Author Topic: P175 Accuracy Query  (Read 4577 times)

Storm905

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P175 Accuracy Query
« on: August 24, 2012, 08:35:13 am »
P175 specs state deviation accuracy < +/- 2 KHz.  And I understand this won't drift in time, as measuresments are digitally extracted from the FM signal.  Today I measured our FM station with the P175 at our studios (good signal strength, Line of site to tx), and adjusted deviation to peak at 75 KHz on dense program material.  Our Inovonics Mod monitor (model 530) connected to same rx antenna consistently showed a deviation peak of 80 kHz.  Given the Pira is not prone to long term drift, is it reasonable to assume the P175 is accurate in this case, and the Inovonics unit - which is ~6/7 years old - has drifted and needs recalibration?  Thanks

Jan

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Re: P175 Accuracy Query
« Reply #1 on: August 24, 2012, 10:41:34 am »
Well, you opened this interesting theme so let's say some words about it.

I'll start with a bit of theory:

1)
The signal processing in the P175 is fully digital. Basic accuracy is thus derived from accuracy of crystal oscillator which is better than 0.01 % (A/D conversion works on purely frequency basis). There is no analogue demodulator that typically has non-linearity of up to a few % and requires calibration, there's no voltage to digital conversion that is unstable with time and temperature.
Additional inaccuracy is caused by noise of IF limiting circuits and local oscillators (but this is minor), IF filters characteristics (minor for typical broadcast content) and especially by quantization stepping of the A/D conversion (reading actual IF signal period).
In real operation the inaccuracy is formed by a) quantization and b) all other influences incl. interference in ratio of about 1:1.

2)
If the system described above gives accurate results in laboratory, it cannot produce negative inaccuracy anywhere outside. The reason is that FM deviation values are collected using 50 ms MAX HOLD function for all samples. Thus (based on probability and statistics) only values with most positive inaccuracy are collected. In other words the values measured can be higher due to outside influences but cannot be lower than reality.
If some equipment shows 5 kHz higher deviation than P175, I say with absolute certainty that such values are not based on reality.

Thus if the P175 indicates full-quality signal, it cannot show less deviation than the signal really has (respecting the accuracy given in manual). That's guaranteed by principle of operation as given above. Anybody can verify the P175 accuracy using the effect of carrier zeros as described in the manual and compare this with other equipment. Many surprises have already came.
The P175 project has started on university while being continuously tested in real use and it is primarily designed for FM deviation and modulation power measurement. Its precision in these two measurements is comparable to the most expensive equipment on the market and often better than 20 times costlier devices.

Now the key question. Why some other equipment shows higher deviation? I will not talk about individual problems like bad calibration. There are other influences possible:

1) How the accuracy is defined?
Consider speed meter in a car, that's very well for comparison. It is designed to have only positive inaccuracy, i.e. it always shows a bit higher speed. This is to prevent exceeding of the speed limit. While the inaccuracy of the speed radar outside the car is designed different way, it can be either positive or negative, but usually zero in average. Thus in most cases radar shows lower speed than speed meter in car. The P175 is closer to the speed radar in this case, i.e. it is designed to show real deviation with average inaccuracy near zero.

2) Audio clipping in sound processors.
Strong audio clipping is used especially in last years in order to increase the sound density of the program without exceeding the deviation limit. Audio clipping produces a signal that has some characteristics of square wave signal. This is a key fact. Older broadcast equipment has a big problem with this kind of signal due to overshoots that may occur in their signal path. To prevent overshoots the frequency characteristic of the signal path must be absolutely flat in minimum range of 2 Hz to 60 kHz. As well as this is not accomplished by many broadcast equipment, especially made in past, it applies the same way to the FM deviation measurement equipment. As a result, that equipment shows higher deviation, increased by the overshoots that occur in the measurement equipment itself. Moreover it is very hard to prevent overshoots in equipment that does not use direct DSP processing and to ensure temperature stability of entire circuit.

3) Problem of values representation.
If the equipment provides a simple LED bar-graph or MAX Hold function collecting data for a few seconds and it does not provide any histogram or similar complex output, you cannot decide on what really happens. Comparison of such equipment with equipment based on fully digital measurement and complex output is therefor debatable.

As a last point, it is known that professional FM broadcast measuring equipment from different manufacturers differs in output values, regardless of the device price. This issue is so strong that national regulatory offices define not only the limiting values for deviation and modulation power but also a device type that is relevant for the measurement...

Storm905

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Re: P175 Accuracy Query
« Reply #2 on: August 24, 2012, 12:00:43 pm »
Hi Jan,  Thank you very much for an excellent & useful explanation.

On that basis the P175 becomes our reference mod monitor, and I'll remember your reply when recommending the P175 to other broadcasters using analog based mod monitors. 

We'll now be looking to buy more to permanently monitor (FM Guard) at a few FM tx sites via 3G.  Affordable insurance and a highly accurate mod monitor.  Brilliant.

Jan

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Re: P175 Accuracy Query
« Reply #3 on: August 24, 2012, 02:27:23 pm »
According to the information above and since you probably cannot know what equipment will the inspection use :D, it's recommended to adjust MAX not to exceed 74 kHz on the P175 display.