Datum Systems PSM-4900L Application Manual

L-Band IF Application Guide

DATUM

SYSTEMS

PSM-4900L L-Band IF Satellite Modem

Application Guide

1.0 Introduction

The purpose of this document is to provide a quick start application guide to users already
familiar with Datum Systems M5 series of satellite modems, and as an aid to setting up an L­Band IF based small station.

The PSM-4900L modem brings all the advantages of Datum System’s direct modulation and
demodulation design, superior performance and high digital integration for low cost assembly to
the VSAT station. Because this new modem costs little more than a standard 70 MHz IF modem
and significantly reduces the complexity and cost of the up and down conversion equipment, it
promises to provide a new high in performance per dollar. Two expensive parts of the converter
equipment have however been transferred to the modem – that is the high stability reference
oscillator and the ODU power supply.

A significant aspect of small station design using an L-Band interface modem is that all of the
complexity and “smarts” are contained within the modem itself. The Block UpConverter or “BUC”
and the Low Noise Block DownConverter or “LNB” now each contain a single fixed local
oscillator, not required to tune for operation over the entire satellite band covering all
transponders. The PSM-4900L tunes over an extended range of 950 to 1750 MHz (transmit) or
950 to 1900 MHz (receive) in 1 Hz increments allowing it to access 800 MHz of Transmit RF
spectrum and 950 MHz of Receive RF spectrum without any converter settings.

Aside from the many advantages, using L-Band as an inter-facility link frequency results in the
need to carefully consider the components, frequencies and construction techniques used to
insure proper operation. Part of the purpose of this addendum is to spell out those areas where
special care must be used to achieve a reliable station operation.

For the purposes of the remainder of this document the names, acronyms and meanings used
which may be new for this type modem are:

• “Modem” - Refers to the PSM-4900L modem capable of both transmit and receive

operation.

• “IF”. The modems Intermediate Frequency used to connect to the Up and

DownConversion equipment.

• “BUC” – Block Up Converter, Often with an integrated power amplifier for installation

directly to the feed at the antenna.

• “LNB” – Low Noise Block Down Converter. Includes a low noise RF front end and single

down conversion stage to L-Band frequencies. In a VSAT, especially at low data rates,
this is a significantly better device than the typical free running LNB used for video
broadcast reception. A “data grade” LNB must have very low phase noise and a phase
locked LO for proper performance.

• “Bias T Mux”. This is a device that multiplexes power, IF signals and often a reference

frequency onto a single cable going up to the BUC or LNB.

• “Terrestrial” side. The Line or data side of the modem.

• “VSAT” – Vary Small Aperture Station, referring to a station with a small antenna,

typically 1 to 4.5 meters in diameter.

• “LO” – Local Oscillator frequency used for up or down conversion of RF frequenies.

Page L-Band - 1

L-Band IF Application Guide

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 non­volatile 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 L­Band IF frequencies are used, then the user MUST

Page L-Band - 2

determine and set the “Spectrum”

L-Band IF Application Guide

parameter for transmit and receive to that required for the BUC and LNB. Without the LO
frequency the modem cannot determine the spectrum sense.

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
receive must use a subtractive scheme where the IF = 5150 – RF frequency. This will
result in a spectrum inversion on the receive side only. The receive L-Band IF frequency
is 5150 – 3705 or 1445 MHz.

By having previously entered the BUC and LNB LO frequencies we only had to enter the RF
frequencies. These are the same frequencies that we would see on a spectrum analyzer looking
directly at the station transmit and receive RF.

Notice that these common LO examples resulted in L-Band IF frequencies at opposite ends of the
L-Band range for carriers that were almost next to each other on the satellite.

