FM Analysis > PIRA75/P175/P275 Control Software
ESTI EN 302 018-1 V 1.2.1 (2006-03)
Jan:
Please let me know if I'm missing something...
jan de vries:
This is as it should be !!!
Ill make a printscreen of mine then.
jan de vries:
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 >:(
Jan de Vries:
--- 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 ??
--- End quote ---
Jan:
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recommends to verify the deviation if the spectrum lies within the mask or not to use the spectrum mask at all (as it is obsolete).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.)
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