DDS VFO CW Exciter

Here is the next part of my home brew CW transmitter. An Arduino and AD9850 are providing the DDS VFO signal. Q1 is a buffer to isolate the AD9850. I am not sure that it is needed, but its there to stop the gain stage from loading it up. Q3 is an amplifier that takes the 400mV P-P DDS signal and amplifies it to 8/9V P-P or there abouts into a 50ohm load. Thats about 200mW, enough drive to supply something like the QRP Labs 10w Linear. My plan is to design an amp to follow on from this that will provide 30 to 50w. More on that to come. Q2 is a PNP switch, pulling the base to ground turns lets power flow to Q3 thus keying the transmitter on and off. Simple but works.

Here is the schematic.

The PCB as laid out in Ecad.

Here is the board as built, if you are not colour blind and can read the colours on R5 you will notice i stuffed up here HAHAHA. Also Q1 silk screen is backwards.

Signal output into 50 ohm load at J2. This jumps up to about 10V P-P into the QRP Labs Linear amp. Not sure why yet.

Schematic PDF: CW-Buffer-Amp

Board Gerber Files: DDS-CW-Buffer-Amp


6 thoughts on “DDS VFO CW Exciter

    1. Its a common library found on the web. I think AD7C was the first to bring one out. Google him, its on his webpage.

  1. Rob,
    I am getting back on CW, after about about a 60 year abscene. It is my obervtion that a CW transceiver requires a tuneabe RIT function. In my day, we used separate receivers and transmitters so that wasn’t required. Is this your goal?
    73, Jack in the USA

  2. Hi Jack, RIT is really easy to implement in software. In my latest iteration of this kind of thing, I essentially have separate RX and TX VFO’s in software that provides the RIT function. Push of a button allows me to change the RX frequency in respect to the TX. It also allows me to work split on home brew as well.

    I have 2 encoders, one is the VFO and it sets the TX frequency and also sets the RX to be the same. Thats the main tuning knob. The second encoder only changes the RX frequency. The PTT/TX-RX switching tells the micro controller what state its in, either TX or RX and sets the frequency accordingly.

    If using something like the SI5351 where you have 3 oscillators, you can do a superhet CW rig quiet easy, the 3 oscillators become TX vfo, RX vfo and BFO, and you more or less have separate RX and TX rigs like the old days, just controlled with software rather than different physical boxes.

    1. Rob,

      Another CW question for you.
      When you operate in this mode, do you use semi or full break-in?
      If you operate in semi-break-in, do you do that via the Arduino script or via some physical circuitry?

      73s, Jack

      1. Semi break in for me with a physical tx/rx switch, which switches the antenna from the receiver to the transmitter and also mutes the receiver. The sidetone is contained in the memory keyer that I use, also home brewed.

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