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.