DDS Buffer Amp

So, i got some focus back into my home brewing with the goal to complete the receiver. The receiver is direct conversion, the DDS VFO is built, but as it outputs 270mV RMS, it is not powerful enough to drive a ring mixer that needs 7dbm or 1V RMS input. So a buffer amp was needed.

The buffer is pretty standard, common emitter gain stage with a common collector buffer. The DDS output is 200ohm, so a 4:1 transformer was used to drop the impedance down on the input. C1 is needed to block DC flowing into the transformer, as the bias voltage is set by the feedback resistor R4 and the transformer was loading it down so there was no bias voltage to the base of Q1.

Initially the Q1 emitter was to ground, giving about 10x gain, i found that this put me in the 3V RMS / 20dbm power range, much too high for a 7 or 10dbm diode mixer, so R2 was added to reduce the gain to about 3x. This gave me a nice 700mV RMS into a 50ohm load, 9dbm, close enough for now.


When running at close to 10x gain the waveform was full of harmonics, dropping the gain back knocked those on the head and the waveform is now nice and clean.


Looking at the output of the spectrum analyzer shows that first and second harmonics are way down compared to the desired frequency. So the dropping of gain certainly cleaned up the output a lot.


And this is where we are at currently, DDS VFO, Arduino micro controller and buffer amp all assembled and confirmed working correctly. Next job is to add the mixer in, then start work on the front end, JFET LNA (Low Noise Amplifier) to follow the Low Pass Filter.



At anytime, there might be half a hundred dozen different things on my work bench slash radio desk that can take up my interest for a fleeting moment. It might be a new toy, a new component, or just something shiny.


I was reminded of this today when i picked up part of the Direct Conversion CW rig i have been building for what seems like forever. I had some boards build to house a micro controller and DDS unit and I started to fiddle about with it and noticed that the output was very low.

The DDS should be putting out about 250mv rms at 7mhz, but i was only seeing 50mv rms on the scope. Something was up and i had never noticed what or why before. Pulling things apart and checking things separately showed everything was working right, but when plugged into the board it all went wrong again.

Turns out, I was measuring things at different frequencies and amplitude out of the DDS depends on frequency. Measuring 40mhz in the board and 7mhz out of the board, but being distracted i did not notice my error til much later. So, i have now cleared off the work bench, the only things on it now are stages of the DC receiver and everything else i have been tinkering with has been shelved. Time to get some focus and get this things build already. So tonight, i will build the DDS buffer amp and add the mixer as well.

More to come on that soon. 🙂