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ContinuousWave: Small Boat Electrical
Understanding Radio Sensitivity Specifications
|Author||Topic: Understanding Radio Sensitivity Specifications|
posted 04-12-2008 04:23 PM ET (US)
Receiver sensitivity is usually specified in a particular level of signal which is needed to produce a particular level of audio output. The signal level is measured in either microvolts or in dBm. The audio output is not specified in a particular level because the audio level can easily be adjusted by the VOLUME control. Instead, the audio output is specified in terms of the ratio between the desired signal and the undesired signal. The undesired signal is generally considered to be NOISE. Because it is very difficult to measure the NOISE without also measuring DISTORTION, the signal ratio is generally measured as the ratio between the desired signal and the NOISE and DISTORTION. This ratio is known as the SIGNAL to NOISE and DISTORTION ratio, and the acronym SINAD is used as an abbreviation. The ratio is usually expressed in terms of decibels.
For example, my radio is specified to have a sensitivity of 0.25 microvolts for a 12-dB SINAD. This means:
--with no signal present the noise and distortion are measured at a convenient audio level
--a signal is applied which causes the audio output to increase 12-dB
--the level of the signal needed to produce this is noted and is found to be 0.25 microVolts.
I got a good chuckle recently when I read that all VHF Marine Band radios have the same SINAD, that is, 12-dB. Well, that is true because the sensitivity of most radios is compared at a SINAD of 12-dB. To make an easily understood analogy, this is akin to comparing the MPG of various boats by running them at a common speed, say 30-MPH, and then saying the speed of all boats is 30-MPH.
A SINAD of 12-dB means the ratio of the audio power output between the two states is 12-dB. What ratio is 12-dB in more familiar numbers? The definition of dB for power ratios is
dB = 10 LOG (P1/P2)
Solving this for dB = 12 we find
12 = 10 LOG (P1/P2)
12/10 = LOG (P1/P2)
1.2 = LOG (P1/P2)
10^1.2 = (P1/P2) from the definition of LOG
15.85 = (P1/P2)
In this case
P1 is the power contained in the SIGNAL + NOISE + DISTORTION, and
P2 is the power in the NOISE + DISTORTION
Thus a SINAD of 12-dB says that when the radio receives a signal at a level of 0.25-microvolts (which is quite weak) it will produce an audio output in which the desired signal is about 15 times greater in power than the noise and distortion of the radio itself. This level is considered a good level for communication quality reception. In contrast, when most people listen to FM Broadcast Radio signals in their car, they are listening to signals which have a SINAD of about 60-dB, or very much greater. (A ratio of 60-dB is a ratio of 1,000,000 to 1, or a ratio that is 63,000 time greater than our communication grade audio.)
This is not to say that a signal of 0.25 microvolts will always produce this same signal to noise ratio. This measurement is under laboratory conditions where there are no other signals present. There is only the desired signal and the noise and distortion produced by the radio itself. In the real world a signal at this level may not produce a SINAD of 12-dB due to other intereferences. They may be other signals in-band. These may be signals from other transmitters or from local noise sources (like the outboard motor). There may also be other signals out of band which produce harmful effects (like a 500-watt paging transmitter on the shore).
Disclaimer: I don't know much about fishing, but I am very familiar with radio communications.
posted 04-12-2008 08:54 PM ET (US)
A receiver sensitivity is also given in dBm, or decibels relative to one milliiwatt. If the sensitivity is known in microvolts and the antenna impedance is 50-ohms, then a conversion to dBm is
0.25 microvolts = -119 dBm
as explained in
posted 04-13-2008 10:06 AM ET (US)
Most VHF Marine Radio band receivers give their sensitivity specification as being measured by a common standard, TIA/EIA-603. The publisher of this standard notes:
Like many standards, you cannot find the details of this specification on-line, but you can buy it for about $260. My curiosity is not that great, so I will have to take a guess at what the standard specifies.
Most likely, when the receiver sensitivity is specified, the signal being received is probably a 100-percent modulated signal containing a fixed tone, probably around 1000-Hz. This is the signal that provides the 12-dB SINAD in the receiver output.
