Benchtest shootout round 1: Icom F-3GS vs. Astro Saber III

[Elroy Jetson]

Due to some recent fortune, I have been able to acquire some particularly fine test equipment for my test bench,
which is also merged with the test equipment I have at work in my day job...as a radio technician. Big surprise there, huh?

My latest acquisition, and the one that would be most valuable in checking out a radio's receiver, is a Rohde & Schwarz SMY 02 signal
generator, which generates a very pure output with essentially no detectable unwanted harmonics, from 9 KHz to 2.08 GHz,
and able to generate signals as low as -140 dBM and as high as +19 dBM. Its output is VERY uniform and it is in calibration, and
has been cross-checked against my other recently NIST-traceable calibrated equipment, so I know it's dead on. What it says it
delivers is what it's delivering.

I had to program and check out a pile of new Icom F-3GS VHF portable radios today, and by chance one channel that was being
programmed in is also one I have programmed into my VHF Astro Saber III.

Part of my program and checkout procedure always includes a receiver sensitivity test.

I got curious and decided to do a comparison of the two radio types.


To keep it short, the results are pretty interesting.

The Icom radios have analog chassis under digital control. All Astros are fully digital radio chassis.

With the Icoms, I was always able to hear the 1 KHz modulated test tone faintly in the squech noise of
the audio output with the generator at -130 dBM. ONE of the radios was able to manage
this trick at -135 dBM. That's pretty impressive raw sensitivity.

A few points of reference:
2.24 microvolts is -100 dBM.
0.709 microvolts is -110 dBM.
0.224 microvolts is -120 dBM.
0.1 microvolt is -127 dBM.
0.0709 microvolts is -130 dBM.
0.0399 microvolts is -135 dBM.

I found that the Astro Saber got comparable detection of the audio right at -120 dBM, and the changeover was very
abrupt, due to the digital nature of the receiver. But the amount of detected audio was higher at the threshold
than the detected audio at the SAME signal strength level with the Icom radios.

Next, I did the full quieting test. It took -100 dBM for the Icom radios to hit full quieting, but
it took only -106 dBM for the Astro Saber to hit full quieting. Just a quarter the power to hit full quieting.

The Icoms may be more sensitive at the ragged edge of signal detection, but the Astro Saber gets to full quieting
on a smaller signal, and at the edge of detection, the signal to noise ratio is already better on the Astro Saber.

I didn't make a SINAD measurement as I don't have the appropriate adapter cable to interface to test equipment
for the Icom radios.


Next chapter: Adjacent channel selectivity tests on these same radios.


Elroy

[xmo]

"...Next chapter: Adjacent channel selectivity tests on these same radios."
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Wow. Are you sure you're ready to go in the deep water?

Receiver sensitivity. That's a basic measurement covered in the manufacturer's service documentation and commonly performed by just about every two-way tech.

Adjacent channel? A whole 'nother matter.

Lets say you open up your XTS3000 service manual and it gives you an adjacent channel spec of 78 dB. Where does it say how to verify that or the procedure that was used to arrive a that number? The closest thing you might find is a reference to tests being performed in accordance with EIA 316B. Look that up on the internet and you find that it has been superceded by EIA-316-C which has been superceded by TIA/EIA 603.

No matter what the number - these procedure documents - the standards that the manufacturers all use to rate their radios - aren't to be found because they SELL them and apparently actively police their IP rights - Google doesn't seem to find a free copy anywhere.

I just might have tried to stick my toes into the same deep end of this pool [wanting to understand the adjacent channel specs] - so I have done some research. This subject probably rates a knowledge base article when that is up again. Anyway, I have compiled a composite procedure from available documents which I believe [but make no assurance] represents close to the EIA/TIA procedure:


Selectivity and Desensitization:

The adjacent channel selectivity, alternate channel selectivity and desensitization tests measure the ability of a receiver to process an on-channel desired signal in the presence of an undesired signal on a specified nearby frequency, without exceeding a specified degradation of output.



