I'll reply with photos later. In short, the issue with 200 mV ripple at low freq. (700-800 MHz) does not occur with the commercial product (Improve It 5V PGA168 to 5V PGA132 adaptor + another Gainbery voltage interposer) on the same testbed. There is also a measurable high freq. ripple at the FSB freq of 25 MHz, also around 200 mV. The amplitude of the both the low and high freq. ripples appear to reduce once clock double is enabled. The reduction is from around 300 mV to 200 mV. The low and high freq. ripple witnessed from the commercial product is much smaller, like 20 mV.
It was a good idea to connect Vin to the second channel of the scope. When I look at the incoming voltage from the PSU on CH2, I do not witness measurable high or low freq. ripples. Thus I suspect I just need to play around with caps on Vout rather than Vin.
The caps I had ordered in anticipation of issues like this are:
Ceramic 1210
10 nF
47 nF
100 nF
220 nF
Tantalum 1411
150 nF
220 nF
330 nF
Tantalum 2312
0.47 uF
10 uF
22 uF
47 uF
100 uF
Since I already have one tantalum 10 uF cap on Vin and one tantalum 10 uF on Vout, I will leave those in place but add a second 10 uF tantalum on Vout in the centre region to combat the low freq ripple. If it does nothing, then add 100 uF tantalum on Vout.
For the high freq. ripple, I'll first try adding 150 nF tantalum to Vout.
These are my starting points anyway. Any further insight welcome.
Plan your life wisely, you'll be dead before you know it.