Analog Multiplex systems: The basics, Part 3

Active analog mux systems are still in widespread use. An understanding of basic testing and troubleshooting techniques can distinguish a technician.

Equipped with a basic understanding of analog multiplex (mux) systems, two-way radio technicians can take the leap toward extending their duties beyond base station maintenance and into transmission systems.

This three-part series concludes with a brief discussion of mux system connections, VF and HF considerations and practical testing and troubleshooting techniques.

VF jackfield

External circuits that interface with a mux modem should do so through a VF jackfield. The jackfield allows the “end equipment” (base station, telephone, etc.) to be isolated from the mux modem for testing purposes. There should be jack positions for the MOD, DMOD, and E & M leads of each mux channel. (See Figure 1.)

The top jacks are wired to the mux channel modem and are referred to as the line side of the jackfield. These jacks allow VF level testing into the mux modem MOD or out from the DMOD. The bottom jacks are referred to as the drop side of the jackfield.These jacks allow VF level testing into and out of the drop (“customer”) equipment. Without any test cords plugged in, the jackfield internally connects the line side to the drop side. If a test cord is inserted into either the line or drop jacks, the internal connection is broken and the two sides are electrically isolated.

The balanced connections to and from the VF jackfield have a circuit impedance of 600V. When test equipment is plugged into either the line or drop jacks, the instrument should have an internal terminating impedance of 600V. Because the line and drop jacks break (isolate) the two sides of the circuit, this allows the side under evaluation to be properly terminated into the test equipment for accurate measurements. A common mistake is to have the test equipment set up in the bridging rather than terminating mode when plugged into a breaking jackfield. This will not offer the proper termination to the circuit, and it will result in level readings being +6dB hotter than what actually exists without the test equipment in the circuit.

Often, a third monitor jack is located beneath the VF line and drop jacks. Plugging test equipment into this jack will not break the circuit, but it puts the test equipment in parallel with it. In this case, the test equipment must be put in the bridging mode to avoid a double-termination to the circuit under test. A double-termination will result in readings that are 23dB lower than what actually exists when the test equipment is removed.

When measurements indicate levels are 23dB lower or +6dB hotter than expected, the test equipment terminating/bridging mode switch should be checked. The mode switch only affects the receive portion of the test equipment.

VF test equipment

Serious VF test equipment can accomplish more than just frequency and levels checks. Circuit performance parameters can be obtained from use of a transmission impairment measurement set (TIMS). Test results indicate various aspects of circuit quality. The telephone industry has established standards for many tests such as PAR (peak-to-average ratio), message circuit noise, noise-with-tone, signal-to-noise ratio, 3-level impulse noise, phase jitter, envelope delay, noise-to-ground and non-linear distortion, to name a few.

HF test equipment

As the channel modems translate VF signals up to their assigned baseband frequency slots, a method is needed to measure the signal levels of the individual circuits while in the HF frequency range. A frequency selective device is required that can discriminate between the different frequencies on the baseband. Such a device is called a frequency selective voltmeter (FSVM), sometimes referred to as a selective level meter. This piece of test equipment has a variable bandpass “window” that can be tuned to any baseband frequency. It measures the power level (in dBm) of the energy within the bandpass being viewed. This allows measurement of individual multiplexed channel signals while filtering off all other baseband frequencies. The selective can measure broadband (3kHz) or narrowband (about 200Hz bandwidth) circuit levels and typically has both flat and C-Msg weighted filters offering different response characteristics.

A spectrum analyzer is another useful tool for viewing baseband levels. There are spectrum analyzers made specifically for baseband frequencies that have their horizontal baseline calibrated to read in terms of channel, group, supergroup and mastergroup numbers. Test tone level can be calibrated on the vertical axis (signal amplitude, in dBm) so that the whole baseband can be evaluated at a glance with regard to signal levels. Hot levels can be easily identified and zoomed in on down to the channel level.

As a tool in gain/loss measurements, a bridging HF signal generator may be connected to the baseband and used to inject specific tone frequencies at a specific levels. The HF frequency, when de-multiplexed, will produce a VF tone in the DMOD of the associated mux channel that occupies that portion of the baseband.

Video (HF) jackfield

The multiplex system baseband connects to a microwave radio through separate receive and transmit coaxial cables. There should be an HF or video jackfield placed between the mux system and the microwave radio so that the two can be electrically isolated for testing and troubleshooting purposes. A monitor jack is usually provided in both the transmit and receive directions. This allows a FSVM, bridging HF signal generator or baseband spectrum analyzer to be plugged into the jackfield to gain access to the baseband without “breaking” the connection and disrupting traffic.

Potential problems

Care should be taken to maintain proper signal levels within an analog multiplex system. Whether they be externally injected test tones or “live traffic,” excessive levels can drive analog mux circuitry into non-linear modes that wreak havoc on the baseband. Hot signals can not only splatter over to adjacent channel slots but produce sum and difference frequencies that affect circuits in other areas of the baseband spectrum as well. As mentioned previously, there should not be any signal on the baseband that exceeds TTL.

Another cause of baseband interference relates to poor shielding or bad solder connections in a mux/baseband assembly. Occasionally, local AM broadcast stations, which operate in the same frequency range as the baseband, can be heard on the baseband along with the mux signals. Although such interference typically affects only one or two mux channel slots, it can render those slots useless for “live traffic.”

Analog mux modems also require periodic maintenance. The operating frequencies of their on-board oscillators should be held to within +/-5Hz to eliminate problems with translation error.

Conclusion

FDM systems have been in use for a long time. Although a lot of the latest wireless applications are employing digital technologies, active analog mux systems are abundant and employers need qualified technicians to maintain them. Many of these employers have microwave, mux and base stations at their hilltops and would much prefer having one technician “do it all.” With a basic understanding of analog multiplex systems, two-way technicians will find themselves better postured in today’s job market.