selecting/designing a quadrature tank for a fsk demodulator

[robismyname]

quadrature tank design

I found this well written, easy to read application note written by FreeScale on designing quadrature tanks for FM/FSK detectors. Im using the SA636. On page 6 of the application note, the right column, under "DE-QUING FOR INCREASED BW" the capacitor value for the tank is 150pF. How was this value derived?

Im asking because the reference circuit for the SA636 page 11, figure 9 says that my capacitor value (C9) should be 39pF. Both Quad tank are designed for 10.7MHz but the cap values are different. I cant tell which design is more credible.

 

[vfone]

fsk quadrature

A tank circuit is formed by a C capacitor and an L inductor.

In Freescale app note, C=150pF in parallel with L=1.47uH (see below C formula) resonates on 10.7MHz.

Philips uses for the tank circuit C9=39pF and L4=5.6uH

Both tank circuits resonates on 10.7MHz.

 

[robismyname]

sa636 app

A tank circuit is formed by a C capacitor and an L inductor.

The freescale app note and the SA636 data sheet both design their tanks with a tunable inductor. Im curious to know could you design a tank with a tunable cap like a trimmer and a fixed/non tunable inductor?

 

[vfone]

fsk quadrature demodulator

Yes, you can.