Radio control line instrumentation, measurements

Remember the good old days when remote base stations were controlled with dc over metallic lines? Metallic lines were simply lines that would provide dc continuity from end to end, allowing a dc current to pass through the radio control loop. It was a problem keeping the control current at the correct level to perform the various control functions.

Changing the routing of the control line, something that a telco always seemed to find delight in doing (they called it re-engineering), would change the loop resistance and therefore the control current.

Later, dc remotes were designed with current-regulated supplies — which greatly reduced the problem of improper current levels for the control relays. However, telcos also seemed to delight in reversing the polarity of the control pair. This reverse happened so frequently that we installed polarity-reversing switches on our equipment so that correcting the reversed wiring came down to simply flipping the switch.

After a while, metallic lines went by the wayside. Telcos could no longer guarantee that we would have dc continuity from end to end. Then came tone remote systems. Better, right? Well, not always. Now, instead of different current levels for control functions, different tone frequencies are used for various control functions. For example, to key the transmitter, a tone frequency of 2,175Hz is sent down the line to the tone termination panel. A special notch filter circuit is used to remove the 2,175Hz tone so that it is not passed on to the transmitter exciter.

Units of measure

Certain units of measure are encountered with line testing and associated equipment.

dBm — This is an absolute unit of measure based on 1mW in a 600Ω impedance. Thus, 0dBm refers to a signal level of 1mW in a 600Ω impedance. This is equivalent to 775mV across 600Ω. Using the formula for power:

P = E²/R

We can rearrange that to yield:

Substituting, we have:

This is usually rounded off to 0.775V or 775mV.

Voltage across 600Ω can be converted to dBm thus:

and dBm in 600Ω can be converted to voltage by the formula:

dBrn — This is generally used to refer to line noise level. 0dBrn equals -90dBm, which is equivalent to 1 picowatt in 600Ω. To convert dBrn to dBm, simply subtract 90dB from the dBrn level to get the equivalent dBm level. For example, 10dBrn = 10 – 90, or -80dBm.

C-message filter — This type of filter is often used in voice circuits. The filter response covers the frequencies used for voice communications. Any noise or interference at frequencies lying outside this band are not detrimental to voice communications.

Three-point slope test — Normally, in telephone-line work, test frequencies are offset by 4Hz. For example, 1,004Hz is used instead of 1,000Hz. Other test frequencies are 404Hz and 2,804Hz. For the three-point slope test, the level at 1,004Hz is taken as the reference and the level at the other two frequencies is given as the slope as referenced to the 1,004Hz level. For example, if the level at 1,004Hz is measured at -5dBm, the level at 404Hz at -10dBm and the level at 2,804Hz at -14dBm, the slope at 404Hz is -5dB and the slope at 2,804Hz is -9dB.

Testing remote control lines

You should be equipped to do some testing of the remote control lines yourself, and testing should be done before reporting any problems to the telco. Otherwise, if the problem is not with the telco lines, you might be charged for the time it spends in checking the line. Testing the line from end to end will require two technicians, one for each end, with necessary test equipment. The following discussion is for a typical two-wire voice frequency base station tone remote control line. All this line needs to do is to pass the audio frequencies from 300Hz to 3,000Hz without severe attenuation.

Photo 1 on page 22 shows different test instruments that can be used for troubleshooting remote-control line problems. The small box (Helper Instruments Lineman) shown on top is the one we normally use for basic line testing. It is simple, small and easy to use. The larger box shown at the bottom is the Model 806A Transmission Impairment Measurement Set from Convex (Simulcast Solutions). On the World Wide Web see www.simulcastsolutions.com and visit www.convexcorp.com/PRO_LIST.html. For a good tutorial on using the Model 806 for simulcast work, go to the bottom of the Web page and click on “simul 01.pdf.”

One recent remote control line problem we experienced took the telco five days to clear. Dealing with the telco on such line problems has been one of the greatest sources of frustrations I have experienced on this job. The telco never seems to “get it.”

As it turned out, my associate was first dispatched to the remote site to check out the problem. He found dc on the line from the telco battery. It was not supposed to be there. He tried to explain to the telco technician that our equipment doesn’t need battery voltage to operate. An entire day was wasted while the telco goofed around with the problem.

