Analog Multiplex systems: The basics, Part 2

Part 2 – Technicians need familiarity with modem circuitry, signaling formats and proper signal levels when servicing multiplex systems.

Today’s two-way radio field technicians have increased responsibilities that extend to maintenance and installation of multiplex equipment. As such, they encounter terminology and a variety of circuitconfigurations that are foreign to the two-way industry.

Four-wire & two-wire circuits Most communications circuits are designed to send and receive information, often at the same time. Analog multiplex (mux) systems accomplish this full-duplex operation through use of a four-wire (4W) mux modem. The modem has separate transmit and receive circuitry, providing isolation between the two. The term 4W comes from the number of VF wire connections made to the mux channel modem. Because the MOD and DMOD are balanced connections, each requires two wires, totaling four, hence the term 4W. (See Figure 1 below.)

In the case of a telephone instrument or other two-wire (2W) device, the transmit and receive signals are carried on a single, balanced pair of wires. A 2W circuit must be converted to 4W when being applied to a transport system having separate receive and transmit circuitry. This conversion is accomplished through a 2W/4W hybrid circuit that is a three-port transformer arrangement. The telephone connects to the 2W port, while the mux modem connects to the 4W receive and transmit ports.

Signaling-in-band & out-of-band Circuits usually require supervisory information (signaling) to be transmitted along with VF. The two basic signaling types used in frequency-division multiplexing (FDM) systems are in-band and out-of-band signaling.

In-band signaling occurs when supervisory information is sent within the normal 300Hz to 3,400Hz circuit passband. Using a subscriber telephone circuit as an example, a 2,600Hz tone can be sent from the far-end mux channel to the central office (CO), indicating the phone is “on-hook.” When the phone is taken off-hook, the 2,600Hz signal is dropped. The CO equipment senses the absence of the tone and sends dial tone back out to the far-end.

In the case of remote base station keying, a 2,175Hz tone is frequently used. Presence of the tone activates the transmitter. The 2,175Hz signal is present for the duration of the radio broadcast to maintain the radio push-to-talk (PTT) function. The tone is within the normal VF passband of the mux circuit and is therefore considered in-band signaling. Because the tone is present during the entire broadcast, the tone must be notched out prior to the base station exciter stage, or it will be transmitted out over-the-air along with the voice.

Out-of-band signaling is achieved by moving the signaling tone frequency outside the 300Hz to 3,400Hz VF passband (but still within the 4kHz channel slot). A common example of this is the use of a 3,825Hz signaling tone.

Out-of-band signaling can also be accomplished by re-insertion of the suppressed carrier from the mux modem’s first balanced modulator stage. Raising and dropping the level of the suppressed carrier on one end of the circuit can activate and de-activate signaling functions on the far-end. In this case, the signaling frequency will be located on the extreme edge of the allotted 4kHz channel slot.

Carrier leak is a term relating to the amount of residual suppressed carrier left in the mux modem transmit signal when the suppressed carrier level has not been raised for signaling. Since this carrier level can be detected and used in the signaling scheme, it is important to keep it suppressed as much as possible to prevent “falsing” when signaling is not activated. Often, the mux channel modem will have an adjustment for minimizing carrier leak when signaling is not activated.

Signaling-E- & M-leads The mux modem connections that relate to signaling are the E- & M-leads. The E-lead connection is dedicated for receive signaling use, while the M-lead is for transmit.

Typically, the mux modem transmits signaling to the far-end when a voltage is applied to its M-lead connection. The far-end modem detects the signaling and activates its E-lead. Usually, the mux modem is equipped with an E-lead relay that supplies external supervisory equipment with a dry set of relay contacts. Figure 2 above offers an example of E & M signaling. A radio dispatch console is connected to mux so that when the console operator wishes to key up a hilltop transmitter, a button on the console is pressed. This applies a voltage to the mux channel M-lead connection, causing the modem to send M-lead signaling out to the hilltop mux via microwave. The coordinate mux modem on the hilltop detects the M-lead signal and energizes its E-lead relay. The E-lead relay contacts close and apply a ground to the radio PTT line. The hilltop base station keys up, and the dispatcher can make the broadcast.

VF transmission levels It is important to maintain proper signal levels within an analog multiplex system. The term test tone level (TTL) describes the maximum signal level that should exist at any point within an analog mux circuit to avoid creating intermodulation distortion components throughout the baseband. Test tone level is sometimes also referred to as test level point (TLP).

Levels of “live traffic” can fluctuate greatly, depending on the circuit application. For this reason, they are not often a useful reference when making critical circuit level measurements and adjustments. Instead, maintenance personnel must inject an external, constant-level test tone (typically, 1,004Hz) into the circuit. However, this is an intrusive method that disrupts normal circuit operations. As circuit gain and loss tests are performed, levels exceeding expected values are referred to as hot, while those which are low are referred to as either low, cold or long.

As shown in Figure 1, TTL for a 2W circuit in the transmit direction is 0dBm, which produces a 216dBm level at the mux MOD. If 17dBm is delivered from the mux DMOD, it should produce a receive level of 22dBm on the 2W side of the hybrid. The standard test tone level for a 4W channel modem is 216dBm into the MOD and 17dBm out of the DMOD.

In transmission systems, the term dBm0 is frequently used to refer to the difference (in decibels) between TTL and the absolute level (in decibels relative to 1mW) being measured. For example, if a base station status tone that has an absolute level of 226dBm is applied to the MOD of a mux channel, the level is said to be “210dBm0” at that point in the circuit. Another way to express this is to say the level at the MOD is “10dB below test tone level.” Assuming proper system alignment, this status tone will come out of the far-end mux DMOD at the same 210dBm0 level. Since TTL at the DMOD is always 17dBm, the absolute level (in dBm) of the status tone can be calculated: 17dBm210dB 5 23dBm. Even though the absolute input and outputs levels of the MOD and DMOD are different (226dBm vs. 23dBm) they both retain the same reference to test tone level (210dBm0). For this reason, the term dBm0 is handy to use.

The third and final part of this series will consider VF and HF connections to analog mux systems, as well as typical test equipment found in the analog transmission systems environment.