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.

bufferamp
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.

oscilliscope

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.

anayser

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.

IMG_20160209_195616

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.

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6 thoughts on “DDS Buffer Amp

  1. This seems to be exactly what I need. I have all the components apart from a ready made transformer. I’m guessing you made your own 12t – 4t transformer and wondered if you could give me an idea of how you constructed it, or point me to a web recourse that will do so.

    Best Regards

  2. Hi Rob!, great article!. I have an observation, the 4:1 tranformer should be 8t and 4t to make 200 ohms to 50 ohms (4:1 impedance ratio). With 12t and 4t you have a secondary impedance of 22.222 ohms and the input impedance in this amplifier should be between 50 to 60 ohms . 73! de LU1MAW

    1. Hi Alejandro, you are 100% correct, It should be 8:4 I honestly cannot remember what I was thinking and looking at the pictures I even built it 12:4, so i guess i stuffed up my math somewhere. I do like the homebrew bitx 40 you have on your QRZ page. Very nice looking rig and well built. Congratulations.

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