If I am specific, the mains voltage is given by Un 230V 50Hz. The tonal voltage that I need to drive is between 0.2 - 1.2% Un, frequency 170-1600Hz, actually several selected frequencies 180 and higher.
The first attempts at pure software detection looked like this:
1VA transformer 230 / 6.3V, 100nF isolating capacitor + resistive divider voltage 1.65V + - 1V converted to ADC sampling at 1kHz. Using the Goertzel algorithm, the presence of a tone signal at the selected frequency is detected.
It works, but I'm afraid there will be problems with the extremely low level of the tonal signal, so I want to strengthen the weight of the tonal signal. Active filter is a nice thing, but relatively expensive and kills the magic of software solutions.
That's why I proposed a passive solution.
Large expensive transformer will be replaced by 2 pcs SMD C NPO 10nF 2000V, followed by the mentioned Twin-T Notch Filter and then ADC
According to the simulations (ideal component values), the attenuation is at 50Hz -83dB at 180Hz approx. -27dB ,in other words, the weight of the tone signal at the input to the ADC increases by 56dB .
How to compare?
The extreme case of tonal voltage is 0.2% of the mains . Ut=460mV Un=230V
In the first case, the transformer has a conversion of 1:36 to "ADC" is Ut = 13mV and Un=6,3V , ratio 1:485.
In the second case,the Ut = 22mV and Un=0,36V, ratio 1:16.
At first glance, this should work great.
But I have no practical experience with this and I am usually very careful in anticipation of very strong filters.