this unit. The gallery below presents some angles on the module.

The module is built on an aluminium angle and sheet half-chassis.
A collection of components and breakout boards — Arduino Nano, si5351 breakout, RTC, MCP9809 precision temp sensor, control logic, VFO clock buffer/squarer.
In transmit mode, right display alternates between over timer and temperature.
Map of the display segments to allow new characters to be coded in ‘raw format.
Nano and si5351 breakout.
Clock buffer MMBT3904 and components.
12v DC switching circuits for band change relays, T/R relay, PWM enable lines.
Displays have a clear front face so they can be mounted flush with the aluminum angle front panel.

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14 thoughts on “Low-band AM Tx: Digital VFO/Controller

  1. Hubert F6DUK says:

    You also suffer from compulsive Transceiver building😊. Always nice reading your publications 73s Hubert F6DUK


  2. glenn says:

    Hi Paul….. I thought Covid might spur you to even greater projects ! Looks great.
    Committed a sin here, bought my 1st commercial rig since the 70’s ! IC-705. Now have to learn to drive it.
    glenn vk3pe


    • Paul Taylor says:

      Hi Glenn, I was thinking of you the other day, David VK3KR was using the HP Sig gen. The AM transmitter for 160 and 80m is working, it’s on air several times a week, but not boxed up or with the final power supply yet.

      Maybe a call sometime to see what you’ve been up to.

      73 Paul VK3HN.


      • Glenn says:

        Hope the sig gen is working for you guys.
        Not upto much. Some projects moving slowly. A simpleSA (based on tinySA) is working including tracking gen. I take no credit for the software. Currently on an amp and lpf to go with the IC-705. And a RF to band select outputs using a PIC working to go with it.


      • Paul Taylor says:

        Forgot to say, congrats on the 705, it looks like a superb package. You can build a top notch amp for it, what devices will you use? Missing ARV HB meetings along with many other things in life. Stay safe, PT.


      • Glenn says:

        Hi Paul, I have an amp with the IRFZ24’s which is good for about 50W on the lower bands. But I have a pcb and MRF454’s for 100W amp too. 4-5W out of the ‘705 would suit it. I have a mating LPF pcb for 4 or 8 bands. 4 would match the IRFZ amp.

        Yes i miss the meetings a lot. I guess many of us should have lots of projects during Covid to show and tell hopefully late this year or early next!

        Lots of feature sin the IC-705. I bought one of those Lithium 12v 7aH batteries for it in case i wander over to the local park for /P operation sometime.


      • Paul Taylor says:

        FB. Side by side with one of your HB receivers, say a Picastar, and ignoring all the fancy digital stuff and conveniences, is the 705
        a) much better receiver
        b) noticeably better
        c) hard to tell


  3. glenn says:

    Paul, I haven’t done aside by side test but with the sig gen it would seem the ‘705 is more sensitive. The ability of the 705 to vary pass band tuning is good too. The Noise Reduction is vastly better than Picastar although G3XJP has been working on improving it.


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