RF2861 datasheet(13/14 Pages) RFMD

13 / 14 page

8-13

RF2861

Rev A2 040107

Differential IF Matching

L1 and C1 are chosen to resonate at the desired IF frequency. C1 can be omitted and the value of L1 increased and uti-

lized solely as a choke to provide VCC to the open-collector outputs, but it is strongly recommended that at least some

small-valued C1 (a few pF) be retained for better mixer linearity performance. R is normally selected to match the input

impedance of the IF filter. However, mixer performance can be modified by selecting an R value that is different from the

IF filter input impedance, and inserting a conjugate matching network between the Resistive Output Network and the IF

filter.

C2 serve dual purposes. C2 serves as a series DC block when a DC path to ground is present in the IF filter. In addition,

C2 may be chosen to improve the combine performance of the mixer and IF filter. L2 should choose to resonate with the

internal capacitance of the SAW filter. Usually, SAW filter has some capacitance. Otherwise, L2 could be eliminated.

A practical approach to obtain the differential matching is to tune the mixer to the correct load point for gain, IIP3, and NF

using the single-end current combiner method. Second, use the component values found in the single-end approach as

starting point for the differential matching. The two-shunt capacitors in the single-end could be converted in a parallel

capacitor and the parallel inductor in the single-end need to be converted in two-choke inductor. Third, set the DC block

capacitors (C2) in the differential-end matching to a high value (i.e., 100pF) and retune the resonate circuit (C1 & L1)

and the resistor (R) for optimal performance. After optimal performance is achieved and if performance is not satisfac-

tory, decrease the series capacitors until optimal performance is achieved.

33 nF

IF+

IF-

IF Saw

IF-

IF+

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