Sunday, 20 August 2017

More AD8307 Testing

Did some further testing today and realized that for some of yesterday’s tests the Arduino sitting close by was powered up. So I re-did all the tests and then some. 

I installed a new thru hole 47uf cap and for the 1st test bypassed the lpf. 2nd test was with the lpf in circuit and ran tests with and without an Arduino running. 3rd test was with the Arduino running and the front end compensation circuit bypassed. For the graph presented here is with the Arduino connected to the AD8307 except for the front end bypassed series.  Of note here are any differences to the slope with the Arduino connected or not is minimal.

After extensive testing, I make the following observations:

LPF has no or minimal effect on output performance.

Adding in the frontend compensation components does have a detrimental effect on the bottom end slope. Use of a jumper to bypass the compensation components improved performance by 5dBm and I would think further improvements will be realized when a trace replaces the jumper. Adding in these components if HF frequencies are the main level of interest is unnecessary. Testing conducted went up to 70mHz and the max error would be about 1.5dBm worse case. 

Effective shielding is a must. An Arduino 6” away effects results as does your hand near the 8307.  The effect from the Arduino is noticeable in the -70 and lower range.

In conclusion, if the frequencies of interest are approximately 1 to 50mHz, then I would not include any of the compensation components. Improvement in the bottom end performance is at least 5dBm.  Earlier testing shows another 5dBm improvement can be had with shielding.  This gets you solidly into the -80 range and perhaps even lower.

Still to come with the 8307 will be to put into a proper case, change out the thru hole regulator for a smt version, and make appropriate connections to the Arduino input and then retest. Another test will be to study the effects on the ADC when mixing low levels of frequencies along with the DC output of the 8307.  Interest here is to see if a lfp is really needed on the 8307 output and its effectiveness.


Saturday, 19 August 2017

AD8307 Version 3 Board

It took a while but I finally got a board that is close to a final version. Version 3.   

Version 2 board was etched and I started to populate but after about 4 components I realized that there were too many short comings and it was not work the effort to complete.  

Version 3 is a redesigned board and all components are now smd.  New is a lpf on the output of the 8307 to protect the adc from any rf.  I think the filter may need to be reworked somewhat but that will be later on and after running some simulations in LT Spice.  As well I installed the additional components in the front end to extend the range up to the 500mhz level.  While I’m not that interested in frequencies that high, I wanted to see if there was any improvement in the HF range.  From 1mHz to 30mHz there was a drop of 1dBm and although that’s not much in the scheme of things, why not try and see if I can make an improvement.

The board was a little challenging to build as I went with smaller traces. But, the tighter clearances were the main issue.  So as I installed each component I checked for shorts with an ohm meter. Probably not a bad idea anyway but adds to the time. The lpf resistors are 603’s and that was an interesting exercise for installation. They are a little cockeyed but since they weren’t shorted to ground I was happy and left well enough alone.

Results so far are ok. The bottom end is not as good as ver 1 and have a couple of things to check for that.  Front end component that I added, the lpf, or maybe even the 8307 chip.  The chip is a smt version from eBay and the ver 1 board had a dip package type I bought from Digi Key.

Due to the 15uf cap causing a large draw of power, I let the smoke out of the smt regulator. In the meantime I had wired up the TO-92 type until I figured out the problem. The traces are now a little worse for the wear unfortunately. As well the pinout is incorrect on the board but I can get around that by putting the regulator on its back. Correct that in version 4.