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Hi. I just bought the NanoVNA and want to make some tests on some LNAs.I have 2 SMA 20db Attenuators.How to setup the VNA & how S11 is important in the Tuning.Very thanks for answers.Regards Sid.

in order to measure S11, you're needs to put 50 ohm load on LNA output and connect CH0 to LNA input. But first, make sure that your LNA can handle 0 dBm level! If it cannot, then there is no simple way to measure S11. If your LNA cannot handle 0 dBm on the input, do not connect it directly to NanoVNA, because you can damage LNA input.

in order to measure S21, you're needs to connect LNA input to CH0 through 20 dB attenuator. And LNA output to CH1 through 20 dB attenuator.

It's better to use 30-40 dB attenuator between LNA output and CH1. But 20 dB also may work if your LNA has not so high gain (smaller than 30 dB).

For high gain LNA (more than 30 dB) you will need 40 dB attenuator between LNA output and CH1, otherwise your CH1 input on NanoVNA may burn out. Do not connect high gain LNA output directly to NanoVNA, you can damage NanoVNA!

You're needs to add attenuators sum to the measured S21 values. In your case you will need to add 40 dB to measured S21. For example, if it shows S21=-30 dB, it means that LNA gain is -30+40=+10 dB.

Very thanks for quick answer qrp.ddc. I made the 2nd manipulation with 20db connected to ch0 & ch1 and turned the Orange & blue traces & have had the S21 & S11.I think i was wrong.
The amplifiers are 20-24db 2m band.
I made a calibration with the 2 Attenuatos connected to have a zero instead of 40db.
Very thanks for infos & 73s dr qrp.ddc

Said,
I posted this procedure earlier that I used to check the gain of a RF preamp. The device under test was a Mini-Circuits preamp with a handwritten note saying, "27 dB gain, 1-1000 MHz".

Test procedure used:
1. Turn the amplifier on to allow it to warm up and also to guard against turn on transients damaging the nanoVNA during testing.
2. Turn on the nanoVNA, set the display to CH1/LOGMAG only, 10 dB per DIV, reference position at 4 divisions to be able to see 27 dB gain.
3. Connect a 20 dB attenuator to CH1 and a 6 dB attenuator to CH0 to closely match the expected 27 dB gain of the preamp.
4. Connect attenuators together using a cable. The nanoVNA should be displaying a 26 dB loss over the frequency range. If not your attenuators are not flat
5. Change to the calibrate menu and select "reset" and then perform a "through" calibration. The new reference should be 0 dB at 4 divisions.
6. Connect the output of the 6 dB attenuator to the input of the amplifier and output of the amplifier to the 20 dB attenuator.
7. Read the amplifier gain in dB directly off the nanoVNA using the marker display.

The above only checks the S21 performance of the preamp. That was all I was interested in at the time.

Hi hwalker.Very thanks for answering my post & i read before your procedure. So the S11 isn't interesting in the LNA Performance.
Regards.

On Thu, Sep 19, 2019 at 03:17 PM, hwalker wrote:

and a 6 dB attenuator to CH0

6 dB is too small, the output level may be very high for LNA and may overload it, so you may get incorrect measurement. I'm using 30-60 dB attenuator on the LNA input for measurement.

On Thu, 19 Sep 2019 at 17:19, Said via Groups.Io <cn8li@...>
wrote:

Hi hwalker.Very thanks for answering my post & i read before your
procedure. So the S11 isn't interesting in the LNA Performance.
Regards.

From all four S-parameters it is possible to work out the Rollett
Stability Factor K



which will allow you to determine if the amplifier is unconditionally
stable. So to say S11 is no use is incorrect.

On a professional VNA you can add an attenuator up to about 20 dB on a port
and perform a 1-port calibration. 20 dB is about the limit. I don’t know
what you might get away with using the NanoVNA.

Dave.

--

Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales.
Company number 08914892.
Registered office:
Stokes Hall Lodge,
Burnham Rd,
Althorne,
Chelmsford,
Essex,
CM3 6DT,
United Kingdom

All my low noise RF preamps have a maximum input of 0 dbm and can handle the maximum output power of the nanoVNA without an attenuator. You have to know what your system budget is and choose your attenuators accordingly. I always look up the specs for the maximum gain and maximum input power of the amplifier under test prior to testing. As an example, today I tested an HP8447D RF preamp. It has a specified 25 dB gain from 100k-1.3GHz and a maximum input of 0 dBm. Knowing those facts, I chose to use at least 25 dB of attenuation in-line during testing to offset the gain of the HP8447D, and stay within the 40 db S21 dynamic range of the nanoVNA at 900 MHz. If you are testing below 300 MHz you can get away with a lot more attention in-line (S21 dynamic range is spec'd at 70 dB). I ended up using a 10 dB and 20 dB attenuator in-line. The 10 dB attenuator was connected to the CH0 output of the nanoVNA and the input of the HP447D. The HP8447D can handle the output of the nanoVNA without an attenuator, but it is good practice to use an attenuator to lower the mismatch and provide isolation. The 20 dB attenuator was connected to the output of the preamplifier and the CH1 input of the nanoVNA . As with the other RF preamps I've tested using the same procedure, the system operated linearly over the 50k-900 MHz test range and the measured gain of the HP8447D was 26 dB =/- 1 dB. This result was verified using a calibrated HP8753C VNA.

The above is my typical S21 measurement procedure for verifying the gain of a low noise RF preamp, such as one that might be used to improve the noise floor of a receiver or spectrum analyzer. When testing a block LNA more attenuation, as suggested by qrp.ddc, may be in order.

On Fri, 20 Sep 2019 at 00:12, hwalker <herbwalker2476@...> wrote:

All my low noise RF preamps have a maximum input of 0 dbm and can handle
the maximum output power of the nanoVNA without an attenuator.

Whilst an LNA can probably take 0 dBm without damage, it would be driven
well beyond its linear region.

I just took as quick example, one of the Minicircuit amplifiers I have here



1 dB compression point +5 dBm
Gain > 24 dB

That means a signal of 5-24 = -19 dBm would drive that into compression. A
typical amateur radio LNA would almost certainly have a 1 dB compression
point of < 0 dBm.

I reckon something around a 10 dB attenuator on the output of port 0 would
be desirable before an LNA, and another 20 dB or so after the amplifier.

A typical amateur LNA would use a FET. The largest 1 dB compression point
would be achieved with a high drain current and lowest noise at a smaller
drain current.


Dave

Dave.
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales.
Company number 08914892.
Registered office:
Stokes Hall Lodge,
Burnham Rd,
Althorne,
Chelmsford,
Essex,
CM3 6DT,
United Kingdom

Tks for mentioning that Dave. I forgot to add that besides the amplifier gain and max input, the 1 dB compression point should be taken into account when determining the system budget.

When I am testing amplifiers, using one of my network analyzers, I normally start at -30 dBm output from the signal source, into the device under test. You don't to come anywhere close to saturation, or the results will not be accurate.
A good indication that a linear amplifier is going into saturation can be determined by watching the bias current. As you raise the input level, the current will start to rise as the device saturates; however you need a resolution of at least 10 ma on the ammeter. The inexpensive Chinese bench power supplies, with digital meters are very good for amplifier testing. For the most part, an analog meter is useless.
Stuart K6YAZLos Angeles, USA

Very interesing to read as this my first VNA to have in hand and i want to discover its possibilties with my ARR,PA3BIY and G4DDK 2m LNAs for EME.
I checked a professional cavity BPF (IL=-1.6db) to install after the LNA.
Regards Sid.