To make a long story short, two more days were wasted while I sat on the mountaintop waiting on the telco. This has been a typical experience. The telco likes to do line testing from some remote spot with automated test equipment and then tell you there is no problem with the line. (Never mind the fact that you can’t get a tone from one end to the other.) If you try getting two of the telco’s techs together to do end-to-end line tests, good luck.

In most cases, I like to do a complete line-response test after a line has been repaired to establish a new benchmark for future reference. Generally, the test involves setting the tone level to 0dBm at the sending end and measuring the level at the other end. The beginning reference frequency is 1,004Hz. The control line is isolated from everything except the test equipment on each end. The measuring instrument on the receiving end is placed in the “terminate” position. This places the proper 600Ω termination impedance on the control line. The level of the 1,004Hz tone is measured and recorded. If it measures -10dBm, then the line loss at 1,004Hz is 10dB. Similarly, the line loss is measured at other frequencies (404Hz, 2,804Hz and all the tone control frequencies), especially the transmitter keying frequency of 2,175Hz. Strange notches in the frequency response can occur, and if one of these notches falls on a control frequency, you might lose one of the control functions. The test is performed in both directions, reversing the sending and receiving ends, because the response can be different with non-metallic lines.

The Lineman from Helper Instruments (now owned by Zetron) provides many of the control tone frequencies at the flip of a switch, including the keying tone (guard tone) of 2,175Hz. This switch makes it simple to measure the line response at key frequencies. An intercom feature allows communication between techs at each end to coordinate the tests.

The Model 806 TIMS from Simulcast Solutions is an excellent instrument. However, for most types of line testing work with which we are involved, it is overkill. The types of tests for which we used the Model 806 were much simpler than the maximum capabilities of the instrument. In photo 2 on page 22, the Model 806 is placed in the bridge mode and connected across a remote-control line. It is set to measure noise through a C-message filter. A graph of the C-message filter response of the Model 806 is shown in Figure 1 on page 26. The passband of the C-message filter is considered the “meat” of the voice frequencies for communications purposes. Photo 2 shows that the residual noise level measured on the control line with a Model 1806 TIMS is 21.9dBrn. Remember, 0dBrn equals -90dBm. Therefore, 21.9dBrn equals 21.9 – 90= -68.1dBm. In Photo 3 on page 26 the measurement unit is switched from dBrn to dBm and reads -68.2dBm. You might encounter the term dBrnc. The c indicates that the noise is measured through a C-message filter.

Photo 4 at the left shows the Model 806 TIMS measuring the audio tone from an audio-function generator. As shown at the left in the photo, the input is set to terminate the line in a 600Ω impedance. First, the output level from the function generator is set to 0dBm and then the frequency is set to 1,002Hz. Another nice feature of the Model 806 is it indicates the frequency of the tone being applied to the instrument. The C-message filter is switched in and the Model 806 is set to measure S/N ratio.

As indicated on the display, the S/N ratio of the tone applied to the instrument is greater than 50dB. The instrument is designed to measure S/N at a tone frequency of 1,004Hz. This is the normal reference frequency used in line work. If the tone frequency is moved up or down from 1,004Hz, the notch filter in the instrument will not completely remove the tone, and it will be measured as noise or distortion. Photo 5 above shows that a tone at 940Hz will have a S/N ratio of about 10dB.

Distortion in percent can be converted to S/N by the formula:

where D is distortion in percent.

The percentage of distortion can be calculated from S/N by the formula:

Photo 6 at the right shows the measurement of noise with tone. The noise level is shown to be -39.4dBm. If the tone level is at 0dBm, then the S/N will be 39.4dB. Photo 7 at the right shows the same measurement but in units of dBrn. Note that -39.4dBm is the equivalent of 50.6dBrn (90 – 39.4 = 50.6).

The Convex Model 806 TIMS will do so much more than was needed in our situation. The unit also features an automatic sweep function to do a swept frequency response of the line. The instrument can also test digital lines. More features are available if the unit is linked to a computer serial port. On the down side, no intercom feature is provided to help the tech communicate from opposite ends of the line. The red LEDs are also difficult to read in bright light.

Overall, the Convex Model 806 TIMS is an impressive instrument, capable of doing much more than we attempted. (We barely scratched the surface.) The basic unit sells for about $2,500. Options are available at extra cost.

Until next time…stay tuned!

Contributing editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. His email address is [email protected].

For more information on the Convex Model 806 TIMS check out the Web sites listed earlier or contact: