# About how to roughly measure LNA NF contour - RF Cafe Forums

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duplicity
 Post subject: About how to roughly measure LNA NF contour Posted: Mon Apr 19, 2010 11:05 am
 Lieutenant

Joined: Mon Apr 19, 2010 10:51 am

Posts: 2

Hi RFCafe senior engineer,

Good Day.

I have a serious issue about LNA NF measurement, and really need your kindly support.

What I want to do is try to draw LNA NF and Gain circle by measurement, and compare with simulation.

Attach related file, especially NF circle.

So, I set up a simple measurement, as shown below:

Here, I got several problem,

1. What value of inductance I should use with power choke inductor, refer to application circuit?

2. How I define output M.N.? refer to application circuit?

3. If I do a simple output conjugate matching, should I turn on LNA, if yes, is power choke a part of matching network?

4. Here comes important problem,

4.1 What I can measure is total system(50ohm) NF, and I want to determine LNA's NF by equation.

4.2 For special case, that's Гin1= Гout1= Гin2, the FriiS equation is O.K.

4.3 But actual situation is, Гin1≠ Гout1≠ Гin2, right? So how to modify FriiS equation.

Itay
 Post subject: Re: About how to roughly measure LNA NF contour Posted: Tue Apr 20, 2010 3:19 pm
 Lieutenant

Joined: Tue Apr 20, 2010 2:35 pm

Posts: 1

Hello duplicity,

Quote:
1. What value of inductance I should use with power choke inductor, refer to application circuit?

The inductor value should give you a reactance of several hundreds ohms (at least 300 ohms) in your lowest frequency of operation. remember that as the frequency increases, XL increases as well.

In addition, you should choose the inductor such that its SRF (Self-Resonance Frequency) would be at least twice higher than your maximal frequency of operation in order to avoid a capacitive reactance.

Quote:
How I define output M.N.? refer to application circuit?

Usually for LNA, you define the output matching network as one for conjugate match. This is because the input matching for noise figure degrades the return loss at the input and reduces the overall gain (mismatch), and as a result you loose power.

Quote:
If I do a simple output conjugate matching, should I turn on LNA, if yes, is power choke a part of matching network?

You should design the output matching only after your input mtaching network is fully designed and connected to the LNA, because the input matching network will affect your S22. The reason for that is that the amplifier is not unilaterlal, i.e. there is a certain feedback from the output to the input S12.

The choke (power inductor as you named it) if properly chosen, should slightly influcnece on the output mathcing network. Actually, it isolates between the supply voltage to the matching network. Usually from RF signal point of view, it only slightly increases the gain.

So your output matching newtok should be designed to the conjugate of the modified S22 (With the influence of the input matching network).

You should turn on the LNA, in order to measure the output power.

____________

In general, when designing matching networks, pay attention to the stability of the amplifier.

It is very important to check that your required reflection coefficients are located within the stable region.

You can perform stability simulation or calculate K (Rollet's Factor). For unconditional stability K>1

duplicity
 Post subject: Re: About how to roughly measure LNA NF contour Posted: Thu Apr 22, 2010 10:56 am
 Lieutenant

Joined: Mon Apr 19, 2010 10:51 am

Posts: 2

Quote:
In addition, you should choose the inductor such that its SRF (Self-Resonance Frequency) would be at least twice higher than your maximal frequency of operation in order to avoid a capacitive reactance.

I will kind in my mind, thanks for your lesson learn.

But actually, the reference design is around 3nH(LQP03TN3N0B04), whose SRF is over 6GHz, and impedance is only 38.46ohm at 1990MHz.

Quote:
You should design the output matching only after your input mtaching network is fully designed and connected to the LNA, because the input matching network will affect your S22. The reason for that is that the amplifier is not unilaterlal, i.e. there is a certain feedback from the output to the input S12.

But how about isolation?! The input M.N. should not influence S22 load.

How i tine the S22 M.N. is tep by:

1. S11 should be ready first.

2. tune choke to get higher gain.

3. tune second componet for conjugate matching.

Quote:
You can perform stability simulation or calculate K (Rollet's Factor). For unconditional stability K>1

IR
 Post subject: Re: About how to roughly measure LNA NF contour Posted: Thu Apr 22, 2010 2:57 pm