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ABC guide to... Step-by-step guides to understanding digital TV
June 2010
Using signal meters Want to see more?
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Accurately aligning your dish is a lot easier with a signal meter and you don’t have to splash out on an expensive professional model Using a signal meter when installing a satellite dish or terrestrial aerial will usually get the job done quicker and produce a more accurate alignment – but only if you know what aspects of the signal the meter is measuring and how it’s presenting the information. Do you need a meter?
01 You already have a very powerful signal meter in your receiver. All Sky Digiboxes, Freesat receivers, most Freeview receivers and the vast majority of independent satellite receivers include a menu page, which displays both the strength and the quality of the signal being received. If you only need to readjust a dish or aerial’s alignment after it has been blown or knocked off kelter, or even if you want to install a single antenna from scratch, it may not be worth buying a meter especially for the job.
Provided you can work on the antenna in sight of a TV screen displaying the receiver’s signal strength page, or have a helper relay the information from the TV screen, then you will likely be able to install the antenna without a meter – but it will be a longer and more frustrating job without the right equipment. Will a simple meter
02 be sufficient?
The most affordable, simplest signal meters measure the level of the carrier signal across the whole spectrum of frequencies transmitted. This is usually displayed on an LED or moving-needle meter, and requires manual adjustment of the range to accommodate the signal level received. The very simplest meters provide no (above) A simple tone-based signal-strength meter can often be enough for domestic fixed-dish systems
aerial
strength quality
transmitter
aerial
strength transmitter
2 What Satellite & Digital TV June 2010
quality
(left) A received signal that includes a component reflected – from nearby buildings, hills, etc – and delayed, (bottom) can produce as high a measured signal strength as the direct signal only (top) and only the signal quality reading will reveal the resulting poor picture
abc guide to... (left) Digital meters can identify satellites as well as measuring signal quality
The signal quality reading is especially useful for the final tweaking of alignment of an antenna and for the LNB skew adjustment of a satellite system. What about signal noise?
04 Many meters will also measure the visual indication of signal level but rely on the pitch of a sounder to indicate signal strength. These meters usually require the receiver to be connected in order to provide power for both the LNB and the meter. Such simple meters can be easily obtained for satellite systems (costing around £15) but are not available for terrestrial installations. Far more convenient in use and more accurate for alignment duties are so-called digital meters. These tune to a particular frequency and decode the digital transmission – to identify the satellite or transmitter. Although a connected receiver will also do this, it is far faster and more convenient to access this information at the antenna. A digital meter will also measure the signal strength, with adjustment-free auto ranging, and display the strength both graphically and as an accurate actual figure for the level of the carrier – usually in dBμV for satellite reception and often dBmW (usually given as just ‘dbm’) for terrestrial and fibre optic satellite systems. The dbm reading can be a little confusing at first as it is a negative number so –5dbm is a stronger signal than –15dbm. As with a receiver, a digital meter will give a measurement of the signal ‘quality’. What is signal quality?
03 For digital TV antenna installations, signal quality is a better yardstick than the strength of the radio carrier signal. The quality measures the reception of the actual transmitted digital data that comprises the TV broadcast. Individual data bits can be lost or corrupted and the
extent of this is measured as the number of errors in the digital data stream: the bit error rate (BER). The BER is the number of data bits received incorrectly as a proportion of the total number of bits received. Most meters will display this both graphically and numerically. The graphical display is actually the inverse of the BER, so a poor signal with a lot of errors shows as a low percentage, while a perfect signal with no errors received is displayed as 100%. The numerical reading is a very small number so is often presented in scientific notation (eg 2.53E-3, which means 0.00253 or 0.253%). In some meters, you can set whether BER is measured before (pre-) or after (post-) Viterbi forward error correction is applied to reconstruct the lost or incorrect data. Of course, a corrected signal has a lower BER than an uncorrected raw signal but the exact value of the BER is not normally important for antenna alignment; the requirement is simply to reduce the errors to the absolute minimum or to maximise the quality reading.
signal-to-noise or carrier-to-noise ratio of the signal. This is essentially an analogue measurement of the quality of the radio carrier and represents the ratio of the level of unwanted noise introduced into the signal along the way, to the level of the wanted signal, as is expressed in dB. The nearest equivalent to signal-tonoise for a digital modulated signal is the modulation error ratio (MER). You should not confuse the signal-tonoise ratio with the noise figure of an LNB. The aim is to maximise the signal-tonoise ratio of the signal whereas the noise figure of an LNB is the amount of noise introduced into the signal by the LNB, and the lower this value, the better, but you are unlikely to have much use for noise measurement for a straightforward antenna installation. How to use a meter’s more
05 advanced features
(below) The bit error rate (BER) of a signal reflects the degree of corruption of the signal but it is reduced by the error correction that is built into the broadcasting system
Some meters offer many more features than measurement of signal strength and quality. A spectrum analyser will display the signal level at each frequency across the range of frequencies received, so you can observe the whole satellite’s transmissions in one image. This is useful (with experience) for identifying a satellite or transmitter before you tune to and decode a transmission and for finding a frequency in use at the time. Many advanced meters also act as receivers. They tune into a transmission not only to identify the transmitter and broadcaster but also to display the TV channel(s) being broadcast on an LCD screen. This brings the advantages of using a receiver’s output with the convenience and safety of a handheld instrument at the antenna, and the ultimate confirmation that the antenna is performing as it should n Geoff Bains
Glossary
Section of signal received without transmission errors BER = 0
Section of signal with transmission errors, before error correction BER = 0.3
Carrier signal
Radio frequency transmission modulated by manipulating its amplitude, frequency or phase to carry analogue or digital information.
dBμV
Logarithmic scale of signal level relative to 1μV.
Section of signal with transmission errors, after (partial) error correction BER = 0.1
dBmW
Logarithmic scale of signal power relative to 1mW.
June 2010 What Satellite & Digital TV 3