What does the circle with the “X” and the cross mean?

Question

I am taking a look at a DSP block for a the Xilinx ZCU111 FPGA and I am getting held up on a simple question. I come from physics so I am not too used to looking at block diagrams yet, but what do the highlighted components in the block indicate within the fine mixer?

Answer 1

They are mixers and summing amplifiers.

_{(Image source: Microwave Journal - Complex RF Mixers, Zero IF Architecture, and Advanced Algorithms: The Black Magic in Next-Generation SDR Transceivers, Figure 3. Basic architecture of a complex transmitter)}