how to decouple RF LNA for desired frequency of 140MHz?

[robismyname]

how to decouple RF LNA

My frequency of interest is 140MHz. How do I decouple this part properly for my desired frequency of 140MHz?

 

[vfone]

Re: how to decouple RF LNA

Looking to the S11 Smith chart (Zin_approx@140MHz = 90 – j40), seems the amplifier needs an external match to improve the return loss at 140MHz.

Try to use a High-Pass matching topology to decrease a little bit the gain at low frequencies, which is very high. Use WinSmith to find the components.

S22 shall be fine, but for 140MHz probably needs to increase the value of the output RF choke L1 up to 500nH.

For decoupling add another 1nF capacitor in parallel with existing ones, to get better decoupling at VHF frequencies.

 

[robismyname]

Re: how to decouple RF LNA

Looking to the S11 Smith chart (Zin_approx(at)140MHz = 90 – j40), seems the amplifier needs an external match to improve the return loss at 140MHz.

I got 80-j35 looking at the 5V smith chart. Did you look at 5V or 3V?

Try to use a High-Pass matching topology to decrease a little bit the gain at low frequencies, which is very high. Use WinSmith to find the components.

When you say matching topology you dont mean impedance matching, you're refering to designing a fillter network correct?

S22 shall be fine, but for 140MHz probably needs to increase the value of the output RF choke L1 up to 500nH.

S22 looks fine because 140 MHz is close to 50 ohms match?
At 150nH Inductive reactance is 131ohms at 140Mhz. If I use 500nH my inductive reactance goes to 439ohms. Shouldn't my inductive reactance be smaller at my desired operating frequency (140MHz)? What did I mess up here?

For decoupling add another 1nF capacitor in parallel with existing ones, to get better decoupling at VHF frequencies.

Is this point a matter of Inductive Reactance, allowing lower frequencies to pass and blocking higher frequencies?

 

[vfone]

Re: how to decouple RF LNA

J35 or j40 is not too much difference.

Matching networks could have different topologies: Low Pass, High Pass, etc. There is no filter.

S22 is close to 50 ohms at 140MHz, so in this case the load choke has to have high reactance at respective frequency.

Decoupling capacitor must have a low reactance at working frequency.