While trying to understand all the pins of the AD9850 board (marked "SR-HC08"), I could not find the square wave output. A good sign was that QOUT2 was at +5 V and QOUT1 at 0 V. A simple adjustment of the onboard trimmer (duty-cycle control) activated the AD9850 internal comparator and there appeared the square wave.
Therefore output pins in my board are:
- ZOUT2: sine wave through an LPF
- ZOUT1: direct sine wave
- QOUT2: square wave
- QOUT1: square wave (180° shifted)
If you don't need the square wave output disable it with a fully (C)CW rotation of the trimmer.