Draft Recommendation O.xx(0.6)


               1 020 Hz REFERENCE TEST FREQUENCY


1.     Introduction

       The intent of this Recommendation is to specify a  single  nominal
reference frequency  of  1  020  Hz  in  order  to  provide  guidance  to
manufacturers and administrations in the design and operation of new equipment and 
systems. This Recommendation is not intended to have an effect on existing 
equipment or systems except where modifications are required to allow for 
interworking, e.g., an older analogue exchange would need to be  provided
with new reference frequency capability if circuits were provided between 
it and digital exchanges.

2.     Test frequencies on circuits routed over PCM systems

       The testing of circuits routed over PCM systems  provide  a  major
consideration when selecting a suitable reference test frequency. An error
in level measurement can arise on circuits routed over PCM systems if the test 
frequency is a sub-multiple of the PCM sampling rate. This error can be nearly 
as great as +0.15 dB at 800 Hz and +0.20 dB at 1 000 Hz with a sampling rate of 
8 000 Hz employing 8-bit coding. In addition, errors, in other parameters, such 
as total distortion, may be even more significant.

       Therefore it is recommended that the use of a reference test  frequency
that is a sub-multiple of the PCM sampling rate  should  be  avoided.  Studies
within CCITT reveal that some administrations have employed nominal reference test 
frequencies offset from 800 Hz or 1 000 Hz by varying amounts but  within  the
ranges 804 - 860 Hz or 1 004 - 1 020 Hz. These studies have confirmed that where 
interworking is not required, no significant problems in maintenance have been 
encountered by administrations and existing test procedures and equipment  may
continue to be used.

       In the case of interworking and for  new  equipment  and  systems,  the
administrations expressed a strong preference for the selection of a reference test 
frequency of 1 020 Hz.

3.     Considerations for new measuring equipment specifications

       The  following  should  be  considered  for  new  measuring   equipment
specifications in the O-Series Recommendations:

       i)   A reference test frequency of 1 020 Hz  is  recommended  for  test
frequency generating circuits  or  instruments  that  provide  reference  test
frequencies. The specified frequency tolerance should be +2 to -7 Hz1. 

               

1  The negative tolerance of 7 Hz is intended to allow the use of digitally 
   generated test signals that are generated by a sufficiently high number of 
   samples to achieve the measurement accuracy specified in certain 
   O-Series Recommendations (e.g. Recommendation O.133).

       ii)  The nominal level of the reference test frequency when used on in-service equipment should not be greater that -10 dBmO = 1 dB.

       iii)      Measuring circuits or instruments which utilize the reference 











test  frequencies  should  provide,  if  possible,  for  measurement  of   any
frequencies within the nominal range of 1 000 to 1 025 Hz.

       By agreement between the administrations concerned, in the  absence  of
the required sending or measuring apparatus, the use of a measuring frequency in 
the range of 800 to 860 Hz  is  admissible.  Other  considerations  about  the
deployment and use of reference test frequencies are given in Recommendation M.20[1].



                                  REFERENCES
                                       
1.   CCITT Recommendation, Maintenance philosophy for analogue, digital and 
     mixed networks, Volume IV.1, Recommendation M.20.