Recommended procedure for setting up a known deviation

[electron]

Dear Jan in fm scope user guide, description for proper frequency setting up deviation at 75Khz is 3rd carrier null (8,67khz). My question is why we must use this frequency instead 31187,6Khz? Is there any limitation regarding your fm analyzer?

[Jan]

In FM broadcast, single tone above 15 kHz cannot cause 75 kHz deviation.

[electron]

In breakaway site there is different opinion regarding null calibration.
https://www.claessonedwards.com/41-bafm/index.php?option=com_content&view=article&id=53&Itemid=67

Which is your comment about this?

[Jan]

If better option is not available, you may probably use such testing configuration (31.2 kHz @ 75 kHz deviation) with similar result on most equipment. However such configuration has at least three disadvantages:

• it does not represent an FM broadcast testing signal because such condition cannot occur in real FM broadcasting (neither theoretically nor intentionally - with exception of SSB coders)
• accuracy is affected because in real MPX signal most energy is below 15 kHz so adjustment should be made using a signal from this region
• requirements for the FM carrier output spectrum are exceeded during the test

[electron]

Thank you for your analysis.