The RF2 filter is sometimes fitted to a line to mitigate the effect of radio frequency interference causing degradation to telephony audio quality. They are normally fitted as a result of a customer complaint and when strong levels of radio signals have been identified as the cause of noise on PSTN service. They are normally fitted at the customer premises but can also be found at the exchange MDF. The RF2 is designed to block High Frequency (HF) radio signals (both common mode and transverse mode). Because ADSL is a transverse HF signal (25kHz to 1.1MHz) it is inhibited by the RF2. A new design, the RF3, has replaced the RF2 which is ADSL friendly – it blocks only the common mode HF signal.
The effect of the RF2 on ADSL Broadband is to increase the downstream loss by 31dB and so can cause No Synchronisation and Loss Of Synchronisation type faults. Typically modems will train but loose synch after a few seconds.
