Detection of two different frequencies using oscilloscope
Might be a stupid request but try zooming out on the time scale to see the 660kHz signal. If you're just seeing a few cycles of the 80 MHz bits, you're not going to see the 660 kHz signal. Also, use the FFT function if you have it on your scope, because that offers a larger dynamic range.

If this doesn't work, you need to filter your signal. Using the 20/40MHz filter on your scope (most scopes should have this), which may help enough. If that doesn't work, a discrete filter may have to be built. Since 660kHz and 80MHz is far apart, a simple 1 pole RC filter will probably be enough.