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ContinuousWave: Small Boat Electrical
DSC Receiver Sensitivity
|Author||Topic: DSC Receiver Sensitivity|
posted 05-16-2009 10:54 PM ET (US)
The typical VHF Marine Band radio generally includes a specification for receiver sensitivity for voice FM reception. The receiver sensitivity is generally in the range of 1-µVolt down to 0.25-µVolt for 12-dB of signal to noise and distortion in the recovered audio. This is a good estimate of the minimum signal that can be copied by a human listening carefully to the radio output.
For reception of digital data via the digital selective calling (DSC) receiver, a receiver sensitivity specification should be given, too. I have found a few products which include a specification for the sensitivity of their DSC receiver, and I list a few below, including on-line sources for the information:
SAMYUNG ENC Model STR-580D
This is a commercial ship radio.
JRC Model JHS-32B Radiotelephone
Also a commercial ship radio.
Furuno FM 8500
Also a commercial ship radio.
Furuno FM 8800
I could not find any specification for the sensitivity of the DSC receiver in any ICOM or STANDARD HORIZON VHF Marine Band radios, although I only checked a couple of the top-of-the-line radios. If anyone happens to see a specification for the sensitivity of the DSC receiver in an ICOM or STANDARD HORIZON VHF Marine Band radio, please point it out for us.
We now look at DSC receiver sensitivity and compare it to voice receiver sensitivity. The DSC sensitivity for less than 1-percent error is almost the same as the voice radio sensitivity for minimum 12-dB signal to noise and distortion (SINAD). The difference is typically less than 3-dB between the rated sensitivities. In another article I have already calculated the expected range of communication between two typical recreational vessels using standard antenna and power, while figuring the path loss using a conservative non-line-of-sight estimate. These calculations used a receive level that was very significantly (18 dB) above threshold sensitivity.
(See http://continuouswave.com/whaler/reference/pathLoss.html for details, in the subheading "Typical Boat-to-Boat Communication.")
Based on this estimate, we see that the DSC receiver should also be very significantly above its threshold sensitivity (by 15-dB or more). This implies a very low error rate for data communications at this same range.
It seems completely reasonable to expect that the range of digital communication between two recreational boats with DSC radios will be approximately the same (or perhaps better) than the range of their voice communications. Since intelligibility of voice communications depends on the ability of the human listener to detect the modulation, the voice sensitivity range is somewhat more variable.
In simple terms, if you can hear another vessel's voice transmission, you can probably exchange DSC data with them.
posted 05-19-2009 09:02 AM ET (US)
Specifications for sensitivity for a radio generally give the radio signal input level at the receiver input which will produce the demodulated signal with a certain fidelity or accuracy at the receiver output. The nature of the specified accuracy or fidelity varies with the type of modulated signal.
In the case of voice modulated transmissions, the sensitivity is generally specified in terms of the signal to noise ratio of the output (loudspeaker) for a given radio input signal. Since it is difficult to measure the noise without including any distortion in the actual signal, the measurement is usually specified as signal to noise and distortion. This ratio is expressed in decibels and called the SINAD ratio. The measurement is typically made with a single audio frequency tone as the modulation which modulates the carrier 100-percent. A tone of 400-Hz is often used. When a voice receiver is specified as having a sensitivity of, say, 1.0-µVolt, for 12-dB SINAD, it means that the receiver can recover the modulation from an input signal of that level and produce an output signal with the specified signal to noise and distortion.
In digital radio systems, the same general technique of specifying a particular input signal level which produces a demodulated output of a specific fidelity is also used. The radio signal is specified in a similar manner by giving its level, either in micro-volts or in decibels relative to one micro-volt (or dBµV). The modulation in a digital radio is digital data, so there is no particular usefulness is specifying a signal to noise ratio for the data, nor is the output a loudspeaker. Instead the fidelity of the demodulated signal is specified by its accuracy to the original. The rate at which an error in the digital data may occur is usually given as a numeric probability.
posted 05-19-2009 10:33 AM ET (US)
Both IEC 62238 and EN 302 025 specify the "maximum useable sensitivity" of the DSC decoder to be 0 dBuV, or +6 dBuV under "extreme test conditions".
However, you can only be really certain that radios tested to either of these standards by an independent lab will meet this specification.
posted 05-19-2009 08:36 PM ET (US)
Hello old sparks and welcome aboard.
Independent verification of all product specifications would be great. There's not anything particularly different about the digital receiver specifications, although I think most testers would have trouble measuring bit error rate with accuracy. I know I could easily measure the F3 modulation (voice using frequency modulation) sensitivity with test gear available to me. I could probably come up with something to measure bit error rate, but the real problem is how to apply the digital modulation to the transmitter. It's not like there is an input jack marked "digital" on the back of a typical VHF Marine Band radio for me to feed a test signal into.
posted 05-20-2009 09:17 AM ET (US)
As it turns out, I do have a copy of the document mentioned above, IEC 62238 Ed.2 CDV
Here is an excerpt regarding the test procedures for the DSC portion of the radio:
6.7 Generation and examination of the digital selective call signal
I don't image too many service technicians have a "DSC generator" on their test bench.
As for the standard test signals, the specified levels for the DSC receiver are:
posted 05-22-2009 09:19 AM ET (US)
For a DSC Test Generator you will need something like this:
It is a product of Sine Qua Non Technologies Holding, Pty., and seems targeted for DSC testing.
--Encoder Function as stated in IEC61993-2 paragraph 10.6 “Encoder for receiver measurements”.
posted 05-25-2009 09:37 AM ET (US)
Looking for more DSC receivers with a rating for sensitivity, I noticed that the ICOM AIS receiver model MXA-5000 is rated for sensitivity. The specifications rate the receiver sensitivity as -116 dBm typical. To convert this to a more familiar rating of microvolts, we use the relationship explained in my REFERENCE article
Conversion of Receiver Sensitivity
For a sensitivity of -116 dBm the equivalent signal in a 50-ohm system would be
Eµ = 10(dBm + 107)/20
Eµ = 10(-116 + 107)/20
Eµ = 10(-9/20)
Eµ = 10(-0.45)
Eµ = 0.3548
Although ICOM does not define the data error rate, we can assume they mean the familiar one-percent error rate (or 10-2). This sensitivity is about the same as their typical voice receiver sensitivitiy for 12-dB SINAD.
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