I've found in ITU-T G.922.3 specification some information about Hlog measurement accurracy (page 113).
The accuracy requirements for the downstream HLOGps (HLOGps_ds and HLOGps_us) shall
apply only for those frequencies where the loop impedance (Z loop ) falls within the following ranges:
• impedance magnitude is between 100 Ω and 120 Ω;
• impedance imaginary component is between –20 Ω and 0 Ω.
The loop impedance (Z loop ) is defined as the impedance seen by the transceiver under test, looking
into the loop, including the transmitting transceiver connected to the loop at the far end
Does somebody know what is about?