On Tue, Nov 8, 2022 at 07:58 PM, Eamon Egan wrote:
What you would care about here would be
a) off isolation vs d-s capacitance, what I said earlier
Once I get time to put this on my bench I hope that "off isolation" will not be a big deal given that the load on another switch is 50 ohm antenna
b) channel to ground capacitance, if the device is large - likely not a huge
issue at HF
As power dissipation will be fairly small I will try NOT to ground the case/heat spreader. Also, should not be a big deal on HF
c) lead inductance - again probably not all that significant at HF
Have that same problem with mechanical relays
For turning the gate on, I'd suggest using a photocoupler with a photovoltaic
output like what you can find here
.
Not sure that will work - Vgs needed to turn the MOSFET on is quite high
This is because these photocouplers have a photovoltaic output that is
isolated from the control (LEDs), and the entire output circuit may may be
able to surf on top of the RF; this is a much easier approach than anything
that involves a direct connection.
I'll start with AA bateries, and then if concept works I have isolated DC/DC converters just for this use :)
For improving off isolation, you could put multiple identical switches in
series. I would start by simply taking the device and measuring its off state,
to see how many I might need to chain up.
Let's see if results are OK with just one. Getting them in series will overly complicate the circuit and add more inductance
Lastly - I was assuming MOSFETs weren't used as RF switches until I asked
Google. Apparently there are such applications. Have you had a look under the
Google search "MOSFET as RF switch"?
All that I found is MOSFET as high speed switch - I need slow speed switch for high frequency circuit :)
