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Trying to use log mag and not working now.
I think I overload S21 with more then 10db.
What is the fix anyone now?
I’m hoping some simple right at the port

On 2/13/22 3:37 PM, Alan Brown via groups.io wrote:

Trying to use log mag and not working now.
I think I overload S21 with more then 10db.
What is the fix anyone now?
I’m hoping some simple right at the port

If you check with a thru and/or a pad, do you get the right values (i.e. did you not damage anything).


You can put 20 dB attenuation at the output of your amplifier under test. Calibrate (Short, open, thru) using the pad.

Hi , in my opinion , calibrate the S11 port , with 20 dB of attenuation applied , make non sense .
Maurizio

I think Jim was saying two things:

1) Calibrate it normally and then measure a known device, like e a 20dB attenuator, and see if you get a sane result.

2) If measuring a (say) 20dB amplifier, put 20dB of attenuation after it, so you don’t overload the input.

73, Willie N1JBJ

On 2/13/22 11:27 PM, Maurizio IZ1MDJ wrote:

Hi , in my opinion , calibrate the S11 port , with 20 dB of attenuation applied , make non sense .
Maurizio

Yes, I'd put the 20 dB pad on the UUT output (i.e. the CH1 port, not the CH0 port).? Then you do a calibration (with the pad in place).? Measure your UUT, add 20 dB to the S21 magnitude. You could also measure the pad separately (with a conventional SOT calibration), measure the UUT and pad, then do the arithmetic.

I’m no wiser after these posts.
S21 does not work as not able to calibrate it.
It has gone bad. Overload.
Is there a fix? I’m measuring gain of a LNA as I have done many times and now the port is bad.
I don’t remember breaking it but I probably did as just takes a few seconds not thinking.
I was about to order a new H4 and this one will work for things I don’t need S21 port for.
What is it I’d have to change is the VNA, resistor chip etc??

On 2/16/22 6:16 AM, Alan Brown via groups.io wrote:

I’m no wiser after these posts.
S21 does not work as not able to calibrate it.
It has gone bad. Overload.
Is there a fix? I’m measuring gain of a LNA as I have done many times and now the port is bad.
I don’t remember breaking it but I probably did as just takes a few seconds not thinking.
I was about to order a new H4 and this one will work for things I don’t need S21 port for.
What is it I’d have to change is the VNA, resistor chip etc??

This is a H4? Original NanoVNA RF design, just with a bigger screen? The first thing after the CH1 input is a resistive pad (I can't remember how many dB, but something like 17-20 dB). I would have thought that's pretty robust against a momentary overload. How much did your LNA put into it? The next thing in the chain is the mixer, but that should be able to handle 0dBm without damage and probably more - it's a bipolar Gilbert cell, and the inputs drive the bases of an NPN transistor.? This isn't some delicate CMOS or MOSFET. So, did you put more than ~100mW in?? Or worse yet, hook it up to the supply voltage without a DC block (e.g. if your LNA expects a bias T or something like that) - 12V across the 56 ohm input resistor is ~200mA or 2.5W - That teeny, tiny SMT resistor won't handle that.? After that goes open, the 12V is then across the 240 ohm series resistors and ultimately the SA612 input

You could probably measure the resistors in the pad to see if one or more is cooked.

OTOH, if it's a NanoVNA2, I believe they have an RF switch as the first thing after the connector, and those are fairly sensitive to overload.

On Sun, Feb 13, 2022 at 03:37 PM, Alan Brown wrote:

Trying to use log mag and not working now.
I think I overload S21 with more then 10db.
What is the fix anyone now?
I’m hoping some simple right at the port

The front end of CH1 just contains some resistors and SA612 mixer. Try looking there for damage first....

Shown here >>>

Full schematic here >>

Roger