3. use the clock signal to drive a switching circuit, built from MOSFETs or an analog multiplexer. The switching circuit would "chop" a continuous voltage (which is easy to adjust) thus the output level would be adjustable. Problem: I haven't find yet an analog multiplexer fast enough with a small parasitic resistance.
That's looking the best approach.
It is a trade off, and there are many Analog Switches that head into sub-ohms territory, but they have large capacitance.
When looking into this trade-off not long ago, I found the USB switches were good at reasonably low resistance without killer parasitics.
Most are 3v3, so I'd suggest a LVC buffer pair as above for a fastest / dedicated 5V output, and then USB switches like Toshiba TC7USB40FT or TC7USB42FT in TSSOP14 etc for your adjustable output
TC7USB40FT claim
Supply voltage: VCC = 2.3 to 4.3 V
Switch terminal ON-capacitance: CI/O = 5 pF Switch ON (typ.) @VCC = 3.3 V
ON-resistance: RON = 4.5 Ω (typ.) @VCC = 3 V, VIS = 0 V
RON flatness: RON(flat) = 1.3 Ω (typ.)@VCC = 3 V
BW RT = 50 Ω, CL = 0 pF, 1500MHz
Or parts like NX3DV221 come in XQFN10 HVSON10, which claim 5.5V analog levels,
6 Ohms (maximum) ON resistance
6 pF (typical) ON-state capacitance
High bandwidth (1.0 GHz typical)
That x221 series looks to be multi sourced, and claims a charge pump so they can use a single N-FET for lower parasitic C.
I did not see typical changeover times, so you may need to experiment.
Addit: I see the OnSemi FSUSB42MUX looks a good fit. Specs typical ton/toff of 12,13ns (3V) so has low skew, and is 3.9 Ohms, 3.7pF, 720MHz
RECOMMENDED OPERATING CONDITIONS VCC Supply Voltage 2.4 ~ 4.4 V VSW Switch I/O Voltage −0.5 ~ 4.5 V
Another candidate would be 74CB3Q3257PW - has higher Con, but does spec a faster sel tpd so you could try that to see what bites first : Con or Tpd ?
It looks to run the charge pump all the time, other parts gate the charge pump on OE. It also specs 5V Analog path switch from 3v3 Vcc
Finally, here are the specifications I try to achieve:
- output frequency: 10 kHz (if needed I'm ready to start at 100kHz or even 1MHz) to 100 MHz (I can accept to go a bit lower, such as 80 or even 60 MHz if the signal gets to distorted).
You should be able to have the full Si5351A register range, ~3kHz to ~225 MHz