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Since this is an antenna switch, one really doesn't have to worry about timing issues. It would be a very simple straight forward design.

On Tue, Nov 8, 2022 at 08:55 PM, Roger Need wrote:

This feature is already in NanoVNA Saver.

Thanks!!
That should be very helpful. I will try it out very soon - several unknown cores to investigate.
--
Doug, K8RFT

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 :)

On Tue, Nov 8, 2022 at 04:06 PM, Jim Lux wrote:

Pick a FET, use a stack of 9V batteries as appropriate to turn it on and off,
and see what happens to S11 and S21.

I'd expect MOSFET to behave in a similar way, with benefit that I have some high voltage (>400V) and decent current (>6A) handy.

Another question that came to mind - how will the the AC (RF is not DC, current crosses polarity) work with established channel - as I recall from theory, in that might be possible as there is no PN layer to go against

On 11/8/22 5:58 PM, Eamon Egan wrote:

Miro:
As far as your "academic" question goes, it's clear that any frequency specs of a MOSFET pertain to how fast it can be turned on and off, not how high frequency current they can pass. In fact, for switching type MOSFETs I don't think I ever see a frequency given in the datasheet. The high-frequency switching performance is, I believe, simply limited by the various capacitances.
What you would care about here would be
a) off isolation vs d-s capacitance, what I said earlier
b) channel to ground capacitance, if the device is large - likely not a huge issue at HF
c) lead inductance - again probably not all that significant at HF
For turning the gate on, I'd suggest using a photocoupler with a photovoltaic output like what you can find here . 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 don't know of any PV output optos that have 10V kind of outputs, which is what you're going to need for Vgs on a big FET.

An isolated supply is fairly easy - as in an off the shelf DC/DC isolated converter. You could use it as the gate drive directly, since you're not looking for high speed.

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.

Or a configuration that has a series switch followed by a shunt to ground.

Miro:

As far as your "academic" question goes, it's clear that any frequency specs of a MOSFET pertain to how fast it can be turned on and off, not how high frequency current they can pass. In fact, for switching type MOSFETs I don't think I ever see a frequency given in the datasheet. The high-frequency switching performance is, I believe, simply limited by the various capacitances.

What you would care about here would be
a) off isolation vs d-s capacitance, what I said earlier
b) channel to ground capacitance, if the device is large - likely not a huge issue at HF
c) lead inductance - again probably not all that significant at HF

For turning the gate on, I'd suggest using a photocoupler with a photovoltaic output like what you can find here . 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.

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.

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"?

Eamon | VE2EGN; AB1NK

On Tue, Nov 8, 2022 at 03:49 PM, DougVL wrote:

Now I'd like to see a more complicated-to-design feature that would analyze,
characterize and hopefully identify toroid cores.
I'm interested in their use as transformers and as chokes.

This feature is already in NanoVNA Saver. Wind a few turns on the ferrite. Select the graph shown below and look for the "crossover" frequency. Then compare that with manufacturer specs for complex permeability "crossover" frequency (each mix is different).

Roger

On 11/8/22 3:49 PM, DougVL wrote:

Antenna experimenting has caused me to wish for another feature addition to the NanoVNA firmware library.
The new "resonance" search is very handy.
Now I'd like to see a more complicated-to-design feature that would analyze, characterize and hopefully identify toroid cores.
I'm interested in their use as transformers and as chokes. I like to work with loops, which need a choke balun, and with random wire antennas, with a need for a good transformer. (The random wire antenna is where the new resonance search feature comes in handy - avoiding any resonance in a ham band).

I'd suggest that as a start, this be a feature to one of the PC applications that controls a NanoVNA. Or a standalone application.

It's hard to fit things into the firmware, and at some point there needs to a triage. The other thing is that one wants to have firmware that is fairly "model indpendent" so you don't wind up in a situation where some of your NanoVNAs run different versions than others.

Actually, a *very* cool thing would be if someone could start writing Apps that use Bluetooth to talk to the VNA. I know some folks have put a BT module on the serial port. Then you can develop all manner of specialized applications that would work on a tablet or phablet or phone (phone would be nice when up on a ladder, roof, or tower)

I'm not sure if a Android phone can be a USB master - it probably can. But iPhones are a bit more locked down, and don't have a USB port on them anyway.

BT serial port also won't raise issues with the "sandbox" provided by the phone mfrs - it's hard to "do bad things" with a serial port.

Thank you!

Antenna experimenting has caused me to wish for another feature addition to the NanoVNA firmware library.
The new "resonance" search is very handy.
Now I'd like to see a more complicated-to-design feature that would analyze, characterize and hopefully identify toroid cores.
I'm interested in their use as transformers and as chokes. I like to work with loops, which need a choke balun, and with random wire antennas, with a need for a good transformer. (The random wire antenna is where the new resonance search feature comes in handy - avoiding any resonance in a ham band).
--
Doug, K8RFT

On 11/8/22 1:31 PM, Donald S Brant Jr wrote:

On Tue, Nov 8, 2022 at 04:06 PM, Jim Lux wrote:

FET would work well beyond it's nominal switching speed.

