Some question about SAW filter.
We used a Maxim 2507 transmitter in our cellular phone.
There is a Saw filter between VGA and PA. Both the output of VGA and the input of PA are matched to 50ohm on chip. The output and input impendance are also 50ohm. In the reference design, There are two capacitances in the connection between the output of VGA and input of SAW, and the connection between the output of SAW and input of PA. One is 220pF, the other is 2.2pF.
I cannot understand the role the capacitance are playing and why the value of capacitances are not identical.
C31 is probably to match output of SAW to 50 Ohm, while C37 is probably to block DC to SAW.
I hope it can help.
Max
Actually the PA_IN of MAX2507 is not perfect 50ohms (as should be), and needs that value of capacitor, to match the 50 ohms output of the SAW filter.
For some units I found that S11 of PA_IN is changing with input power, and is not ok (probably is a self bias in the first PA stage).
Normally ,I see the value of the DC block capacitor is 22 pf in rf circuit . Also ,the match network series compont is inductor ,maybe the capacitor increase the S21 .Do you have any comments ?
C37 is used to block DC to SAW filter. So how do i decide the value of C37?
whatever is ok?
Real capacitors has parasitics; The value of cap to use as DC block is in the range:
-high not to introduce attenuation
-lower than Self resonnat freq.
30-60 pF in the 1-2 GHz range in case 0603/0402 are commonly used as DC blocks.
I hope it can help.
Mazz
Maybe the PA has some low frequency oscillation issues so therefore the coupling cap to the PA is small.