Low noise blocks

Updated June 4, 2006

New for July 2006! And a mess of construction!!! The low noise block is what takes the infinitesimally small signal collected from a dish, amplifies it and mixes it down. It sits in front of the dish in a weatherproof housing.

We speak from experience here, having designed an LNB prototype in 1996 for the Korean market (don't ask how many were sold...) Before you decide you want to compete in manufacturing LNBs, think about this: unless your employees are content to share a single pair of shoes, forget about it. You will never compete in price!

The front end of any DBS receiver is the low noise block. The LNB is an environmentally packaged (read that rain and sun proof) superheterodyne receiver. A two or three stage low noise amplifier sets the noise figure to somewhere under one dB with maybe 20 dB of gain, then a mixer downconverts to an IF frequency. An IF amp boosts the overall gain to 55 to 60 dB.

The picture below is the guts of an LNB. Absent from the field of view is the downconvertor integrated circuit, which is actually on the back of this double-sided board. Somewhere are a voltage regulator and a DC-DC convertor for generating the negative voltage that is needed for the PHEMT amplifier stages.

Covering NA and European apps, downconverter needs to cover 950-2150 MHz

Signals are circularly polarized. need vertical and horizontal amplifier chains. allows twice as many channels! Switching is done at IF frequency to minimize added noise figure

DC power to the LNB is typically 18 volts at up to 500 mA. If a negative voltage is needed, a DC-DC converter is onboard. A positive regulator cleans up the DC supply for the LNA and downconverter functions.

A dielectric resonator oscillator is used to create the LO signal. Only 27 MHz of bandwidth is sent down the RG6 or RG6/U 75 ohm cable. Loss per foot of this cable is xx dB/100 feet (a typical length from receiver to dish).

The input signal is approximately -70 dBm at LNB. 11.45 to 12.75 GHz in NA, Japan and Korea, 10.7 to 12.75 in Europe total cascaded gain is 55 to 60 dB with 1.0 dB noise figure.

An image rejection filter is sometimes use to clean up the noise from the image band. Otherwise an image rejection mixer is used. The mixer and IF amp are almost always combined onto a single monolithic chip using MMIC technology. Often an RF amplifier stage or two is employed in front of the mixer to reduce the downconverter noise figure (which makes it an active mixer). Today's downconvertor ICs are silicon, the only GaAs chips are the discrete low noise FETs.

Low cost circuit board used (not ceramic!) substrates

LNA consists of discrete 0.25 micron "super-low-noise" transistors in ceramic or plastic micro-X X-packages. From Japan (NEC). First stage is more expensive than second stage, screened for lowest NF, with a minimum noise figure of 0.45 dB at 12 GHz. The second stage need only provide 0.8 dB (minimum) for a successful DBS LNB. The combined noise figure of the two stage LNA of the LNB, including matching networks, bias networks and blocking caps needs to be just under 1.0 dB.

When you are designing a two or three stage LNA using discrete FETs, always return to a known impedance (such a 50 ohms) between stages. Don't try any fancy interstages like you might on a MMIC, you will regret it.

The blocking cap between stages should be a 30 x 60 mil surface mount 1.0 pF cap. The idea is that the series resonance of the cap occurs exactly at the DBS band, so it behaves like an RF short. Of course, it should have no parasitic real resistance. You can verify that you have a good capacitor by measuring it's S-parameters across a gap in a fifty-ohm test fixture. You don't need a blocking cap on the input, the antenna presents a DC open.

DRO 10.5 to 11.5 GHz in north America

IF 950-1450 GHz (L-band)

950-2150 Europe

Needs to performed from -40 to +85C

RG6 75 ohm cable has approximately 7 to 10 dB loss per 100 feet at 1 GHz. Also, about 4 ohms resistance in 100 foot