A fellow ham recently purchased and assembled a nice little signal generator kit consisting of a DDS AD9910 signal generator board/shield, Arduino Mega, OLED display, and TCXO option. I had borrowed it from him for the purpose of generating some modulated AM test signals to check the performance of some 20 MHz oscilloscopes I plan to distribute to some of the AM transmitter sites I maintain for clients of my broadcast engineering firm — Alabama Broadcast Services, LLC. My initial testing of the device’s output frequency showed it to be off considerably, so I spent some time adjusting the software settings in the device and got it aligned pretty nicely.
Initial Test Results Before Recalibration
Initial testing showed the signal generator’s output frequency to be considerably different than the selected frequency.
Referring to the oscilloscope trace, I counted roughly 7.7 major graticules. Multiplying that time 0.1 uS and taking the inverse of that value resulted in a measured output frequency of approximately 13.2 MHz — way off from the expected 10 MHz value.
Recalibration of the device for accurate output frequency
For the purposes of adjusting the settings and calibrating the output frequency, I opted to perform the output calibration at a frequency of 1.0 MHz.
I wanted to calibrate the device to be as accurate as possible without the use of an external clock signal, so I changed that setting to “TCXO” so it would use the installed TCXO, after which I worked with the clock frequency and offset settings to calibrate the device for as close to 1.000000 MHz as possible.
Testing the DDS AD9910 at 5.0 MHz after calibration. Let’s see how close it is…
So, now let’s try it dialed-down to its minimum output frequency of 100 KHz…
Now let’s see how it does at some higher frequencies…
Continued testing all the way up to 400 MHz showed the device to be extremely accurate. In fact, at 400 MHz, the frequency measured 400.000014 MHz, an error of only 3.5 to the minus 8th, or — to phrase it another way — 0.035 ppm frequency error. I’d say that’s quite impressive for a device which cost around $160. I’ve seen devices costing up into the thousands of dollars which don’t perform nearly that well.
Oscilloscope and Spectrum Analyzer Views At 100 MHz output setting…
Summary and My Opinion
Overall, I give this device high marks. At the price point of around $160, I have not yet handled or tested another device that’s this accurate in terms of frequency accuracy and stability, although there are probably some out there, given the proliferation of inexpensive yet relatively sophisticated signal generators which have hit the market as of late. The frequency was not difficult to get calibrated and far exceeded my initial expectations. I’d say my friend definitely got his money’s worth. Now… if he’ll just get to work on 3-D printing a case to hold it and make it convenient to use, I just might have to purchase one for my lab bench and tool bag!
PS: Further Testing and Notes After Device Left Running for An Additional Hour Or So After Calibration and Testing…
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