A.2 IF Filter

The recommended IF filter is shown in figure 5. The stop band of the filter is to reject the alias images

around 12MHz, and around 28MHz, where it should have at least 15dBc rejection.

Note that the mixer output is low impedance, (70 Ohms), and the IF input is high impedance (15kOhms),

so considerable voltage gain is achieved in the impedance matching filter.

The filter also sets the bandwidth of the receiver, using the load impedance with the L/C ratio to set the

filter Q.

If desired, an external resistor may be added in parallel to reduce the Q. Note that the

bandwidth must be much wider than the 2MHz needed to pass the power of the GPS signal... it must

maintain linear phase across the 2MHz, even at the extremes of component tolerance.

Fig. 4 Matching Network with four elements

Fig. 5 IF Filter

STB5600

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