Using a High Sped DAC e.g. AD3551R for ADF5709 VTUNE Control

The ADF5709 VCO is typically controlled by the ADF41513 PLL. An alternative being considered is using a high-speed DAC, such as the AD3551R, to drive the VTUNE input directly. The following concerns are also considered : Can a DAC provide stable tuning over a 0–10 V range for 10–16 GHz output? Is a 200 ns settling time achievable with this method? Can the DAC be placed on a separate PCB from the VCO without degrading performance? Answer: Using AD3551R DAC for ADF5709 VTUNE Control The ADF5709 VCO can be controlled using the AD3551R DAC as an alternative to the ADF41513 PLL. This approach may require circuit modifications to meet the specific requirements of the AD3551R. While an exact settling time value is not specified, achieving a 200 ns settling time appears feasible with proper implementation. The DAC can be placed on a separate PCB; however, careful routing of the connection between the AD3551R’s output and the ADF5709's VTUNE input is essential to maintain optimal performance. Proper layout and signal integrity measures should be considered to minimize noise and preserve frequency stability.

Can the ADF5709 VCO be controlled with a fast DAC, such as the AD3551R, for stable frequency control within the 10-16 GHz range, and achieve a 200 ns setting time?

You can certainly drive a VCO open loop with a DC voltage or a ramping voltage if you want to make a chirp. The ADF5709's frequency stability vs temperature appears to be quite good, with minimal variation between hot, cold, and ambient conditions. However, there will be some part-to-part variation, meaning that the same input voltage may not yield the exact same frequency across different VCO units. So you should include calibration in your system to account for this variation. The phase noise performance should match what is specified in the datasheet (Figure 12), but any noise on the DAC output voltage will degrade this performance and slightly modulate the output frequency. The shape of the phase noise will resemble Figure 12 rather than the classic PLL phase noise curve. The settling time of the VCO when controlled by a fast DAC will be much faster than that of a PLL/VCO circuit, which may be advantageous depending on your application.