LNB LO Frequency - 101
What is an LNB and how does it work?
When a satellite signal is reflected into the feedhorn, the signal travels down the feedhorn or a waveguide and is picked up by an antenna probe and fed to the electronics section of the LNB (Low Noise Block). If the LNB has an integrated feedhorn it is called a LNBF (Low Noise Block Feedhorn).
The LNB consists of two sections:
- LNA (Low Noise Amplifier) which boosts the strength of the very weak satellite signal.
- Frequency downconversion circuit that lowers the frequency so it will travel through the coax line to the receiver with minimized losses.
The LNB's down convertor circuit subtracts a specific frequency called an LO (local Oscillation) from the downlink transponder satellite frequency to provide the IF (Intermediate Frequency) signal that is outputted from the LNB. A DRO (Dielectric Resonator Oscillator) type LNB(f) uses a tuning slug to adjust the LO (local Oscillation) frequency and consumer PLL (Phased Locked Loop) types use an internal reference crystal.
Correct LO Frequency:
For this example we will reference a standard KU-band LNB with an LO frequency of 10750 receiving a satellite signal on downlink frequency 11700mhz.
1. The LNB receives the downlink transponder satellite signal at 11700mhz and subtracts the LO frequency of 10750 and outputs an IF signal of 950mhz.
- DL - LO = IF (11700 - 10750 = 950)
2. The IF signal of 950mhz travels down the coax to the satellite receiver.
3. The receiver's tuner then blind scans and logs an IF frequency of 950mhz and displays Signal Level and Signal Quality readings. The receiver also calculates the downlink frequency to display for the user. To do this calculation, the user inputs the LO frequency of the connected LNB then the receiver adds the LO to the IF and displays the downlink transponder frequency.
- LO + IF = DL transponder frequency (10750 + 950 = 11700)
4. If a receiver is pre-programmed with a transponder frequency of 11700 and the user sets the LO frequency to 10750, the receiver's tuner will look for a satellite signal at 950mhz IF. Most receivers have an AFT (Automatic Fine Tuning) circuit that searches for the signal on frequencies near the specified IF frequency. Most receivers will adjust the tuner to find a signal that is 3 - 5mhz away from the center of the specified IF. On GEOSATpro receivers we program the AFT range for up to 10mhz. With a typical receiver, if the IF frequency is not within 2 -5mhz of the specified frequency, the receiver will not find it.
Incorrect LO Frequency:
Why might the LNB LO frequency be incorrect? This may happen on a DRO type LNB due to a misaligned tuning slug at the factory, vibrations loosening the tuning slug or the seated PCB, heat / cooling expanding or shrinking the mechanics of the PCB/tuning slug or the aging of the LNB. PLL units have very little drift as they are referenced to the crystal and will only slightly vary from the factory setting. PLL LO frequency outside specification would be due to manufacturing error or mechanical failure.
Now imagine if the Standard LNB LO frequency had drifted to from 10750 to instead be 10740. The LNB receives a downlink satellite signal at 11700, down converts by subtracting 10740mhz and outputs an IF of 960mhz.
- DL - LO = IF (11700 - 10740 = 960)
The IF signal travels down the coax to the receiver. The tuner now will blind scan and find the signal at 960mhz. The user has inputted that the connected LNBF LO is 10750, so the receiver add 10750 to 960 and displays 11710mhz as the transponder frequency. The receiver will log the transponder in the blind scan mode and be able to display programming, but the displayed transponder frequency will be wrong.
- LO + IF = DL (10750 + 960 = 11710)
A receiver's tuner with a preprogrammed transponder frequency of 11700 and user inputted LO frequency of 10750 will look for a satellite signal at 950mhz IF. Since the LNB is sending the transponder at 960 IF, the receiver will probably not find a satellite signal as it is beyond the tuning range of the AFT circuit.
- Receiver Tuning: 11700 - 10750 = 950mhz IF
- LNB Output: 11700 - 10740 = 960mhz IF
- IF Difference: 960mhz IF - 950mhz IF = 10mhz IF offset
- Receiver IF AFT Range: 5mhz - 10mhz = 5mhz beyond tuner AFT range
Bottom line: When a satellite receiver is looking for a channel on a specific frequency, but the LNB is sending the transponder on a different frequency, no channel will be found.
