The Link below is for the circuit diagram for a good performing Ring Modulator, but instead of using a diode “ring” it uses a simple optocoupler as an unbalanced mixer. This works for two reasons, one; the modulation drives the optocoupler LED and is electrically isolated, eliminating modulation leakage through the mixer. Two, a simple differential amplifier is used to pass the original signal from 100%, all the way to nulling it out entirely, by means of the mix POT. The frequency mixing occurs because of the highly nonlinear nature of the switching action which generates the sum and difference components.
This simple design rivals the performance of a well designed balanced mixer. The optocoupler is driven by a triangle wave. The drive level is important. By setting the drive correctly, the triangle wave peaks are rounded off gently by the LED turning on inside the H11M1. Another LED is put in series with optocoupler to give frequency indication and the output of the oscillator is tailored to do this. It will not work correctly without the voltage drop of the LED. I just grabbed a junk box one. Depending on your LED you may need a different value for R13. Alternatively, you can shift the level down by tweaking the oscillator values some, if you do not want the LED.
The impedance loading the output of the H11M1 is important. If it is to high or to low, the signal will be distorted. The key is to minimize the voltage across the optofet as it is transitioning from off to on. The point being that the differential amplifier resistor values are not arbitrary and need to be adhered to.
Finally, it is important isolate the oscillator from the the rest of the circuit or the modulation tone can leak through to the output from stray leakage. You can tell this is the problem when you disconnect the drive to the mixer but you still hear the modulation tone.
This is just a basic circuit, I am working on a really enhanced version that will do far more – so I will keep updating