When testing with the official EV-ADF4356SD development board, I observed the following phenomenon:
After starting the backend software, with the default configuration, I modified the VCO output frequency to 4915.20 MHz.
Initialization completed. The output waveform appears normal when observed with a spectrum analyzer.
The VCO output is modified to 4915.25 MHz, meaning the frequency point has been increased by 50 kHz.
After initialization, when observing the output with a spectrum analyzer, a 50 kHz harmonic with strong amplitude is visible.
As shown in the marker 2Δ1 in the figure below.
This phenomenon occurs within ±1 MHz when the VCO output is an integer multiple of the reference frequency (122.88 MHz). It disappears when the frequency offset exceeds ±2 MHz.
For example:
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The phenomenon is observed when VCOout = 39 × 122.88 MHz ± 1 MHz.
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The phenomenon disappears when VCOout = 39 × 122.88 MHz ± 2 MHz or beyond.
What could be the cause of this issue, and how can it be avoided?
