I started my first RF Signal Generator design in my garage in 2009. I was just goofing around enjoying electronics design like I have since being a kid.
The generator used an ADF4350, which was a revolutionary chip from Analog Devices that had a PLL and broadband VCO all in one plastic package.
When I found this chip my first thought was, WOW that’s so cool! I designed and assembled 3 units, keeping two for myself and putting 1 on eBay for $250, never really thinking it would sell. One day while working in the lab at NASA my phone alerted of the sale. A wave of euphoria ran through me, and I finally realized how cool it is for someone else to buy something that came from my brain.
I couldn’t wait to make it better and sell another. I never expected more than beer money, but it proved to do much better than that.
An RF Generator creates radio frequency signals using high speed voltage controlled oscillator (VCO) circuits combined inside a phase locked loop (PLL). The VCO oscillates and makes an RF signal that is tuned in frequency by a control voltage. A VCO will drift in frequency and have significant noise called phase noise if it is not nicely controlled. This is the job of the PLL. The PLL is a control system that measures the phase (and frequency) of the VCO while concurrently adjusting the voltage of the VCO so that frequency stability is achieved. The frequency stability is determined by the reference frequency (Fref), which usually comes from a lower frequency crystal based oscillator.