L-Band IF Application Guide
Page L-Band - 2
2.0
Differences Between 70 MHz and L-Band Modems
Since the PSM-4900L modem is closely based on the design of the PSM-4900 70 MHz modem
the vast majority of the operation of these modems is identical. We briefly list the differences
between these modems here and further amplify operating differences in the following sections.
•
The IF Frequency range is changed to 950 to 1750 MHz Transmit, and 950 to 1900
Receive. A BUC and LNB LO frequency may be input, allow setting transmit and receive
RF frequencies directly.
•
Transmit power levels are wide range to cover long transmit IFLink cables.
•
Receive Input Level AGC range is greatly expanded covering demodulator input levels of
–20 dBm to –102 dBm, dependant on data rate.
•
New and Modified Commands available, specifically related to the care and feeding of
the BUC and LNB.
2.1
IF Frequency Range
Typical 70 MHz modems are designed to operate over a 36 (or 40) MHz range representing the
bandwidth of a single transponder on a C-Band (6 GHz uplink/4 GHz downlink) satellite. This
results in the classic 70 MHz IF range of 52 to 88 MHz.
Since it is expected that no tuning is available in the BUC or LNB, then an L-Band modem must
tune over at least the 500 MHz of a typical satellite’s full transponder range. For C-Band this
would be the RF ranges of 5.925 to 6.425 GHz transmit and 3.7 to 4.2 GHz receive. Translated to
an L-Band IF this would represent the typical frequency range of 950 to 1450 MHz. Not all
satellites use the exact same bands of RF frequencies for transmit and receive, therefore the
PSM-4900L is designed to tune over an 800 MHz tranmsit and 950 MHz receive range to
accommodate as many satellite range/converter LO schemes as possible. One scheme seems to
be fairly common for C-Band ODUs using a BUC transmit LO of 4900 MHz, while the LNB uses
an LO of 5150 MHz
The PSM-4900L provides two methods of specifying transmit and receive frequencies. Added
transmit and receive parameter inputs are provided for the transmit BUC and receive LNB Local
Oscillator (LO) frequencies. On the front panel display they are referred to as “MOD Cnvrter LO”,
and “DEMOD Cnvrter LO”.
1. If a zero frequency is supplied here then the user inputs L-Band IF frequencies (950 to
1750 MHz) for the transmit or receive carrier frequency assignment.
2. If a transmit or receive LO frequency is supplied, for example the 4900 MHz transmit LO
and 5150 MHz receive LO, then the modem accepts RF frequency inputs and computes
the actual required L-Band IF transmit and receive frequency. The modem also
determines if the LO is a high side or low side LO, and if a spectrum inversion results,
and then corrects for spectrum inversions within the modem parameters.
The modem’s automatic use of input LO frequencies is independent in the transmit and
receive channels.
As you might imagine it would be difficult to compute the proper L-Band IF frequencies to use
every time a new transmit or receive frequency is desired. The second method is highly
preferable since the LO frequencies are only entered once and the modem stores them in non-
volatile memory.
Note:
If this second method is used it is important to set the “Spectrum” parameter for
transmit and receive to “Normal” Then the modem will set the spectrum sense correctly
for the chosen BUC or LNB LO frequency.
Warning Note:
If the BUC and/or LNB LO Frequencies are set to “0”, and therefore L-
Band IF frequencies are used, then the user
MUST
determine and set the “Spectrum”