Method of Measurement

Terminate the audio output of the receiver in a load specified by the manufacturer. The
audio level control should be adjusted so that the receiver delivers the maximum audio power output without exceeding 10% distortion, or the manufacturer's specified maximum power level, whichever is less.

(a) Connect two RF signal generators equally coupled to the receiver antenna input terminals through a suitable matching network. Set the first RF signal generator to the assigned channel frequency and modulate it with a 1004 Hz tone to 60% peak frequency deviation [±3 kHz for wide band ±5 kHz deviation systems].

Switch the second generator off. Adjust the first RF signal generator level to produce a 12 dB SINAD measurement at the audio-output terminals of the receive path. Record the RF signal level and increase this first RF signal generator output by 3 dB.

(b) Set the frequency of the second RF signal generator to the appropriate spacing above the frequency of the first RF signal generator and modulate it with a 400 Hz tone to ±3 kHz peak frequency deviation.

The appropriate spacing depends on the test being conducted, i.e. adjacent channel selectivity, alternate channel selectivity, or desensitization.

Adjust the level of the second RF signal generator to reduce the SINAD measurement back to 12 dB from the first RF signal generator. Record the RF signal level.

(c) Repeat step (b) with the frequency of the second RF signal generator set to the same spacing below the frequency of the first RF signal generator.

(d) Calculate the ratios, in decibels (dB), of the undesired signal levels measured in steps (b) and (c) to the reference level obtained in step (a). For each case of adjacent and alternate channel undesired input signals, the smaller of these ratios for the above and below channel undesired signals is the minimum selectivity.

OK.

So - now we have a procedure. Go ahead and experiment. See what you learn. That's what a good technical person does. I think you will have some difficulty veriying the manufacturer's specs, but elaborating on the 'why' of that will take some time that I don't have right now - I'll add more in a day or two.

[Elroy Jetson]

Yes, I have noticed that the actual "standard tests" for determining specific performance attributes seem to be less than exactly
well-documented or readily available.

The procedure you described is pretty much what I suspected it would be, or close enough.


My basic adjacent channel test is simply to go off-channel by a defined offset from that channel (Will that be 6.25, 12.5, 25, or 30 KHz?) and
then input a signal and treat it as a receiver sensitivity test.

Then I'll do your desense test. And more.


I'll start looking at transmitter performance after working my way through all the receiver tests I'm able to perform.


I'm doing this because it does seem to me that every manufacturer does it a bit differently so the playing field is not level.


There's a popular (mis)conception that many amateur radios are more sensitive than commercial or public safety radios,
and while that MAY be true based on ONE measurement, it takes more than one measurement to really understand what's
going on and what's REALLY more sensitive to the RIGHT signal (and not the WRONG one) in the real world.

As I noted above, the Icom F3GS has a slight edge over the Astro Saber at the lowest detectable signal level, but at
or near full quieting, the Astro Saber is more sensitive AND, I'm sure, it will prove to be considerably more selective as well.

I hope to do tests like these on MANY radios, using a standardized set of procedures and measurements. New radios,
old radios, any radios.


Elroy

[Hartley]

Hi Guys,

A couple comments..

Actually, the tests (and standards) are available, and have been for some time - but they aren't free- the TIA charges big bux for a copy of the -603 standard, and they imprint every page with the purchaser's ID, so if you foto-copy it, they know who you are..

Regarding the test for Receiver Adjacent Channel Selectivity, the test that xmo gives is from the original version of EIA/TIA-603, now referred to as "EIA-603-92". Starting at revision A, they changed that test significantly (they're now up to revision C, btw) by modulating the interfering signal with two tones - one at 400 Hz, like before, and one at 2200 Hz - both at 1/2 system deviation - i.e., 1.25 kHz dev for a 2.5 kHz "narrowband" channel. and the interfering signal is therefore modulated at 100% of system deviation. So the resultant numbers are much lower than before - if a radio measured 70 dB before, it will be around 45 dB with the new test.
Most spec sheets you will look at, however, use the older test - because it LOOKS better, and the marketing people know that very few customers understand the difference, so they want a "better" number on display.

73 DE Hartley