If you are talking about modem type products, the only attenuation requirement of the filter is to reduce the level of meter pulse signals such that they do not cause distortion through the signal transformer and do not cause the dc gyrator to throw a wobbly. This is enough with modern chipsets to allow reliable rejection and good performance (in our experience).
You also need to meet minimum impedance requirements at meter pulse signals. We always use a simple LC circuit which, with off the shelf component values and tolerances, provides at least 20dB attenuation worst case. As this circuit goes high impedance at the resonant frequency (meter pulse frequency) it also allows the minimum impedance requirements to be met. For type approval purposes now, there are no performance tests done in the presence of meter pulse signals. The only requirement is that the meter pulse signal does not affect the dc performance of the gyrator and that the minimum impedance is met. With good gyrator design and 20dB attenuation meeting these requirements is no problem. BUT, the lab we use for testing has meter pulses on the PSTN line they allow clients to use for modem calls and, if you do not have a meter pulse filter fitted to your modem, it will not connect over this line! However, place the 20dB filter in circuit and you have no problem with all the major chipsets! So, type approval is much easier than it was but some practical problems still exist because meter pulses are still out there. Best Regards Nick Evans Genesys
