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Varactors and DROs are completely different species, you certainly can't use a varactor as a DRO but you might be able to use a varactor to pull DRO frequency. It depends on many other factors but if you want to pull a DRO very slightly, you might be able to get it to work. I doubt the manufacturer will provide information on such a specific requirement though. I use DRO at frequencies between about 3 and 13GHz and they behave quite differently to other components around them.