## Comparing VHF Communication Receiver Sensitivity

jimh
Posts: 5994
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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### Comparing VHF Communication Receiver Sensitivity

Let's compare the receiver sensitivity of three top-line VHF Marine Band Radios. I won't be actually testing and measuring the sensitivity, I will rely on the manufacturer's specifications as accurate. I have chosen:

--ICOM M506
--Standard Horizon GX5500
--SAILOR 6216

We will start with the ICOM and the Standard-Horizon. Here are the manufacturer's specified receiver sensitivity data:

ICOM M-506
Sensitivity (at 12dB SINAD) 0.22µV typical

Standard-Horizon GX5500
Sensitivity
20 dB Quieting = 0.35 μV
12 dB SINAD = 0.30 μV
Squelch Sensitivity (Threshold) = 0.13 μV

We compare at the common specification, input signal to give 12-dB signal to noise and distortion ratio. This is easy because both use the same unit of measurement, microvolts at the 50-Ohm antenna input. The results is that ICOM rates their receiver to be 0.08-microVolts more sensitive, but qualifies that as "typically." The difference in sensitivity looks small. Now let's look at the difference in signal power. First we convert the two signals to be in dBm.

ICOM = 0.22 microVolt
ICOM = 20 LOG Eµ - 107
ICOM = 20 LOG (0.22) - 107
ICOM = -120.1515 dBm

SH = 0.3 microvolt
SH = ICOM = 20 LOG Eµ - 107
SH = ICOM = 20 LOG (0.3) - 107
SH = -117.4576 dBm

The difference in signal power for same sensitivity is (-117.4576 dBm -[-120.1515 dBm]) or 2.69 dBm in favor of the ICOM. That is more impressive that I thought it would be with only a 0.08-microVolt difference in sensitivity.

In order to appreciate a 0.08-microVolt or 2.69 dB difference in sensitivity, the desired signal would have to be just at the threshold of being detected. That means the squelch circuit would have to be backed off to no-squelch and the signal was just barely above the noise floor. I don't know where you can get signals like that to perform tests, unless you have a signal generator. You don't get signals like that by randoming tuning around the marine band.

The Standard-Horizon specifications are more informative. They show that with an increase in signal to 0.35-microVolt the recovered modulation will have a very nice 20 dB SINAD--that means the recovered signal is 100-time stronger than the noise.

ICOM does not give us any data on how fast its receiver quiets down with increasing signal strength.

Now let's throw the SAILOR radio into the comparison. For their 6216 radio the sensitivity is rated as:

First, we see the SAILOR is rated at a 20-dB SINAD. Whoa--we must keep that in mind, as we are getting a much better signal-to-noise output than with the other two sensitivity ratings at only 12-dB SINAD. And we have to convert dBm to microvolts at 50-Ohms; that's not too hard. The relationship is

microVolts = 10^(dBm +107)/20
microVolts = 10^(-119 +107)/20
microvolts = 10^-0.6
microvolts = 0.25

Heya--that is a rated sensitivity of 0.25-microVolt for a 20-dB SINAD, so that is actually better than both the ICOM and the Standard-Horizon. However, the SAILOR radio probably costs more than either the ICOM or the Standard-Horizon.

ASIDE: for the math behind the conversion of dBm to microvolts, see my derivation at

http://continuouswave.com/whaler/reference/dBm.html

ASIDE: you can also compare the sensitivity in dB using the two voltages. The power ratio in dB for two voltages is

dB = 10 log (V1/V2)^2

For 0.3 and 0.22 microvolts, that works out to

dB = 10 log (0.3/0.22)^2
dB = 20 log (1.3636)
dB = 2.69

jimh
Posts: 5994
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Perhaps a better comparison of the three receivers will be to compare their sensitivity in signal level in dBm for a common recovered modulation signal-to-noise ratio (SINAD). Because the only figure the sailing is giving us SAILOR is for 20-dB SINAD, we will use that level for comparison. The ICOM and Standard-Horizon radios give us sensitivity in voltage levels, so we have to convert the data into dBm for comparison. The Standard-Horizon does give us a rating for 20-dB SINAD, but the ICOM does not. We will have to make an estimate for the ICOM sensitivity at 20-dB SINAD, and we can use the Standard-Horizon as a guide, because it is the only radio of the three that gives both figures. So we start with the Standard-Horizon.

Here is the manufacturer's specifications for the GX5500 radio:

Now we convert those to dBM. (I'll skip the details)
GX5500

We see that an increase in signal strength of 1.34-dB gave an improvement to 20-dB SINAD from 12-dB SINAD.

Now we convert the ICOM sensitivity at 12-dB SINAD:
ICOM M506

Since we don't have a figure for the ICOM radio at 20-dB SINAD, we will add 1.34-dB of signal--the figure we got from the other radio--as a reasonable estimate of how the ICOM will quiet down with more signal.
ICOM M506

Now we have all three radios with their sensitivity expressed in the same units at the same SINAD; here is the comparison:

Signal in dBm to produce 20-dB SINAD recovered modulation
Standard-Horizon = -116.12 dBm (specified)
ICOM = -118.81 dBm (estimate)
SAILOR = -119 dBm (specified)

Don't be confused by the negative numbers, the MOST sensitive radio is the SAILOR, the ICOM is just about identical, and the Standard-Horizon is 2.7 to 2.8-dB less sensitive.

In practical applications, in order for the greater sensitivity of any receiver to be used, the local radio frequency NOISE FLOOR must be below the desired signal level. When you start to get down to signal levels of -120 dBm, the signals are very weak, and in order to be able to receive these signals and demodulate them by FM methods to recover the intelligence modulated on them, the local radio-frequency noise floor at the receiver site must be below -120 dBm, and below it by quite a bit. If the local radio-frequency noise floor is not well below -120 dBM, then the difference in receiver sensitivity that we see in the above three radios makes no difference at all. All three receivers will have difficulty receiving any signal at their rated sensitivity if that signal is buried in noise.

In installation of radio on small boats with outboard engines using spark-ignition, with active SONARs, and with other boat electronics in operation, the receive antenna for the radio is going to be relatively close to all those noise sources. The result may be that the radio-frequency noise floor in the VHF region on a small recreational boat is not going to be ultra-quiet. It is common to hear interference from spark ignition on even fairly strong VHF signals being received, or to hear effects of SONAR emissions on VHF signals. The effect of a fairly noisy environment on a small boat is to reduce the influence of the ultimate sensitivity of any receiver, and to limit all receivers to signals that are above the noise floor.