Because the FET is not really switching things at the RF rate, the gate being DC biased; the channel is just being opened or closed by the gate and looks like a bias-dependent resistor (plus all of its parasitic elements) on the RF side.
Best regards, Don Brant

In fact, that may actually help.. If the gate circuit is slow, that's great. The bane of switches is when the RF signal causes the switch state to change (a classic problem with diode switches, which is why biasing is so important). So you'd look at whether RF voltage can (capacitively) couple to the gate and move it from the desired state.


Pick a FET, use a stack of 9V batteries as appropriate to turn it on and off, and see what happens to S11 and S21.

(I seem to recall people using FETs, or attempting to do so, for spark gap replacement on a Tesla Coil - the switching speed required is slow (~100 Hz) and it's carrying a 100-300 kHz RF current.)

For a Tesla coil, one wants to be able to turn the switch off at exactly the right time.

On Tue, Nov 8, 2022 at 04:06 PM, Jim Lux wrote:

FET would work well beyond it's nominal switching speed.

Because the FET is not really switching things at the RF rate, the gate being DC biased; the channel is just being opened or closed by the gate and looks like a bias-dependent resistor (plus all of its parasitic elements) on the RF side.
Best regards, Don Brant

On 11/8/22 1:14 PM, Miro, N9LR via groups.io wrote:

On Tue, Nov 8, 2022 at 03:06 PM, Jim Lux wrote:

Since you have a NanoVNA, hook one up and try it.

Yep, that's the next on my list :)

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)

This is interesting value, will need to read a bit more, appears to represent N channel as RC comprised of Coff and Ron

It's not a circuit model, just an arbitrary number to allow comparing switches.

The linked articles talk more about the theory.

On Tue, Nov 8, 2022 at 03:06 PM, Jim Lux wrote:

Since you have a NanoVNA, hook one up and try it.

Yep, that's the next on my list :)

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)

This is interesting value, will need to read a bit more, appears to represent N channel as RC comprised of Coff and Ron

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)

On Tue, Nov 8, 2022 at 10:38 AM, Jim Lux wrote:

That will be the challenge - off isolation. (and the usual issues with
biasing, just like using diodes as switches)

Let's say that my use case is the one where "cross channel leakage" and "off isolation" are not overly critical (HF antenna switching).

The biasing - to be honest, in my "thought experiment" I completely ignored that my signal is AC (RF AC), so some creativity will be required or even render my scenario i possible :)

You might look at T/R switch literature - It's a similar problem - slow
switching, high RF frequencies. You probably aren't as concerned about
the gate capacitance or Miller effect, since you're going to drive it
pretty hard.

Yes, switching is slow (minutes, not nano seconds), so gate capacitance is not a big problem here

Note this: "PIN diodes are offered by many suppliers of RF components.
For example, M/A-COM's MA4AGBLP912 is an AlGaAs PIN diode with just 4 Ω
“on” resistance, low capacitance, and an extremely fast 5 nsec switching
speed (Figure 8). It can be used up to 40 GHz in a shunt configuration,
with an operating bias of +10 mA for the low-loss state, and 0 V for the
isolation state, using a simple +5 V TTL gate driver."

In antenna switching 4 ohm is a lot! Also, powers I want to run through are not of a "signal level PIN diodes" (1KW). There are PIN diodes for high power switching, but I was wandering if MOSFET can do the same

So you have a diode switch 40GHz, but the diode itself is a 50-100 MHz
kind of part (5ns switching speed)

That's exact analogy I'm asking about - MOSFET works differently then PIN diode, even BJT saturation is not the same, but at least what you are saying keeps my question alive :)

Figure 13 has S11 and S21 plots, like the ones you're thinking of
measuring, eh?

I knew my question is appropriate for this group - we just added S11 and S21 to discussion :)

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

Brent, see this topic. It shows the jig I used to measure the ferrite beads:

/g/nanovna-users/topic/79953788#20089

But I calibrated it with a 50? SMD resistor, rather than the leaded resistor 50? load shown there, to get better accuracy into the VHF range.

See this too:

/g/nanovna-users/topic/85417716#24627

Manfred

NanoVNA-F is not compatible with NanoVNA App because the -F does not use a binary
transfer protocol.

Thank you Roger
Here's why it seemed to connect at low level and then it didn't work anymore. So that solves my notice problem :(
--
F1AMM (Fran?ois)

-----Message d'origine-----
De : Roger Need
Envoyé : mardi 8 novembre 2022 19:32

NanoVNA-F is not compatible with NanoVNA App because the -F does not use a binary transfer protocol.

Roger