Ad8307 Testing up to 500mHz

Ran another linearity test this time from 60mHz to 500mHz. Power level was set at -70dbm and -30dBm.  

Two conditions for each power level. Compensation components in circuit and components bypassed.  Results were pretty interesting for the bypassed assessment. It was about a 160mv drop, which would represent about a 6dBm loss, over a 50mHz span and 205mv over the entire range.

Since I soldered in a jumper in order to bypass the compensation components, I wonder if it is having the effect that is responsible for the rapid falloff.  I suppose I could put in another jumper of a different size/shape and see if there is any change in the curve. Even though I’m not all that interested in in frequencies this high, the curiosity factor alone is enough to try it.  Besides the whole idea of these experiments is to learn all I can about this device first hand.

The compensated response shows a 94mv drop over the range from 60mHz to 500mHz.  That would be close to a 4dBm drop. Not too bad and certainly can be easily taken into account either by software or a “cheat sheet”. 

As well I injected various hf frequencies into the Arduino input along with the 8307 output. I used a .1 cap to isolate the signal generator. Long story short here was, unless you inject a signal at the level of 0 to 5 dBm, there was no effect on the readings of the dvm.  Even then it was only 1 or 2mv. However the important thing here was not how the dc output was performing but how the Arduino behaved to any harmonics that may bleed through and get into the adc. The readings being displayed by the Arduino were steady and never once changed values no matter the frequency or power level. So for me the debate begins on how necessary is the lpf in that path? 

 

Linearity Test for ver 3 Board

Ran a linearity test from 400kHz to 50mHz. Power level was set at -70dbm.  I’ll do another run at a higher power level later, but previous testing showed an even better response curve.

Surprisingly, the circuit over this range of frequencies without the frontend compensation components had better results. 32mv compared to 40mv.  Over the hf range the variance was 12mv for no compensation and 20mv for the circuit with the compensation components added in. Maybe the circuit has better results on frequencies between 100 to 500 mHz.   Since a change of 1dBm results in a change of 25mv, this represents about .5dBm difference when not using the compensation components within the hf frequencies.

 

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 1^st test bypassed the lpf. 2^nd test was with the lpf in circuit and ran tests with and without an Arduino running. 3^rd 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.