Ku-Band LNBFs

Introduction

The existence of inexpensive low-noise block downconverters (LNBF) for the satellite reception is fortunate as the bands they cover include the transition frequencies for two species of masers - the 12.178 GHz Methanol maser line and the 22.235 GHz Water maser line. However - the very reason why they are available means that reception of those maser lines can be subject to increasing levels of interference from those satellite signals.

The 'low-noise' part of LNBF refers to a low-noise front end amplifier (LNA) working at the sky frequency.

The 'down-converter' part of LNBF refers to a circuit which 'mixes' a local oscillator (LO) with the incoming sky frequency signal from the LNA - thereby 'down-converting' the sky frequency to a lower frequency - usually in the 1 to 2 GHz range.

This page deals with those Ku-band LNBFs which cover the 12.178 GHz Methanol maser transition line rest frequency.

Ku-Band LNBFs

Nomenclature for various satellite front-ends is a bit loose. Terms which mean one thing in one part of the world market can mean something else in another. Coupled with not uncommon errors in the specifications given by online sellers, it is very much a case of 'caveat emptor'. In some cases the true nature of the device advertised can only be determined by biting the bullet and ordering one to test. Fortunately they are not overly expensive - ranging from about USD$10 to somewhat less than USD$100.  Usually those experimenters who play with such devices have a bunch on hand - largely made up of units which were thought to be useful in their default state or after modification - only to turn out to be neither. I am looking at myself in the mirror as I write this...

Different Types Suitable for Methanol Masers

Generally speaking the 'F' in the term 'LNBF' refers to an integral feedhorn.  The electronics ride piggy-back on the waveguide-type feedhorn and upon feeding power - and some control signals - to the LNBF, the down-converted signal is available at one or more F-type female sockets mounted on the LNBF at a sufficient level to be transported to a receiver down a relatively inexpensive coaxial cable (75 ohm) run. These LNBFs come in a variety of configurations - but basically can be sub-divided into a number of types. The following is just commentary - as terms can differ with market region.

• 'Standard' vs 'Universal' - can signify that only one band is covered - 'Standard', or two bands are covered - 'Universal'. What satellite services are available in a particular market region will influence whether retail outlets carry one, or both of these types. The full range of sky frequencies which may be encountered - depending on region - is 10.7 GHz to 12.75 GHz - a bandwidth of 2050 MHz. The actual satellite receivers in the house are designed to accept from 950 MHz to 2050 MHz - a bandwidth of only 1100 MHz. Therefore to cover the whole range a 'universal' LNBF will house two local oscillators (LO) to effectively split the 2050 MHz whole bandwidth into two bands - called the 'low' and 'high' bands. As we are interested in receiving  the 12.178 GHz Methanol maser transition line frequency, a universal LNBF would be operated in its 'high' band state. To get the LNBF into the 'high' band receiving state it is necessary to inject a 22 kHz control via the output signal cable - which is a little inconvenient. Some examples...
  
  Universal LNBF
  
  RF Input Frequency Range
  Low Band: 10.7 – 11.70 GHz
  High Band: 11.7 – 12.75 GHz (22 kHz injected)
  
  IF Output Frequency Range
  Low Band: 950 – 1950 MHz
  High Band: 1100 – 2150 MHz (22 kHz injected)
  
  Local Oscillator
  Frequency Low Band: 9.75 GHz
  High Band: 10.6 GHz (22 kHz injected)
  
  Standard LNBF
  
  RF Input Frequency Range
  11.70  – 12.75 GHz
  
  IF Output Frequency Range
  1000 – 2050 MHz
  
  Local Oscillator
  Frequency 10.7 GHz
  
  For convenience a 'standard' LNBF might be preferred - as it doesn't require a 22 kHz control tone as does the 'universal' type. Also the 'universal' type will down-convert the 12.178 GHz Methanol line to 1578 MHz - which is too close to the maximum RF input frequency for the garden-variety RTL-SDRs. The 'standard' LNBF with a 10.7 GHz LO frequency will output the line at 1478 MHz. Better still look for a 'standard' LNBF with a 10.75 GHz LO. This will output the Methanol line at 1428 MHz - this is close to the 1420 MHz frequency of the Hydrogen Line and so lends a possibility of connecting an existing Hydrogen Line chain as the backend.
  