2.1.1 Some Other Block Converter Schemes

In a single conversion UpConverter from L-Band there is also the possibility of using a “high side”
LO for both C and L-Band transmit frequencies. For a C-Band BUC using a High side LO going
from 950 – 1450 MHz to 5925 – 6425 MHz the LO frequency would be 7375 MHz (950 + 6425
MHz). There would be an inversion in the transmit output spectrum. Notice also that the highest
transmit output frequency results from using the lowest L-Band modem transmit frequency.

The same schemes are possible at Ku-Band frequencies, where either a high or low side LO may
be used. The following table summarizes the straightforward low and high side LO frequencies for
Block Up and Down Converters.

Band Up/Down Freq Range

(MHz)

LO LO Freq.

(MHz)

Spectrum

Inversion

Notes

C Up 5925-6425 Low 4900 No Common
C Up 5925-6425 High 7375 Yes
C Up 5850-6350 High 7300 Yes Brazilian

C Down 3700-4200 High 5150 Yes Common
C Down 3700-4200 Low 2750 Yes Not used

Ku Up 14,000-14,500 High 15,450 Yes
Ku Up 14,000-14,500 Low 13,050 No Common

Ku Down 11,700-12,200 Low 10,750 No Common
Ku Down 11,700-12,200 High 13,150 Yes ?

Of course there are many possible frequency ranges used for satellite stations in different parts of
the world and we make no attempt to show them all here. This table is simply to list some of the
possibilities. The PSM-4900L tunes over more than the typical 500 MHz (800 MHz transmit, 950
MHz receive), so it is also possible to use an LO frequency that allows a single modem and
Converter to cover multiple frequency ranges. For example, a 4800 MHz C-Band Low side LO
would translate the 950 to 1750 MHz range (available in the PSM-4900L) to 5750 to 6450 MHz.

Page L-Band - 3

L-Band IF Application Guide

2.2 Transmit Output Power Levels

The PSM-4900L has a very wide range of power levels available from the transmit output. This is
to accommodate direct connection to a standard BUC including significant cable loss without the
need for inline amplifiers or attenuators, and to accommodate transmit combiners. The PSM­4900L can output from –35 dBm to +5 dBm in 0.1 dB steps. This 40 dB range can accommodate
a wide range of cable length and BUC gain. Assuming for example that with a BUC gain of 60 dB,
and a 4 Watt maximum output (+36 dBm) the required BUC input to achieve full output power
would be –24 dBm. The modem then could drive up to a maximum of 29 dB of cable/connection
losses. This could be a maximum of 100 to 500 feet or more depending on the size and type of
cable used. More about cable selection is provided in Section 3 below on designing and setting
up an L-Band station.

2.3 Receive Input Power Levels

The PSM-4900L has increased the range of power levels acceptable to the receive input. This is
to accommodate direct connection of a standard data grade LNB including significant cable loss
or inline splitters without the need for inline amplifiers or attenuators. The PSM-4900L can accept
a window of approximately 55 dB at any given data rate. The input level range changes with data
rate. When considering the full data rate range of 1.2 kbps (BPSK, rate ½) to 4.92 Mbps (QPSK,
rate ¾ or 7/8) this results in a total range of approximately from –20 dBm to -102 dBm. The
modem automatically adjusts the range for the data rate used and the user is warned if the level
is marginal. Of course, if the level is below the AGC capability then the modem will not acquire
signal lock. This approximate 55 dB range at any particular data rate can accommodate a wide
range of cable length and LNB gains. The LNB gain minus the cable loss should always fall within
the range of 40 dB to 70 dB of overall gain. As long as this gain is achieved, the demodulator will
function properly at all data rates from 1.2 kpbs to 4.92 Mbps requiring no further system level
engineering. For example a typical data grade LNB has a gain of approximately 60 dB. This
would allow for up to 20 dB of cable loss at any data rate. Like the transmit this allows a
maximum cable length of approximately 100 to 400 feet depending on the size and type of cable
used. The LNB gain and cable loss variations due to temperature changes are less important on
the receive side as long as the overall gain range above is met at all times.

