On 11/8/22 12:50 PM, Miro, N9LR via groups.io wrote:
On Tue, Nov 8, 2022 at 10:23 AM, Eamon Egan wrote:
Even if we assume that you can bias the MOSFET properly in the presence of RF
and try to use it as a switch, and even if we assume that it behaves pretty
well in the ON state given its pretty low Rds, you will need to check what OFF
isolation you can achieve given the device's drain to source capacitance.
Fair point - my use case is more forgiving - I'm "exploring" options to replace relay based HF antenna switch, where "OFF isolation" is marginally critical.
But back to "academical" side of the question - is frequency characteristic of MOSFET in triode or BJT in saturation commensurate to the equivalent in active regions, or extends to higher frequencies
I've used a lot of MMIC FET switches over the years, but not actually designed them, so I can't speak to "bandwidth of the part" vs "bandwidth of the switch". I'm going to guess, based on the diode notes cited in another post, that the FET would work well beyond it's nominal switching speed.
Since you have a NanoVNA, hook one up and try it. Pick a FET with Vds and Ids bigger than you'll be using with your transmitter or receiver, and just breadboard something.
has a lot of interesting info on MMIC switches.
In particular:
has some modeling examples
And
talks about Switch Figure of Merit, with a note that SwFoM * 10 is the highest frequency it will work at as a switch.
So, FoM = 1/(2*pi*Coff*Ron)
