Pira.cz Technical Forum

FM Analysis => P275 FM Analyzer => Topic started by: electron on October 23, 2018, 11:18:41 am

Title: Recommended procedure for setting up a known deviation
Post by: electron on October 23, 2018, 11:18:41 am

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?

Title: Re: Recommended procedure for setting up a known deviation
Post by: Jan on October 23, 2018, 02:24:09 pm

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

Title: Re: Recommended procedure for setting up a known deviation
Post by: electron on October 23, 2018, 08:40:29 pm

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?

Title: Re: Recommended procedure for setting up a known deviation
Post by: Jan on October 24, 2018, 09:48:08 am

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

Title: Re: Recommended procedure for setting up a known deviation
Post by: electron on October 24, 2018, 11:55:14 am

Thank you for your analysis.