[G0HZU_JMR]

I have quite a few of the classic (sadly obsolete) Avago Schottky diodes here plus some low capacitance Infineon alternatives.

I have several HSMS-282x types and I could tack together a basic detector using SMD parts to show you the response if that helps?

I have various sig gens here with excellent amplitude flatness, typically <0.08dB error across LF through 1GHz and it should be possible to prove the response of a 'contender' for your sig gen detector. My guess is that (to offset the droop vs frequency in the step attenuator) you would want maybe 1.5dB to 2dB 'droop' in the detector by 1.1GHz and this would require a diode with about 0.5pF capacitance when driven by a 200R source. The hot end of the 200R resistor will be a voltage source or 'virtual ground' because of the ALC action so I think that only the 200R resistor sets the resistive part of the time constant for the droop. There may be other parasitics that influence the droop/rolloff of a practical design and the layout of the detector components will be critical here. If old school leaded parts are used instead for the detector components then all bets are off with respect to the frequency response. Depending on how sloppy the layout gets the detector could easily have an undesirable peak somewhere up towards UHF. This would give the opposite effect and any positive peaking of the detector at 1GHz (say 2dB) would cause the sig gen output level to appear very droopy by 1GHz. It could easily droop by 4dB by 1GHz in this example.