The PSM-4900L Receive input provides a direct impedance match to a typical 75 Ohm LNB.
More about cable selection is provided in Section 3 below on setting up an L-Band station.

The user does not have to specify the input power level. The modem AGC locks to the signal and
reports the receive signal level as a front panel parameter under “DEMOD INPUT LEVEL”

2.4 New/Modified Commands

New Commands relative to the 70 MHz modem are all directly related to L-Band operation. Each
is represented by a new “parameter entry” in the front panel matrix. All of the Modulator BUC
commands are contained within one column of the Modulator parameter matrix, and all of the
Demodulator LNB commands are contained within one column of the Demodulator parameter
matrix Two new binary packet commands are also included in the command protocols. Modified
commands have modified entry parameters from the 70 MHz modem commands.

2.4.1 New Commands

“MOD BUC – Power” – Transmit BUC Voltage Enable/Disable.
“MOD BUC – Voltage Out” – Reading of Voltage sent to BUC when it is enabled.
“MOD BUC – Voltage Min” – Alarm setting for minimum voltage sent to BUC. Can be used to

warn if the voltage is below the minimum.
“MOD BUC – Current Out” – Reading of current sent to BUC when it is enabled.

Page L-Band - 4

L-Band IF Application Guide

“MOD BUC – Current Max” – Alarm setting for maximum current sent to BUC. Can be used to
warn or if the current is above the maximum.

“MOD BUC – Current Min” – Alarm setting for minimum current sent to BUC. Can be used to
warn if the current is below the minimum.

“MOD BUC – 10 MHz Ref” – Transmit BUC reference output enable/disable.
“MOD BUC – LO Frequency” – Transmit BUC Local Oscillator Frequency – Input to a non-zero

value allows direct RF frequency entry (see Section 2.1)

“DEMOD LNB – Power” – Receive LNB Voltage select and Enable/Disable.
“DEMOD LNB – Current Out” – Reading of current sent to LNB when it is enabled.
“DEMOD LNB – Current Max” – Alarm setting for maximum current sent to LNB. Can be used to

warn if the current is above the maximum.
“DEMOD LNB – Current Min” – Alarm setting for minimum current sent to LNB. Can be used to

warn if the current is below the minimum.
“DEMOD LNB – 10 MHz Ref” – Receive LNB reference output enable/disable.
“DEMOD LNB – LO Frequency” Receive LNB Local Oscillator Frequency – Input to a non-zero

value allows direct RF frequency entry (see Section 2.1)

2.4.2 Modified Commands

Modulator Carrier Frequency

Was: 50 to 90 MHz, 4 bytes in binary command
Is: 950 to 1750 MHz, 5 bytes in binary command
OR 800 MHz of RF frequency range when the LO input not = 0.

Demodulator Carrier Frequency

Was: 50 to 90 MHz, 4 bytes in binary command
Is: 950 to 1750 MHz, 6 bytes in binary command
OR 950 MHz of RF frequency range when the LO input not = 0.

3.0 Designing and Setting up an L-Band Station

The equipment complement at any station site almost always consists of transmit and receive
equipment including Modem(s), UpConverter and Downconverter, Power Amplifier and Low
Noise Receivers as well as the antenna itself. In an L-Band IF station the locations and
complexity of these items is changed. The basic station diagram below shows the typical
equipment complement for an L-Band based VSAT. The station can be expanded by adding
combiners and splitters in the IF to feed more modems.

Attempting to build a multi-modem L-Band station does bring up one of the difficulties of using an
L-Band IF Link. The power and reference signals to a BUC and LNB may have to be combined in
a separate BIAS-T/Mux device. This is because special combiners and splitters would be needed
to pass the current for the typical BUC or LNB. The lower cutoff frequency of a combiner or
splitter may not also pass a 10 MHz reference signal. A “Wilkinson” type combiner and splitter are
the types needed.

Page L-Band - 5