IEEE 1631:2008 pdf free download – IEEE Recommended Practice for Measurement of 8-VSB Digital Television Transmission Mask Compliance for the USA
4.1 General
Iln the USA, DTV transmission is specified by the FCC.The critical emissions standards, the subject ofthis recommended practice, are wholly specified by the FCC itself,whereas the detailed specification ofthe 8-VSB signal itself is specified by the FcC by reference to the ATSC*s standard [FCC47CFR673.682(d)J.’For the reader’s convenience, the applicable portions of both the FCC’s regulationsand the ATSC’s standards are repeated here. Although correct at the time of writing, this informationmay have been superseded and should not be considered authoritative.The reader is urged to referencethe source documents, especially the FCC’s DTV emissions rules,to ensure that the correct FCC rulesare applied to the measured values.
4.2 Pilot frequency
The 8-VSB pilot is normally 309.441 kHz above the lower edge of the channel assigned to the DTVtransmitter, except when the DTV transmitter is required by the FCC to offset its pilot frequency in orderto minimize interference caused to a lower adjacent channel analog TV station. In that case, the pilotfrequency is required to be 5.082,138 MHz (±3 Hz) above the visual carrier of the analog station [FCC47CFR573.622(g) ]-6
The exact pilot frequency of an offset 8-VSB transmitter is thus dependent on the frequency of the analogtransmitter.Because the nominal frequencies of some analog stations are offset 10 kHz lower infrequency, whereas others are offset 10 kHz higher [FCC 47CFR$73.606(a).’the pilot frequency of anupper adjacent channel 8-VSB transmitter will be about 12.697 kHz, 22.697 kHz, or 32.697 kHz higherthan nominal.Operating with an offset makes it more difficult for a DTV transmitter to meet thecmissions mask. A quantitative analysis of this problem is given in Annex B.
4.4 Units for 8-vSB emissions amplitude measurements
The FCC specifies the attenuation of emissions in terms of measurements made with a 500 kHz referencebandwidth, expressed in decibels with respect to the total power transmitted in the transmitter’s 6 MHzchannel (including the pilot’s power).Because of this quantity ‘s complexity and its use in allmeasurements and regulations, it is given a special designation, dBpTv, in this document.
NOTE—The amplitude of a measurement at a single frequency may also be expressed in dBprv.With the exceptionof the pilot signal，the 8-VSB signal is noise-like. Therefore，measured amplitudes vary directly with themeasurement bandwidth.To express an amplitude measurement in dBprv, first scale that amplitude to a 500 kHznoise bandwidth using the procedure given in 6.1.1.Then divide that result by the transmitter ‘s total average powermeasured within its channel and express the result in decibels.For amplitudcs measured in dBm,measurc theamplitude across a 500 kHz band or adjust the amplitude to a 500 kHz equivalent bandwidth using the procedure in6.1.1 and then subtract the total average 8-vSB signal power (in dBm) to obtain the amplitude in dBprv.
4.5 Source of signal to be measured
The FCC specifies that emissions are to be measured on a signal derived from the transmitter’s output ata point beyond any filters that may be employed [FCC 47CFR$73.622(h)]J. Typically，a coupler orsampling device to obtain this signal is inserted in the transmission line(coaxial cable or waveguide)between the transmitter (including the mask filter) and its load or antenna. During measurement, thetransmitter may be operated into either an antenna or a dummy load; the dummy load is preferred, as itminimizes possible problems with off-air signal ingress.As an option,the sampling device can be asimple attenuator of the correct impedance and one that can accommodate the transmitter’s output power.Regardless of type,to make measurements possible,the sampling device should provide themeasurement instrument with a signal of the appropriate amplitude.(See 6.3.2.1.)
To enable accurate measurements of adjacent near-channel emissions, the sampling device should exhibitno more than a 0.5 dB peak-to-peak flatness error over a minimum range of 18 MHz centered at thetransmitter’s output channel. The sampling device should exhibit at least 18 dB of return loss over the18 MHz range centered at the transmitter’s output channel.(See 5.4.1.) The cables and connectors usedto make connections between the sampling device and the test instrument should avoid the generation ofpassive intermodulation (PIM) distortion.