Note that U4, a 10 MHz MEMS oscillator, is also very close to the microcontroller’s clock input pin. It’s always a good idea to minimize the length of traces carrying high-frequency digital signals. First of all, there are noise benefits: a shorter, more direct trace reduces the amount of noise that would otherwise be coupled into adjacent traces, and a shorter trace also reduces electromagnetic interference (EMI) because it is less effective as an antenna. The second issue is related to transmission-line effects. Minimizing trace length is a simple way to avoid problems related to signal reflections. However, reflection is not a significant concern at frequencies in the 10 MHz range, unless you are dealing with long interconnections or a very large PCB.
I always prefer to maintain a logical flow of signals and functionality in my PCB layouts. I think that this approach results in an easier design process and a better board. The AWG layout progresses from left to right: from power input and communication input/output, to the power supply circuitry, to the processor, to the DAC (which is controlled by the processor and constitutes the board’s transition from digital to analog), to the analog signal-conditioning circuitry, and finally to the output connector. Parts that don’t have a specific position in the functional flow—such as the oscillator (U4), the DAC’s passives (C16, R5, etc.), and the voltage reference (U3)—are arranged near the components that they support.
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