The Cranky Sysadmin A world of technology, fun, and ignorant rants.

March 2, 2013

A Buffer and Amplifier for a DDS Signal Generator

Filed under: Electronics,ham radio — Cranky Sysadmin @ 11:02 am

A google search for DDS buffer amplifier turns up some nice results in the first two hits. These circuits are very similar. My rendition of this circuit replaces the 2n2222 with 2n3904’s since I have scads of them. I also lower the value of the feedback resistor to 470 ohms (R3 in my schematic). This lowers the output voltage a bit, but it also seems to cut back on the harmonics. With 1k feedback I was seeing the second harmonic at 10db down from the original signal. With 470 ohms, the second order harmonic was down between 34 and 40 db over the whole range of tuning.

Here’s the schematic of my version which is based off the two links above. No matter how hard you look, you won’t find C2. That was an input coupling capacitor that I removed.dds_amp

And here is a picture of the prototype construction.

IMG_0338

I created a power rail by dividing the board in two and placing the positive power in the smaller section. I’m beginning to love ugly construction. It allows me to experiment almost as easily as a breadboard without the stray capacitance.

4 Comments »

  1. I need to drive a resistive SWR bridge with an AD9850. I have one of the eBay $8 specials. What output voltage can you get driving a 50 Ohm load? Yeah, I could build it and measure it but I thought I would do this the modern way and simulate it with SPICE. Results are ugly, either severe clipping or less than unity gain depending on the feedback resistance.

    Comment by Karl — August 23, 2013 @ 3:10 pm

  2. I don’t recall the exact measurements with the buffer. I took a different route and drove my resistive bridge directly from the DDS. As I recall the voltage out from the dds was around 1.5 volts. I used an opamp with a gain of 3 to bring the voltage up so I wouldn’t lose resolution in the AVR’s adc.

    Comment by Cranky Sysadmin — August 23, 2013 @ 8:32 pm

  3. I built the circuit and got results similar to the SPICE simulation – either bad clipping or poor gain. Not sure what I am doing wrong but it seems very sensitive to small changes in resistor values.

    There may be a small error in the schematic. R3 should connect to the base of Q1 rather that the emitter.

    Comment by Karl — August 30, 2013 @ 9:33 pm

  4. I think you’re correct about the schematic mistake Karl, at least judging by my photograph. I did test with the circuit as photographed (and maybe not as in the schematic), and I didn’t see any clipping. I connected to the filtered output of the module, which is several DB lower in power. That might make a difference.

    I’ll have to dig out my DDS stuff and see if I can reproduce the experiment.

    Comment by Cranky Sysadmin — August 30, 2013 @ 10:12 pm

RSS feed for comments on this post.

Leave a comment

Powered by WordPress