Direct downconversion (IQ downconverter) with ASK modulation-- what kind of Balun do you need?
You could make your intentions clearer about &quot;using direct downconversion for carrier interference cancellation&quot; to &quot;move the carrier interference to DC then DC block&quot; in your scheme of &quot;[d]irect downconversion ... with ASK modulation&quot;, had you attached the circuit drawing.
But, for a prescribed application of MAX2021 as GSM/CDMA signaling demodulator, you need a power combiner -- in fact, three power combiners: two for differential components in the $I$ and $Q$ paths and one to combine the single-ended $I,Q$ signals into one baseband signal. The &quot;power&quot; determinant should not confuse you: the name is inherited from a nomenclature of microwave power devices (e.g., microwave power amplifiers, attenuators, etc.), but, when used in the context of modulators/demodulators, designates exactly the component you need to combine $I_{+}/Q_{+}, I_{-}/Q_{-}$ outputs into single-ended signals. Besides, microwaves101.com directly indicates a correspondence between baluns and power combiners in the section 180 degree combiners.
Also, the baseband output of MAX2021 is not &quot;square pulses&quot; with zero rise/fall times; after output filtering according to a circuit shown in Figure 4. Baseband Port Typical Filtering and DC Return Network, baseband outputs look more like an &quot;analog&quot; signal with a frequency range defined by the symbol data rate.
In Figure 5. Demodulator Combining Diagram (page 15), the MAX2021 datasheet of your reference directs you to use Mini-Circuits' 180° power combiner ZFSCJ-2-1 and 0° power combiner ZFSC-2-1W-S+. ZFSCJ-2-1 features wideband operations, 1MHz to 3GHz. If your symbol data rate requires frequency below 1MHz, there are available on the market lumped-element/resistive power dividers/combiners of other suppliers, with a lower frequency value down to 4KHz and even DC.
Read also the MiniCircuits blog article and the application note AN10-006.