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Topic Summary

Posted by: Jan
« on: December 05, 2013, 05:01:13 pm »

It is already mentioned in the manual: "If the spectrum plot is outside the mask on any frequency, the transmission device may be considered as overmodulating. If the spectrum plot is within the mask, additional peak frequency deviation measurement must be always made to verify if the station exceeds the peak deviation limit or not."
In other words, exceeding the mask limit gives a final information that the transmitter does not pass the test. A curve that is entirely below the limit does not bring any information (in relation to ITU-R SM.1268-1).
Posted by: Jan de Vries
« on: December 05, 2013, 04:21:10 pm »

So i cannot do the meassurments conform ITU-R SM.1268-1, although you have the mask drawn in the spectrum analyser, and the mask on the  analyser is there but not related to anny meassuring methode.
Am reading your comments right ??

Posted by: Jan
« on: December 05, 2013, 11:09:53 am »

There's nothing to adjust but these methods are not comparable on that devices. The P175 does not implement the mask method as it has been originaly described because it is not a spectrum analyzer. There's no sweeping, no RBW, no VBW. The spectrum is showed using FFT digital method, calculated from samples of the IF signal. Since the method and purpose is different, the result also differs in some cases. The methods give same result only for "slow" or periodic modulation signals. The P75/P175 manual recommends to verify the deviation if the spectrum lies within the mask or not to use the spectrum mask at all (as it is obsolete).

The mask method does not measure the FM deviation nor real spectrum. Based on decision whether the resulting curve is within the limits, the station is considered as OK or overmodulating. The sweep time is relatively long compared to filter's bandwidth so the curve is more likely a function of time instead of frequency. This helps to detect short peaks but each point represents a spectrum component sampled at considerably different time so the total spectrum curve is not real. The time coverage is about 1/34, i.e. it does not cover 100 % of the time so many peaks will never be detected, two identical instruments may give different results due to different start time of the sweeping. This can be partialy compensated by long period of measurement.

The FFT method brings real spectrum averaged over a time interval which is about 1 ms in the FM Scope. The longer time interval, the better resolution but the lower sensitivity to short peaks. The time coverage is about 1/150. The spectrum tool may detect transmitter failure like strong spurious near the carrier or hard limiting in the MPX signal path. For deviation measurement a completely independent DSP method is implemented.

For an imagination, I took a sample yesterday, it's the same station at the same time on different equipment:
(The spectrum is relatively thin on both equipments although the station modulates to 75 kHz. It's because the modulation power is limited in our country to 0 dBr so strong audio compression can't be used.)
Posted by: Jan de Vries
« on: December 04, 2013, 09:08:16 pm »

Quote from: jan de vries on December 04, 2013, 09:02:57 pm
Well been a while, but finaly had some time to do some tests.
Did some meassurments at the transmitter site of dutch radio Station Sky Radio.
This station is to be known as verly loud en fill the mask almost within an eye blink.
Meassured about 15 minutes, but it does not fill the mask,  with the P175.

Is this an adjustment issue ??


Posted by: jan de vries
« on: December 04, 2013, 09:02:57 pm »

Well been a while, but finaly had some time to do some tests.
Did some meassurments at the transmitter site of dutch radio Station Sky Radio.
This station is to be known as verly loud en fill the mask almost within an eye blink.
Meassured about 15 minutes, but it does not fill the mask,  with the P175.

Is this an adjustment issue ??

Tryed to insert a picture  >:(
Posted by: jan de vries
« on: September 09, 2013, 04:06:17 pm »

This is as it should be !!!
Ill make a printscreen of mine then.
Posted by: Jan
« on: September 09, 2013, 08:41:43 am »

Please let me know if I'm missing something...
Posted by: jan de vries
« on: September 08, 2013, 10:48:53 pm »

Take a look at the latest version of the fm-scope software.
The lines (mask ) on the RFcarrier window, really stop at 80Khz instead of 74 Khz, as they should.

Anny possible way to adjust those lines, according to the ITU standard??, otherways they serve, to my opinion , no use.
Posted by: admin
« on: September 08, 2013, 07:11:52 pm »

It is implemented already. I think there never has been any "80 kHz" modification.
Posted by: jan de vries
« on: September 08, 2013, 06:00:55 pm »

Is there anny poss to adjust the mast to the ITU-R SM.1268-1 ??
It is now set to 80 Khz deviation, mainly used is 75 khz dev.

maybe this document is usefull : http://www.processingfreakday.com/docs/measuring_freq_dev.pdf

I shurely hope this adjustment can take place, nice piece of meassurment tool !
Posted by: Jan
« on: October 21, 2012, 06:54:36 pm »

The mask defined in this document applies to out-of-band emission limits for FM sound broadcasting transmitters. It's valid only for dedicated measuring method using a spectrum analyzer, psophometric voltage meter and a set of predefined modulation signals. All transmitters on the market are guaranteed to comply with this standard.

The P75/P175 does not measure out-of-band emissions.
Posted by: piboonvit
« on: October 21, 2012, 05:56:56 pm »

In my country my regulator they use ESTI EN 302 018-1 V 1.2.1 (2006-03) standard , and the mask for RF Carrier is different.
Any thing that can do for change the limit line.