Greetings gentlemen,
I'm trying to improve on this 40 year old design by HP. The ALC section is supposed to maintain the RF source output level across a range of 4Mhz to 1300Mhz constant to within 1dB. Unfortunately, due to a combination of component ageing and a restricted design budget at that time, it doesn't meet this spec; not even close. There is "considerable room for improvement" as they say. :) I'm inclined to believe there are better solutions available today with better frequency and temperature compensation than that afforded by the handful of discretes shown; possibly something at least partly monolithic? Suggestions welcome...
Here's the schematic; you can see the relevant signal path is shown by dashed lines: