Analog multiplex systems: The basics
Part 1 – Multiplexing combines multiple discrete signals into one composite transmission. Familiarity with direct-to-line systems enhances a technician’s repertoire.
A common complaint for employers in today’s communications industry is not being able to find qualified technicians. Years ago, two-way field technicians might have been considered “qualified” by merely having experience with Micors or Master IIs-not so today. Employers are looking for people whose experience extends to microwave and multiplex systems. Those seeking employment must be willing to expand their knowledge base if they expect to be marketable. Two-way technicians who are currently employed must be willing to expand their responsibilities to maintain job security.
With this in mind, let’s take a brief and practical look at the subject of analog multiplex systems, which can prove valuable to those yet unfamiliar with such transmission systems equipment.
Analog mux-FDM Frequency-division multiplexing (FDM) is, and has been for quite some time, a common analog method of combining or encoding multiple discrete signals into one composite transmission signal. Analog microwave radio systems are the typical transport mechanisms for FDM systems. The multiplex (mux) equipment provides the means for interfacing the individual circuits (radio, voice or data) with the microwave radio. For every circuit transported over the microwave system, a corresponding mux channel modem will be on each end of the circuit.
In general, there are two types of FDM systems: basic and direct-to-line. Basic FDM systems preceded the arrival of direct-to-line equipment by decades. The architecture of these older systems typically requires multiple equipment racks to accommodate all the individual, interconnected units necessary for each step of the multiplex process.
Over the years, advances in technology and miniaturization of circuitry made possible the direct-to-line method that offers the same end result with significantly less equipment. Direct-to-line systems allow companies with minimal channel requirements to obtain multiplex equipment on a per-channel basis rather than as a full-blown system made to accommodate hundreds of channels, whether you need them all or not.
Because of their cost-effectiveness, direct-to-line systems are probably what today’s communications technician will encounter. For this reason, basic FDM systems will only be mentioned briefly as a backdrop to the more modern and popular direct-to-line systems.
VF and signaling Multiplex and microwave systems are often referred to as transmission systems equipment because they provide a method of transmitting broadband signals from one point to another. A common term used in the transmission systems field is “VF,” short for “voice frequency.” A VF signal is the audio information being carried over a circuit. The VF transmit connection to the mux channel modem is referred to as the MOD while the receive connection is the DMOD. (See Figure 1 on page 72.)
In addition to VF signals, the mux circuit often carries information called signaling, which relates supervisory information from one end of the circuit to the other. Using a subscriber telephone circuit as an example, a method is needed to signal the central office as to whether the phone is on- or off-hook, or to activate ringing. Radio circuit signaling is used to convey the push-to-talk (PTT) function from the dispatcher console to the remote base station. VF and signaling can be accomplished over the same multiplex channel at the same time.
The primary mux modem signaling connections are referred to as the M-lead (transmit) and E-lead (receive). Although modem-strapping options usually accommodate alternate arrangements, transmit signaling is typically activated by applying battery (the modem’s operating voltage) to the M-lead connection. When signaling is detected from the far-end mux modem, the local E-lead is activated, which most often applies a ground to its external connection. As with the M-lead, onboard E-lead strapping options usually allow for alternatives such as the internal battery voltage to be sent out to the external connection. In some cases, mux modems are equipped with an F-lead that allows some other external voltage level to be applied to the modem, which can then be gated out the E-lead when signaling is detected. This is an advantage when the external device activated by the E-lead requires a specific voltage that is not obtainable from the mux modem itself.
VF frequency response The frequency response of speech can range from about 100Hz to 8kHz. However, the primary components necessary for speech recognition are contained below 3kHz. The typical analog mux channel has a circuit frequency response of 300Hz to 3,400Hz. Frequencies above and below this range are rolled off with filters.
FDM-direct-to-line method In direct-to-line systems, the whole multiplexing process takes place in one unit, the mux channel modem. (See Figure 2 below.) Signals are transmitted to a far-end location by applying analog circuit information (VF) to the MOD of the mux channel. The modem processes the signal through a limiter stage to prevent excessive levels from overdriving the following circuits. The VF signals are then routed through a high-and-low-pass filter to give the desired 300Hz to 3,400Hz frequency response. A VF amplifier stage follows and allows adjustment of the transmit VF levels. The VF signal is applied to one port of the first balanced modulator while an IF oscillator (typically in the 3MHz to 6MHz range) is injected into another. The resultant mix translates the original 300Hz-to-3,400Hz VF signal into the RF spectrum as a double-sideband, suppressed-carrier (DSBSC) signal. A crystal filter then eliminates the unwanted sideband, leaving a single-sideband, suppressed carrier (SSBSC) signal occupying a 4kHz bandwidth.
This modulated signal is then applied to a second balanced modulator stage that again produces a DSBSC signal whose unwanted sideband and suppressed carrier are eliminated by filtering. The final result is a SSBSC signal modulated with the original VF circuit information. At this point, the signal is considered HF (high frequency), rather than VF.
Each mux circuit has the same first IF frequency. The second modulator, however, has a different and unique frequency injected for each channel in the system. This prevents one mux channel from interfering with another by ending up on the same frequency in the HF spectrum. To accommodate this, the second oscillator (which typically covers the 4MHz to 8MHz range) is a dip-switch programmable, phase-lock-loop circuit allowing frequency adjustment in 4kHz steps. Spectrally, the channels form a collection of adjacent HF frequency slots, one for each circuit. Each slot has a bandwidth of 4kHz and contains the modulated VF signals unique to that circuit. The spectral composite of all circuits in the system is referred to as the “baseband.”
The mux channel modems on both ends of the same circuit must have their pro-grammable oscillators set to the same frequency so that one end can communicate with the other. The baseband channel slots follow a CCITT-recommended scheme that establishes a standard for channel placement within the baseband spectrum.
Once multiplexed to the proper frequency slots on the baseband, the circuits may be transmitted to the distant-end by means of an analog microwave radio transmitter. The mux system baseband is connected to the transmitter through a coaxial cable, which in this case would be referred to as the transmit baseband cable. At the distant-end, the baseband signals are demodulated through the microwave receiver. They are then applied, via the receive baseband cable, to a complementary FDM system. Here, the HF baseband signals are de-multiplexed through a process identical to the multiplexing of the opposite end, only in reverse. The end result is a recovered VF waveform the same as that applied to the mux channel MOD on the distant end.
Groups, supergroups and mastergroups Basic mux systems accomplish the multiplexing process through a standardized series of intermediate steps within a variety of separate, but interconnected, units.
Channel units are shelved in groups of 12. Each unit in the shelf has a different carrier frequency injected into its modulator stage by external channel carrier generator units. The result is a group of 12 channels translated one above the other with respect to frequency, occupying 48kHz of bandwidth between 60kHz and 108kHz. At this point the 12 channels are referred to as a basic group. Because each basic group occupies the exact same frequency range, they must be cabled separately to the next stage in the process to avoid interfering with one another.
These basic groups are modulated up to yet a higher frequency range, five at a time. Each group has a different injection frequency applied from its respective group carrier generator unit. The result is five groups (of 12 channels each) stacked one above the other with regard to frequency. The five groups are now referred to as a basic supergroup and occupy 240kHz of bandwidth between 312kHz and 552kHz.
Each basic supergroup occupies the same frequency spectrum and must, as with the basic group equipment, be cabled separately to the next stage.
Basic supergroups are then applied, in sets of 10, to higher-level translation equipment. Different injection carrier frequencies are again supplied, this time by supergroup carrier generator units. The results are 10 supergroups stacked one above the other on the baseband, now referred to as a mastergroup.
The translation processes can continue beyond the mastergroup level. For industrial applications, however, this is typically the greatest required channel capacity (600 channels).
Each translation process produces sum and difference frequencies that correlate to upper and lower sideband signals. As seen in Figure 3 on page 74, the CCITT modulation scheme dictates upper sideband (USB) for Supergroup Two while the others are lower sideband (LSB).
Unassigned spectrum is left available below 60kHz and can be used for additional channel assignments. The 12kHz-60kHz portion of the baseband is referred to as Group A. Like Supergroup Two, Group A comprises USB channels. The 0kHz-4kHz portion of the baseband is used as a maintenance communications channel on the microwave system, called an order wire. It allows communication between technicians at different microwave sites sharing the same baseband. Two more channels can be used in the 4kHz-12kHz spectrum. Often, remote alarm monitoring & control systems occupy one of these channel slots.
Synchronization In both basic and direct-to-line FDM systems, pilot tones can be placed on the baseband for synchronization between the central office and the remote-end mux locations. A pilot is a specific frequency tone such as 60kHz, 64kHz, 308kHz, 512kHz or 564kHz, sent at a specific level. Once received at the remote site, the pilot tone frequency is divided down and used as a reference for the remote-end multiplexing and demultiplexing processes.
Synchronization can also be achieved through master/slave strapping on the mux channel modem. A channel modem, strapped as the slave, locks onto the master’s signaling tone frequency. The master tone frequency is used within the slave channel modem for the multiplexing and demultiplexing processes.
Whichever method is used, the purpose of synchronization is to eliminate translation error between the two ends of the circuit. Excessive translation error on a voice circuit makes the party on the other end of the circuit sound like Donald Duck. Those familiar with tuning VFOs on SSB receivers will recognize the sound.
