11. This results in a 100 kHz difference frequency proportional in level to the bridge unbalance. 12. Parametric sources use the non-linearity of water to generate the difference frequency between two high frequencies. 13. The difference frequency signal is amplified by a filter amplifier combination and is applied to a null meter. 14. This beat signal, detectable with a photodiode, includes a difference frequency component, which is the carrier envelope offset frequency. 15. Hence, narrow electronic filtering near the difference frequency is highly effective at removing the remaining, generally broadband, noise sources. 16. Also, the animation contains both up-and down-converted spectra, which is due to a frequency mixer producing both sum and difference frequencies . 17. In contrast, the harmonic and subharmonic mixers form sum and difference frequencies at a harmonic multiple of one of the inputs. 18. A heterodyne bat detector combines the bat call with a constant internal frequency so that sum and difference frequencies are generated. 19. A simple mixer will pass both of the input frequencies and all of their harmonics along with the sum and difference frequencies . 20. An alternative to stabilizing the carrier envelope offset frequency is to cancel it completely by use of difference frequency generation ( DFG ).