A SINAD of 12-dB should provide a comfortable margin for copying voice communications. A skilled listener can probably copy voice signals which have a signal to noise ratio of much less than 12-dB. Very skilled listeners can copy voice signals which are at or below the noise level.
posted 04-13-2008 10:24 AM ET (US)
The regulations governing ship radios are given in
PART 80--STATIONS IN THE MARITIME SERVICES
Part 80.201 gives the technical standards. Most defined standards relate to the transmission of signals, not the reception of them.
posted 04-14-2008 12:04 AM ET (US)
In addition to a figure for sensitivity, most radio specifications include a figure of merit for SELECTIVITY. Again, the precise details of this measurement are not given, just the mention of the test specification. The figure of merit mentions INTERMODULATION and REJECTION. A typical number is 75 to 80 dB.
Here the meaning of these numbers is the ratio between a desired signal and an undesired signal in decibels. A receiver that is rated at 75dB, like mine happens to be, is supposed to be able to maintain clear reception of the desired signal in the presence of very much stronger but off-channel signals.
My receiver is rated at -119 dBm sensitivity, so with a 75 dB selectivity figure, the radio ought to be able to maintain reception in the presence of a signal that is -119 + 75 = -44 dBm.
How strong is a signal of -44 dBm? Let's convert that to microvolts. We use the relationship mentioned earlier:
dBm = 20 LOG Eu - 107
and solve for Eu:
Eu = 10^(dBm + 107)/20
Eu = 1,412 microvolt
or a lot stronger than the 0.25 microvolt signal we are trying to receive
Let's also look at what it takes to produce a signal of -44 dBm at our receiver. Using the example presented in detail in my article on VHF Communication Range, we see that a 25-watt transmitter at 10 miles will produce a signal of -52 dBm. Every time we halve the distance we get 6dB more signal. So if we move to 5-miles, we are up to -46 dBm. Cut the distance in half again and we are up to -40 dBm. Hey, now we've produced a signal that is strong enough to begin to affect reception of the desired signal, and we are still 2.5-miles away!
The significant factor is that most of the time we are not trying to copy signals at the absolute threshold of sensitivity of the receiver. As the desired signal gets stronger, the receiver can tolerate stronger undesired signals. And the undesired signals have to be at certain frequencies (which will tend to combine with other signals in an undesired way to produce the intermodulation).
Look at it this way, if the guy you're trying to communicate with is only a mile or two away, his signal is going to be in the -50 dBm range, so now you have 75 dB more headroom. That means your receiver will have enough selectivity to reject a signal of +25 dBm--which is very, very strong. Actually, your radio probably will give up due to other consideration, as you will likely reach its blocking range (not specified put typically around -10 dBm for a common radio receiver).
On some of the better receivers the SELECTIVITY figure of merit increases to 80dB, about 5dB better. This means our interference can get 5dB stronger before it affects us. Using the same example we've been running, a 25-watt transmitter could be half-again as close, about 1.25 miles, before it would start to interfere at our minimum signal threshold.
Again, this is just an informed guess at the actual test conditions, as I don't have the full TIA-603 specification available to see the actual test parameters. But this analysis should give you some idea of what the numbers cited in the specifications mean.
This signal analysis also shows that you should follow the FCC regulations and use LOW POWER to communicate with a vessel that is in sight. If you can see the other boat, you don't need 25-watts to speak to them. Two boats a few hundred yards apart and transmitting at 25-watts are hitting each other's receivers will extremely strong signals.
I have only had one occasion when I noticed my 75-dB-rated receiver showing any artifact of interference. That happened when a boat that was literally right alongside was transmitting at 25-watts on an adjacent channel. His transmission broke through on the channel I was listening on.
Do you need the 80 dB rated receiver? If you do most of your boating in a crowded commercial harbor where there are a lot other boats transmitting, the better receiver may be worth the investment (which often is only a $50 increment in the price). I may buy myself one this summer--a fellow can never have too many radios.
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