A quick ramble about decibels.
A decibel is one tenth of a Bel, the bel being a large unit, tenths of a Bel are more convenient for practical use.
The decibel is the ratio if two powers, expressed logarithmically.
So we can use decibels to express the gain or loss of an amplifier, attenuator, transmission line etc.
0dB represents a gain of 1, ie the same power output as the input, a short length of cable will have a gain of 0dB, the power coming out is the same as the power going in.
+10dB represents a gain of 10, 1 watt in and 10watts out = a gain of 10dB
-10dB represents a loss of 10, 1 watt in and 100mW out = a loss of 10dB or a gain of -10dB
When stating the bandwidth of an amplifier or some other system, it is usual to state the bandwidth between the "half power points", ie where the power has dropped to half its mid band value.
-3dB is a loss of 2, half the power, hence the expression "3dB bandwidth".
Useful ratios to remember:
0dB 1:1
3dB 2:1
6dB 4:1
10dB 10:1
20dB 100:1
30dB 1000:1
To find the overall gain or loss of a system, it is easier to add and subtract the dBs rather than multiplying and dividing the ratios.
dBs can also be used as a measure of absolute value rather than being used as a ratio.
dBm = dBs relative to 1 milliwatt
dBW = dBs relative to 1 watt
There are others but these are the ones which are most familiar to me.
An example:
An audio signal has a level of +3dBm (power = 2mW)
it passes through an amplifier with a gain of +16dB
along a transmission line with a loss of 18dB (it is a very long line)
at the far end ther is another amplifier, this time with a gain of +10dB
What is the signal level at the far end?
Code:
+3dBm (the original signal level)
plus +16dB (the near end amplifier
plus -18dB (negative dB, the line has loss)
plus +10dB (the far end amplifier)
+3 + 16 - 18 + 10 = +11dBm.
So, getting back to your original question (one of them anyway)
but why is the db measurment say -40 etc. ALL minus signs?
Because there is a lot less coming out of the filter(?) than going in, it has lots of loss.
This is probably good on the skirts of the filter, what the loss is mid band I cant easily see as I am typing this.
I hope this has been helpfull.
There are some people on this forum who think that decibels are a device of the devil invented by some twisted mathematical pervert for the torture of students.
This may be true!
But, give them a chance and they are very useful.
JimB