Direct downconversion (IQ downconverter) with ASK modulation-- what kind of Balun do you need?

Question

I have a simple but confused question.

I am trying demodulate a 915MHz ASK modulated signal with direct downconversion. So what I am building up is: Antenna --> SAW bandpass filter @915MHz --> LNA --> Direct IQ down-converter (max2021 datasheet LO = 915MHZ) --> balun? --> decoder(sdr)

The max2021 downconverter is a 4-ports output: I+,I-,Q+,Q-. To put the differential signal back to single-ended, I was taught this is a balun's job.

But when I trying to build this system the balun concept is confusing. If successfully down-converted and DC blocked , the IQ outputs are just unmodulated differential square pulses-- so what kind of balun do I use and at what frequency? Do I use a digital buffer in this case instead of an analog balun?

ps: I am using direct downconversion for carrier interference cancellation(move the carrier interference to DC then DC block) and design simplicity; and IQ downconverter to avoid phase ambiguity.

Answer 1

You could make your intentions clearer about "using direct downconversion for carrier interference cancellation" to "move the carrier interference to DC then DC block" in your scheme of "[d]irect downconversion ... with ASK modulation", 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 "power" 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 "square pulses" 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 "analog" 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.