As I noted on my earlier blog Quality Choice: Motorola Radios vs. Budget GRMS & FRS, two-way radio isn’t primarily about how much power you transmit, it’s about your ability to receive the relatively weak signal in an ocean of EM (electromagnetic) noise. While consumer marketing likes to promote high transmit power as the key performance consideration, receiver performance is actually what makes the difference.
Think of a transmitted signal as a small boat adrift somewhere offshore. You scan the horizon and see nothing but sunlight reflecting off the waves. You certainly can’t see a small boat. But there are things you can do to help you spot it:
1) You can use binoculars or a telescope to increase the distance you can see;
2) You can use tinted/polarized lenses to reduce the glare from the reflected sunlight.
We have just increased our SENSITIVITY with the binoculars and increased our SELECTIVITY with the polarized lenses. A radio receiver works much the same way.
What does it mean to transmit a signal
Our objective is to receive a transmitted signal on a specific channel and the signal at our receive antenna will be millions of times weaker than it was at the transmit antenna by the time it gets to us. The receiver therefore must do all it can to amplify and discriminate the signal on our channel while rejecting any signals or noise on any other channel. The ability of a receiver to perform these critical functions are measured as Sensitivity and Selectivity respectively.
For the nerds among us, Motorola’s SL300 portable is guaranteed to perform to the following specs:
Analog Sensitivity: 0.3 uV (microvolts) for “12 dB SINAD”
Digital Sensitivity: 0.25 uV (microvolts) for “5% BER”
Motorola is reporting the signal level presented at the antenna connector on the radio. For analog signals, the established industry standard of signal readability is 12 dB SINAD which is a relative measurement of the desired signal against noise and distortion. For digital signals, Motorola is using a 5% Bit Error Rate as the standard for an acceptably readable signal.
Adjacent Channel Selectivity: 45dB at 12.5 kHz, 70dB at 25kHz measured per TIA Standard 603D
The “Adjacent Channel” is the very next channel up or down the line from our channel, just like the channels adjacent to 101.1 MHz on your FM dial are 100.9 MHz and 101.3 MHz. (FM channels are 200 kHz wide versus our 12.5 kHz channels.). This selectivity spec is telling us that the receiver will suppress (eg. filter) signals on the next channel by at least 45dB. (45dB means the adjacent channel signal is suppressed to a level about forty-thousand times smaller than it would be if it were on our channel).
Side Note: 45dB isn’t really that much suppression… a signal at that level could still be received on our channel as interference, messing with our ability to receive the signal we want to hear. But check out this transmitter spec for the SL300:
Adjacent Channel Power: 60dB at 12.5 kHz, 70dB at 25kHz
This is a measure of how “clean” the transmitter’s signal is: the transmitter suppresses any signal it generates by one-million times on the adjacent channel (for our 12.5 kHz channels). So we add this 60 db to the receiver’s 45 dB for a total Adjacent Channel suppression of 105dB. This is enough to keep any signal on the adjacent channel from interfering with our channel unless the transmitter and receiver are in the same room.
Marketing keywords does not mean tested quality
As I said earlier, you don’t need to understand the technicalities of these specifications. But you should note that very few manufacturers are willing to publish these specs in this kind of detail for comparison to competitive products. Instead they opt for subjective marketing terms like “rugged” and “high power”, etc.
GMRS/FRS radios will operate on their radio channels but they are poor at refining signals for optimum readability. For example, their lack of sophisticated noise cancellation features will ensure background noise is carried along with your voice rather than optimizing for clear voice communications. While suitable for recreational family use, they are unlikely to meet the demands of busy work environments where message clarity is paramount. Motorola’s range of two-way radios incorporate advanced noise suppression technology, automatically reducing background noise to ensure your messages are transmitted with crystal-clear clarity, even in noisy surroundings.
A little off topic but just as relevant: The portion of the spec sheet beneath the transmitter section is a dizzying array of physical specifications Motorola products are designed to. The certification tests conducted to obtain and verify these specifications include “Accelerated Life Tests“. Many Motorola radio products are tested over a simulated extended lifecycle to ensure our customers can expect these products to serve them for many years. For a manufacturer who wants to back up their claim, this is what “rugged” is supposed to look like.
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