Author Topic: Maximum Deviation Measurement in UK  (Read 3279 times)

radiohead

  • Newbie
  • *
  • Posts: 5
Maximum Deviation Measurement in UK
« on: April 30, 2013, 08:11:28 pm »
Great bit of kit, very pleased with it.

Our local regulator (Ofcom in UK) states a method for measuring peak deviation that seems slightly difference to that stated in the P175 manual (see below).   Would it be possible to make this method available in future software? (Not sure if that is Firmware or Scope SW or both!)

Transmissions must be compliant with ITU-R Recommendation 450-3, adhering to
certain options and additional provisions therein, as follows:

i) The maximum frequency deviation applied to the radio-frequency carrier
must not exceed ±75 kHz by more than 5 positive-going or 5 negative
going excursions in any 5 second period of programme service. Where an
excursion above ±75 kHz exceeds 10 msec duration, it shall be divided
into discrete 10 msec periods (rounded up) and counted accordingly.
Under no circumstances is the deviation to exceed ±80 kHz other than by
anomalous behaviour

Jan

  • Hero Member
  • *****
  • Posts: 564
Re: Maximum Deviation Measurement in UK
« Reply #1 on: April 30, 2013, 09:48:40 pm »
This method stated in UK is well known. There's however no serious reason for implementation in the P175 and I'll try to explain why.

1) The P175 is not a reference measuring device used by Ofcom. The P175 is primarily intended for broadcasters.
2) Broadcasters must adjust their transmitters in order to generate no peaks (overshoots) above 75 kHz. It is not possible to permit for example 4 peaks above 75 kHz, say that it is ok and go away. The number of peaks is in relation with only actual broadcast content. Nobody can guarantee how many peaks will generate another broadcast content, another song, another speaker. A transmitter that provably generates at least one peak above 75 kHz that is in relation with the broadcast content, is assumed to be set incorrectly, because another content may result in almost any number of peaks. It's just a matter of time, and you cannot know when Ofcom will make measurements...

Conclusion:
The Ofcom's method can be used for inspection purposes only. It provides a tolerance band in favor of the broadcaster, suppressing inaccuracies and occasional influences. The number of peaks received is exclusively related to the time and conditions of the measurement, it has no general validity and it is not comparable.
This method cannot be taken into consideration when adjusting the transmitter. The broadcaster must adjust a modulation level that results in zero peaks above 75 kHz.

radiohead

  • Newbie
  • *
  • Posts: 5
Re: Maximum Deviation Measurement in UK
« Reply #2 on: April 30, 2013, 10:32:52 pm »
Thanks Jan.

So you are saying that the P175 will always be more pessimistic/cautious than the tools Ofcom use then?   If so, then fair point.

However I am aware that the Ofcom monitoring station at Baldock actually does own one of your P175 devices.  Maybe if it were possible to use their preferred measurement method they could be persuaded to purchase some more and setup some automated monitoring in all the major cities of the UK.... Just a thought.