How to connect LNA to VNA

[robismyname]

I would like to measure the gain of my LNA using a VNA.

I understand the concept behind:


b1 = S11a1 + S12a2
b2 = S21a1 + S22a2


S21 = forward transmission = b2/a1 = signal out of port 2/signal incident at port 1

S12 = reverse transmission = b1/a2 = signal out of port 1/signal incident at port 1

S11 = input reflection = b1/a1 = reflection at port 1/incident at port 1

S22 = output reflection = b2/a2 = reflection at port 2/ incident at port 2

What I dont understand is what ports of the VNA goes to my LNA input and what port of my VNA goes to my LNA output for S21, S11, S12, S22 measurements?

On my VNA it has:

port 1

FWD --------------->

REFL FWD------------
-
-
-
<---------------------


port 2

<-------------TRANS REV

-------------- REFL REV
-
-
-
----------------------->

So if I wanted to measure S21 (forward transmission) how would I connect my IN/OUT of my LNA to the VNA Ports 1/2?


So if I wanted to measure S11 (input reflection) how would I connect my IN/OUT of my LNA to the VNA Ports 1/2?


So if I wanted to measure S12 (reverse transmission) how would I connect my IN/OUT of my LNA to the VNA Ports 1/2?

So if I wanted to measure S22 (output reflection) how would I connect my IN/OUT of my LNA to the VNA Ports 1/2?

 

[albbg]

The symbol S11 means reflection measurement did at port 1, and S22 at port 2.
S21 means a transmission measurement from port 1 to port 2 and, of course, S12 means from port 2 to port 1.
Thus if you connect port 1 to the input of LNA and port 2 to the output, you will measure:

S11 LNA input reflection
S22 LNA output reflection
S21 LNA gain
S12 LNA isolation

 

[tony_lth]

Be careful, some LNA IIP1 is only about -20dBm, but LNA normally S1 port output is -5dBm, that would cause your LNA dead.

 

[robismyname]

S12 LNA isolation

The S12 isolation measurement between the output and input of the LNA has no relationship with measuring the LNA's linearity (IP3), 1db compression (P1db) or point and LNA BW correct?

---------- Post added at 01:50 ---------- Previous post was at 01:29 ----------

Be careful, some LNA IIP1 is only about -20dBm, but LNA normally S1 port output is -5dBm, that would cause your LNA dead.

Are you implying that P1db = IIP1?

The LNA under test (RF2472G) has a P1db of -10dbm and a IIP3 of +10dBm.

 

[tony_lth]

Normally P1dB is output P1dB, but IIP1 is input P1dB

 

[robismyname]

So i dont have to ever place port two of the VNA into the input of the LNA and port 1 of the VNA into the output of the LNA?

 

[enjunear]

So i dont have to ever place port two of the VNA into the input of the LNA and port 1 of the VNA into the output of the LNA?

You can make the measurements that way, but the normal convention is that port 1 is connected to the "input" of the DUT (Device Under Test) and port 2 is connected to the output of the DUT. Thus, S21 is always the gain of the device... when talking about amplifiers, in general. S11 is then input return loss and S22 is output return loss. If you switch it around, you'll confuse a lot of people because of the port 1-to-port 2 convention.

Agree w/ previous posters... turn down your source power to something that won't cause your LNA to get into compression (stay about 3dB under P1dB, use external attenuators if you have to, and calibrate them out). The lower the power, generally the better for amps, but at the expense of accurate measurements at the low end of the power range (e.g. not much signal is injected, so very little signal is reflected back... there is very little signal for the VNA's receiver's to pick up, accurately).

 

[albbg]

You don't need to physically swap port 1 and port 2 to measure gain and solation. Just connect port1 to input and port2 to output.
RF2472G has a typical input P1dB=-10 dBm. This meas that if you VNA is generating -10dBm you will see a gain 1 dB lower than the actual (of course we are supposing P1dB of your device is actually -10 dBm).
Since the expected gain of the LNA is 14.6 dB, I suggest you to set the VNA to generate a power of about -20 dBm, that means an output power of your LNA of about -5.4 dBm high enough to have a precise measurement.
You can also set the VNA to generate at a fixed frequency a power sweep so that you can find the actual value of P1dB. Be careful not to exceed the absolute maximum ratings. For instance, you could set the maximum power to be generated to -5 dBm.
If you are interested to linearity you can do the measurement using two generators and a spectrum analyzer. In this way you can input to the device two tones having the same level at different frequencies (let say f1 and f2),. The you can measure the intermodulation product at a frequency of 2*f1-f2 then if the level of the two tones is Ptone and the intermodulation product if lower tahn this level of x dBc, then OIP3 = Pout + x/2.
If for instance you generate the tones with level of -20 dBm and you measure an intermodulation with power -50 dBm, then x = -20 - (-50) = 30 dBc then
OIP3 = -20 + 30/2 = -5 dBm. It is better to place isolators at the output of each generator in order to avoid that they intermodulates.

 

[robismyname]

You can also set the VNA to generate at a fixed frequency a power sweep so that you can find the actual value of P1dB. Be careful not to exceed the absolute maximum ratings. For instance, you could set the maximum power to be generated to -5 dBm.

Thanks for the tips. IP3 measurement im ok with. More interested in the P1db measurement. By definition P1db means the input signal level at which the receiver gains drop 1 dB as you mentioned earlier.

My question is related to the definition of P1dB.

so lets say my input and corresponding output signals via the LNA is:

input output
-20 -10
-19 -9
-18 -8
etc.

I will know i reached P1 dB when my output signal is only 1dB less than my input signal? I.E.

input output
-17 -16

please confirm.

Not familiar with P1dB measurement using VNA, only SA. Are either one of these methods more of a standard in industry? If VNA is more of a standard method for P1dB can you elaborate a little further on the test setup or provide link?

 

[albbg]

Your understanding about the meaning of P1dB parameter is not correct.

Ideally the tranfer function of a linear amplifier is Output = Gain + Input (in dB). This is true in a limited range of input values. Exceeding the maximum input power for which the device is linear it starts to compress due to saturation. If, for instance the gain is G=10 and we measure the following behaviour:

.in (dBm)..out(dBm)....equivalent Gain (dB)
...-20..........-10.....................10
...-19..........-9.0....................10
...-18..........-8.0....................10
...-17..........-7.2....................9.8
...-16..........-6.5....................9.5
...-15..........-5.8....................9.2
...-14..........-5.0....................9
...-13..........-4.2....................8.8
(sorry for the dots, but I was not able to place the numbers in the right column using spaces)

as you can see for an input of -14 dBm we measure an output of -5 dBm instead of the -4 dBm expected for a gain of 10 dB. This means the output is compressed 1 dB that is IP1dB (Input P1dB) for this device is -14 dBm and OP1dB (Output P1dB) is -5 dBm.
OP1dB = IP1dB + G.
You can see that at P1dB the gain is 9 dB instead of 10 dB that is is lowered by 1 dB.
Then to measure the P1dB point by means of a VNA, simply generate a power sweep (not exceding the maimum absolute rating) at a fixed frequency and have a look to S21. You will see a horizontal line that drops when the input reaches a certain level. The line is the gain of the device and when it drops by 1 dB the P1dB point has reached. Simply place a cursor at that point and measure the input power.