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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

On 11/8/22 8:23 AM, Eamon Egan wrote:

Miro,
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.

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

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.




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."

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

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

Miro,

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.

Eamon | VE2EGN or AB1NK

OK, this one is only artificially aligned with this group buy using NanoVNA to actually measure what I expect to be answered in a more "academic" fashion :)

How does MOSFET behaves in the triode region, or BJT transistor in saturation when it comes to the frequency response.

What I'm finding is what to expect in the active regions, but could not find anything when it comes to the "extreme" cases when they are in the "wide open/conductive" state.

Can I use a MOSFET as a replacement for a mechanical relay to switch between my antennas even if MOSFET's max declared small signal frequency is quite lower?

This is not the use case where MOSFET is used for fast switching - I'll turn a MOSFET on or off once a day, and then expect it conducts RF (or not).

Might be different between two - MOSFET is using majority charges, BJT is using both and is acting as fully biased diode.

And, just to justify this topic being asked at this smart group - what would you see if connected to 50ohm dummy load and used NanoVNA to measure S11 :)

Thanks!

Can someone provide the link to pre compiled version of NanoVNA-App for Windows - link

:)
--
F1AMM Fran?ois

-----Message d'origine-----

De la part de Miro, N9LR via

Envoyé : mardi 8 novembre 2022 14:53

Can someone provide the link to pre compiled version of NanoVNA-App for Windows - link provided here does not work, and github page only offers source so one can compile and mess it up with it if wanted :)

Google search did not reveal much more then that.

Thanks!

This is a link to a previous discussion in this group about NanoVNA App
/g/nanovna-users/topic/91143866#28096

Thanks
The url does not point to a use of the nanoVNA-App software

I have "COM9" which is displayed and Connected/Disconnected which switches. If I click "Calibration", [Click to open Calibration Window] I get the message "You need to be connected too your VNA" and then Connected/Disconnected no longer switches (stays on Disconnected).

It may come from the fact that my nanaoVNA-F is not compatible with nanaoVNA-App while it is with nanaovna-saver.

Software without manuals is not my age (73 years old). It's okay, nanovna-saver works fine. I am quite satisfied: in HF I compare the results produced by EZNEC on the basis of a description of a real antenna with the measurements made with my nanaoVNA-F + nanaovana-saver and it is "in the nails".

73
--
F1AMM (Fran?ois)

-----Message d'origine-----
De : [email protected] [mailto:[email protected]] De la part de Roger Need via
Envoyé : lundi 7 novembre 2022 20:24

Roger
Thanks for the annotated screen shot. Very helpful. I am a newbie, lying in the weeds observing what is going on. I have made a copy of your post for future reference.
Thanks again.
Don KC3MPO

Yes. I agree, R&S has published some very good information. My current questions revolve, very specifically, around fixturing and calibration for measuring "soft" ferrites, of which there is much less information. I may be homing in on though!