The transponder on the AO-51 no longer requires the presence of 67 Hz CTCSS sutone in the modulation of the broadcasting station. For operation, it is enough to transmit a pure FM signal on the 2m uplink.
The CTCSS system was supposed to save on-board batteries. In areas where there is interference on the uplink frequency, the transponder would be unnecessarily activated. However, the use of CTCSS caused dropouts in the modulation and generally made it difficult to work through the AO-51.
The AO-51 can also be used with a two-band hand-held FM station, provided that gain antennas are used. However, I still highly recommend it for the downlink preamplifier.