Musings on Measurements & Mathematics in an Analog World
Security Sales & Integration’s Tech Talk columnist Bob Dolph takes a look at some of the terminology and calculations security systems integrators may run across when making measurements for analog systems.
In the May SSI Tech Talk column, we discussed the importance of analog systems in today’s ever-expanding digital world. However, today’s technicians may not understand and/or appreciate how we make measurements in analog systems. Let’s take a look at some of the terminology and calculations you may run across.
In the digital world many young techs are familiar and comfortable with the prefixes Giga (G) and Mega (M). After all, everyone knows a computer file that is 32.5Mb is actually 32,500,000 bytes. But how many techs are comfortable with terms like milli (m) and micro (u)? Better yet, how do you calculate values that are M, G, k, m, or u? Entry-level technicians may say, “Why bother with all of this? I’m not an engineer.”
That may be true, that you are not an engineer. But you are often responsible for knowing, and, even more so, understanding basic electrical circuit calculations such as E=IR or Ohm’s law. And calculating these very small or very large numbers containing many zeros can easily lead to mistakes. That is why methods such as scientific notation and logarithms were created.
Always make sure to request from antenna manufacturers if they are referring to dBd or dBi. They have been known to play games with these values. One popular dB value today is the amount of signal strength you are receiving from cell tower to your smartphone or alarm cell transceiver. Did you know that on many Android phones you can access such values as cell tower strength by simply entering the Android secret code *#*#4636#*#*? If you are seeing better than -10dBm you should be in pretty good shape (see photo). Make sure to check out popular smartphone apps such as OpenSignal for checking cell network signal strength.
In scientific notation a 2,800-ohm EOL (end of line) resistor in an alarm circuit can be represented as 2.8K, 2K8 or 2.8 x 103. Better yet smaller numbers such as a current of 45.6 micro-amps can be 45.6 x 10-6 . The simplicity is that no matter how large or small the number is you are only dealing with powers of 10. As you practice working with these numbers you will want to make a small investment in a scientific calculator for field calculations. An alternative is a scientific calculator on your mobile device or computer. On option is to use free calculator programs such as Calc98.
A popular measurement term in the analog world is the decibel (dB). It is used in calculating everything from sound pressure levels (SPL) to fiber optics and RF signal gain. It is a logarithmic unit rather than linear, as this is how our eyes and ears respond in the real analog world. When first working with values in decibels just remember it is an expression of a ratio between two values of a physical quantity, which is often power and intensity. One of the quantities is often a reference value; after all, it makes sense that you would want a referenced, known value.
The core information that you should take away from these measurements is that they provide a foundation for the simple calculations of power gained or lost within a system by simply adding or subtracting dB values. This could be as a fiber-optic network or a RF transmitter, receiver and their respective antennas.
Basic dB calculation rules are pretty simple; every time you double or halve the power level, you add or subtract 3dB to the power level. A 20dB loss corresponds to a hundred-fold decrease in signal level. Therefore you have small numbers to represent large gains or losses. The dB values come in different flavors such as dBm for milliWatts (mW) where 0dBm is 1mW; others for antennas are dBd for db dipole and dBi for isotropic.
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