| ??? 05/25/07 15:25 Read: times |
#139837 - Don't forget C2, C12 and the stray capacitances Responding to: ???'s previous message |
David said:
They show a parallel LC from Vcc to the output of the PA. The resonant frequency of the LC given their component selection is about 7.1GHz. In every PA circuit I've dealt with you always tune this LC resonant frequency to the frequency of opperation (2.4GHz in my case and in the reference design case). They're calling that LC a 3rd harmonic shunt, but in my mind it's a fundamental shunt (provides a nice path to ground via a 10 pF cap that sits from Vcc to gnd) and a 3rd harmonic pass - when the impedance of the LC is maximized all the RF energy will go to the antenna. In this case that means 7.1 GHz goes to the antenna. 
The LC parallel circuit consists of L1=1.5nH and C4=0.5pF which yields a resonance frequency of f = 1/(2*pi*SQRT(L*C))= 5.8GHz But, don't forget C2, C12, the unavoidable output stray capacitance of chip and the parasitic stray capacitance of L1! These capacitances have to be added to C4. This will push the resonance frequency down to f = 2.45GHz. Take care, even the least change of layout, circuit and components can result in totally unpredictable resonances making your setup appearing to fail. Also, you must use highly expensive SMA cables when dealing with 2.45GHz. And the output must be terminated by exactly 50 Ohm. 2.45GHz is really nasty... Kai |
| Topic | Author | Date |
| RF forum/question? | 01/01/70 00:00 | |
| Physician's Assistant? | 01/01/70 00:00 | |
| Daddy? | 01/01/70 00:00 | |
| Power Amp | 01/01/70 00:00 | |
| this is not a RF forum | 01/01/70 00:00 | |
| Power Amplifier | 01/01/70 00:00 | |
| my intent was not ... | 01/01/70 00:00 | |
| Power Attenuator | 01/01/70 00:00 | |
| What is your problem? | 01/01/70 00:00 | |
| not getting any output | 01/01/70 00:00 | |
| Don't forget C2, C12 and the stray capacitances | 01/01/70 00:00 | |
| a little more.. | 01/01/70 00:00 | |
| Don't take the values too seriously,... | 01/01/70 00:00 | |
| fair enough | 01/01/70 00:00 | |
Electrodynamics instead of electrostatics... | 01/01/70 00:00 |