Measuring DVB-T Signals - Digital Video and Audio Broadcasting Technology - page 392

Approximate Determination of the Noise Power N:

If it were possible to switch off the DVB-T signal without changing the
noise ratios in the channel, the noise marker in the center of the band
would provide information about the noise ratios in the channel. However,
this cannot be done so easily. Using the noise marker for measuring very
closely to the signal on the shoulder of the DVB-T signal will provide at
least a “good idea” about the noise power in the channel, if not a precise
measurement value. This is because it can be assumed that in the useful
band, the noise fringe continues similarly to that found on the shoulder.
The spectrum analyzer outputs the value N' of the noise power density.
The noise power N in the channel with the bandwidth B C of the DVB-T
transmission channel is calculated from the noise power density N' as fol-
lows:

N = N' + 10log(B C /Hz); [dBm]

The channel bandwidth to be used is the actual bandwidth of the DVB-T
channel, e.g. 8 MHz.

Example:

Measurement value of the noise marker: -140 dBm/Hz

Correction value at 8 MHz bandwidth: + 69.0 dB

Noise power in the DVB-T channel: - 71.0 dBm

From this, the C/N value is obtained as:

C/N [dB] = C [dBm] - N [dBm]

In the example: C/N [dB] = -31.2 dBm - (-71.2 dBm) = 40 dB.

In estimating C/N in this way by means of the shoulders of the DVB-T
signal it is important that this measurement is made directly at the output
interfaces after the power amplifier and before any passive bandpass fil-
ters. Otherwise, only the shoulders lowered by the bandpass filter will be
seen. The author has repeatedly verified the validity of this measuring
method in comparisons with measurement results from a DVB-T test re-
ceiver.