Editing Low-noise block downconverter (section)

==Block downconversion==
Satellites use comparatively high [[radio frequencies]] ([[microwave]]s) to transmit their TV [[signal]]s. As microwave satellite signals do not easily pass through [[wall]]s, [[roof]]s, or even [[glass]] [[window]]s, it is preferable for satellite [[antenna (radio)|antenna]]s to be mounted outdoors. However, plastic glazing is transparent to microwaves and residential satellite dishes have successfully been hidden indoors looking through [[acrylic glass|acrylic]] or [[polycarbonate]] windows to preserve the external aesthetics of the home.<ref>{{cite web|url=http://www.satelliteguys.us/threads/322871-Ku-Dish-through-glass/page2|title=Ku dish through glass|work=satelliteguys.us|access-date=November 6, 2013|url-status=dead|archive-url=https://web.archive.org/web/20140407062421/http://www.satelliteguys.us/threads/322871-Ku-Dish-through-glass/page2|archive-date=April 7, 2014}}</ref>

The purpose of the LNB is to use [[heterodyne|heterodyning]] to take a block (or [[band (radio)|band]]) of relatively high frequencies and convert them to similar signals carried at a much lower frequency (called the [[intermediate frequency]] or IF). These lower frequencies travel through cables with much less [[attenuation]], so there is much more signal left at the satellite receiver end of the cable. It is also much easier and cheaper to design electronic circuits to operate at these lower frequencies, rather than the very high frequencies of satellite transmission.

The frequency conversion is performed by mixing a fixed frequency produced by a [[local oscillator]] inside the LNB with the incoming signal, to generate two signals equal to the sum of their frequencies and the difference. The frequency sum signal is filtered out and the frequency difference signal (the IF) is amplified and sent down the cable to the receiver:

;[[C band (IEEE)|C-band]]: <math>f_\text{IF} = f_\text{LO} - f_\text{recv}</math>
;[[Ku-band|K<sub>u</sub>-band]]: <math>f_\text{IF} = f_\text{recv} - f_\text{LO}</math>

where <math>\scriptstyle f</math> is a frequency.

The local oscillator frequency determines what block of incoming frequencies is downconverted to the frequencies expected by the receiver. For example, to downconvert the incoming signals from [[Astra 1KR]], which transmits in a frequency block of 10.70–11.70&nbsp;GHz, to within a standard European receiver's IF tuning range of 950–2,150&nbsp;MHz, a 9.75&nbsp;GHz local oscillator frequency is used, producing a block of signals in the band 950–1,950&nbsp;MHz.

For the block of higher transmission frequencies used by [[Astra 2A]] and [[Astra 2B|2B]] (11.70–12.75&nbsp;GHz), a different local oscillator frequency converts the block of incoming frequencies. Typically, a local oscillator frequency of 10.60&nbsp;GHz is used to downconvert the block to 1,100–2,150&nbsp;MHz, which is still within the receiver's 950–2,150&nbsp;MHz IF tuning range.<ref>{{cite web
 | title = Understanding lnb specifications
 | url = http://www.satcritics.com/sc_tech_lnb.pdf
 | work = SatCritics Technicals
 | access-date = 2011-11-08
 | date = 2002-11-15
 | archive-date = 2012-04-25
 | archive-url = https://web.archive.org/web/20120425150745/http://www.satcritics.com/sc_tech_lnb.pdf
 | url-status = live
 }}</ref>

In a C-band antenna setup, the transmission frequencies are typically 3.7–4.2&nbsp;GHz. By using a local oscillator frequency of 5.150&nbsp;GHz the IF will be 950–1,450&nbsp;MHz which is, again, in the receiver's IF tuning range.

For the reception of [[wideband]] satellite television [[carrier signal|carrier]]s, typically 27&nbsp;MHz wide, the [[accuracy]] of the frequency of the LNB local oscillator need only be in the order of ±500&nbsp;kHz, so low cost [[Dielectric resonator|dielectric oscillators (DRO)]] may be used. For the reception of narrow bandwidth carriers or ones using advanced [[modulation]] techniques, such as [[16-QAM]], highly stable and low phase noise LNB local oscillators are required.  These use an internal [[crystal oscillator]] or an external 10&nbsp;MHz reference from the indoor unit and a [[phase-locked loop]] (PLL) [[oscillator]].