PSM-2100L Satellite Modem Addendum
Page L-Band - 2
2.0
Differences Between 70 MHz and L-Band Modems
Since the PSM-2100L modem is closely based on the design of the PSM-2100 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.
•
IF Frequency range changed to 950 to 1650 MHz, Transmit and Receive.
•
Transmit Output Level expanded to +5 to –35 dBm to accommodate a wide range of
losses between the modulator and BUC.
•
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.
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-2100L is designed to tune over a 700 MHz range to accommodate as many satellite
range/converter LO schemes as possible. One scheme seems to be fairly common using a BUC
transmit LO of 4.900 GHz, while the LNB uses an LO of 5.150 GHz
The PSM-2100L 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
1650 MHz) for the transmit or receive carrier frequency assignment.
2. If a transmit or receive LO frequency is supplied, for example the 4.90 GHz transmit LO
and 5.15 GHz 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 LO frequency.
EXAMPLE:
Using the above LOs as an example, suppose that we wanted to operate on transponder
1 of a C- Band satellite at RF transmit frequency of 5932.1 MHz and a receive frequency
of 3705 MHz (representing a 5930.0 MHz transmit from the other station at a satellite LO
of 2225 MHz). The given transmit LO is used in an additive scheme where the RF
frequency = IF + 4900 MHz. The L-Band IF is then 5932.1 – 4900 or 1032.1 MHz. The
