sometimes installers have to face problems caused by unexpected behaviour of their field strength meters (fsm), which is the basic instrument that they must have available and ready to use at any time.
the most important measurement to assess the quality of the tv signal is the ratio carrier-to-noise c/n.
it does not worth to have a high signal level if the c/n is poor and the noise is literally "eating" the useful signal.
let us illustrate both noise and carrier-to-noise concepts:
- on the left of the pictures it can be seen a signal in the domain of the time.
its amplitude is varying with the time (e.g. picture obtained by an oscilloscope).
- on the right it can be seen the same signal in the domain of the frequency, as a spectrum analyser shows.
signal and noise levels are opposite: the greater the distance between them the better the quality of the signal.
nevertheless, let us consider the case that reading the c/n with our fsm, the measured value was unacceptable and at the same time the picture you are watching is good.
what is happening?
having a better test instrument like a spectrum analyser perhaps the answer will be easy to find.
but this is not the case, and none installer can afford the cost of such expensive instrument.
therefore let us check this scenario by means of a fsm.
one can figure up that is due to the tolerances of the fsm, e.g. +- 2 db; this might be the case that is far away of the truth.
anyhow, thinking deeply and deeply, and having a good background of the situation, the answer is obvious:
to measure the real c/n for a particular tv signal, no matter the fsm one's could be handling, it is essential to be sure that the noise level of the signal under test is higher than the noise level of the instrument used to measure it (floor noise).
if the noise level inherent to the fsm was higher than the noise of the signal to be measured, the fsm cannot measure the c/n correctly: its own noise will mask the signal noise.
the question is, when this situation can happen?
it is obvious that this situation will take place whenever the input level was low, and this input level is a function of the fsm itself and the c/n of the signal:
let us suppose that we have a signal level of 60 dbμv and that its real c/n=26 db.
first of all, let us calculate the noise level of the fsm by making a measurement of none signal.
the reading of the fsm has to be its own noise.
if you do this, the result is about 30 dbμv.
the fsm will measure the c/n correctly because the noise level of the signal is 60 - 26 = 34 dbμv which is 4 db higher than the fsm noise floor (30 dbμv).
what will it happen in case of measuring the c/n of the same signal but with input level 50 dbμv?
in this case the input signal noise level will be
50 - 26 = 24 dbμv.
and in these conditions the fsm cannot measure this noise!!
the fsm will read in this case:
50 - 30 (own noise) = 20 dbμv
and will display c/n>20.
the fsm "knows" that the c/n>20 db but it is not able to say how much it is higher, what is the value.
just make use of a preamplifier to increase the level of the signal.
even good spectrum analysers have the same limitation, e.g. the hp8590, that must be taken into account