  

•  Stability - as the LNBF will be used in a system where the received transition line frequency will be corrected for Doppler to the LSR (giving an identifying relative velocity to LSR), it is important to be able to correctly relate the down-converted IF frequency to the received sky frequency. To do this requires knowing what the LO frequency is to the same degree of accuracy (in ppm) as is required for the end LSR velocity. The  degree of accuracy required for the LSR velocity is influenced by the spectral resolution used in the processing - which, in turn, is governed by the degree of detail required in the displayed maser line profile.  Different masers have different requirements in this regard, and also some compromise on spectral detail might have to be made in poor SNR cases by reducing the spectral resolution. As a rule of thumb a velocity resolution less than 1 km/s seems reasonable. This translates to ~3.5 ppm. The LO stability (drift) is the key factor over time and temperature. Initial offsets can be calibrated out - it is the drift which is the issue. The LO can be generated - in order of increasing stability - directly by a dielectric resonance oscillator (DRO), a crystal-controlled  phase-locked loop (PLL) or an external reference driven PLL. Typical stabilities are...
  DRO:  +/- 300 ppm
  Crystal-Driven PLL:  +/- 3 ppm
  External Reference PLL: same as the stability of the external reference - which could GPS-locked or atomic clock sourced.
  
  It can be seen that the DRO-based oscillators - while much cheaper - do not meet the stability specification required for the 1 km/s velocity  specification. The crystal-driven PLL LOs just squeak in -  but should be fine as the +/- 3 ppm is over a wide temperature range.
  
  

• Polarisation - to allow reception of different satellite feeds on - or close to - the same frequency, LNBFs can receive on two orthogonal linear polarisations - horizontal (H) or vertical (V). Note that some LNBFs for some regions can have left and right circular polarisation feeds. The LNBFs can operate with a DC power input between 12 V to 24 V fed up the signal down cable, with different voltages used to switch polarisations. The probes for each polarisation are placed into the same waveguide space, so to avoid interaction the horizontal probe is placed at the rear (with the rear face of the waveguide acting as reflector), whilst the vertical probe is placed closer to the mouth of the feed horn (with a tuned rod behind it as a reflector). Consequently, the horizontal polarisation probe has to 'look past' the vertical polarisation hardware. There is anecdotal evidence that as a result of this arrangement, the vertical polarisation channel has better performance. The most commonly used voltage ranges for polarization switching are 13V and 18V. When a voltage of 13V is applied, the LNBF selects vertical polarization, and when a voltage of 18V is applied, it selects horizontal polarization. However, some LNBFs may use different voltage ranges, such as 14V and 19V or 15V and 22V.
  
  It's important to note that not all LNBFs use voltage switching for polarization selection. Some LNBFs use DiSEqC signals or tone switching instead of voltage switching. The specific method used by an LNBF can typically be found in its specifications or user manual.
  
  

• Gain - usually high enough to drive the IF signal down a reasonable run of cable into the observatory - but there are in-line amplifiers available for long runs.
  
  

• Noise Figure - perhaps better termed 'Noise Fiction'. Ignore claims of 0.1 dB noise figure - these are fiction IMHO. Any unit quoted as having a noise figure less than 0.8 dB will probably be the same as any unit quoted as 0.1 dB - at least in the sub USD$50 range.

The LNBFs On-Hand

A total of 6 versions of Ku-Band LNBFs are on hand. Their details are shown below for reference. All are single output versions except for the Acer model (twin) and all are standard models (one band) except for the Bullseye model (universal - two bands).