Hello All,
I’m working on making a slightly simplified external power amplifier antenna for the trf7960a.
While looking at the diagrams provided by TI http://www.ti.com/product/trf7960a#technicaldocuments
HF Power Amplifier design guide: in the appnote_trf796x_pwramp_4w: on page 6, I had some questions on why certain values were chosen for the components that make up the filters.
When I did the calculations for the constant K filters, I found that I did not get the same results as were depicted on the TI schematic, but only on the non-shunted components, and they were about a factor of two off which I thought was different. I was wondering what TI was doing that I did not do.
I understand the post-amplifier design, impedance matching -> BPF(high pass -> low pass) -> extremely broad low pass filter with C113, I think, acting as a phase shifter for the receiver. I am assuming the capacitors are split into parallel for robustness.
Before I build the antenna, I want to understand what I was doing wrong. I am a firm believer in understanding a design before you build it instead of trying to fix it later.
L1 = L102: 538nH
L2 = L103: 538nH
C1 = C105: 220pF
C2 = C106: 47pF
Ceq=C1||C2=267pF
I am using the following K values in-order to calculate the components
KL1=7.94
KL2=7.94
KCeq=1590
Star values are my calculated values for the components.
L1*=KL1/F_MHz=7.94/12=0.661 uH
L2*=same calculation =0.661 uH
Ceq*=KCeq/F_MHz=1590/12=132.5 pF
When comparing L1*,L2*,Ceq* with L1, L2, Ceq, I was okay with the inductors, but the capacitor value was off by about a factor of two (Ceq=267pF Ceq*=132.5pF)
For the LPF in the bandpass filter consisting of L104, C108, C110 F_MHz=15, I found
L104*=1058 pF
C108*=265 pF
C110*=265 pF
KL104=15.88
KC108=3180
KC110=3180
Here L104*=1058pF while L104=538pf which is about a factor of two off similar to the in-line capacitor earlier which is why I am asking this question to see why this is the way it is?
The transceiver LPF made using C111,C112, and phase shifter C113 seemed feasible. Doing the calculations without C113 provided okay results.
I just do not see how those values for the components were chosen in order to meet the specifications that the documentation calls for from my current knowledge of filter design and would like to know what I am missing because I've searched and cannot find a reason to assume the calculations above are wrong.