[The Westmorland Flyer]

I've just had a very enlightening conversation with the engineer at Icom UK. Whilst on the phone he tuned the signal generator across a 25kHz channel and it was clear that the bandwidth at -6dB is ±8kHz. As I suspected, the Icom documentation, which states ±3kHz is wrong. We suspect a simple typo, as 3 looks rather similar to 8, perhaps more so for Japanese engineers.

This is as I expected and is great news - the radio will work just fine with offset carrier stations. Case closed!

[tnowak]

When set to 25KHz mode it works as expected (Farnborough LARS etc.)

Tony

[Flying_john]

If the Icom man on the phone says that, then the radio he was testing is either modified or faulty and/or doesn't meet the requirements spec for an Aeronautical Radio.
Have a quick look at this :-

http://antena.fe.uni-lj.si/literatura/Razno/Avionika/rtca/RTCA%20DO-186A%20VHF%20AM%20ATC%20Wave%20Conformance%20Test%20Procedures.pdf

Scroll to page 37 the selectivity tests and at step 8 and step 11 you see the instructions to the test engineer to tune +3khz and then -3khz from centre frequency (for a 25khz channel spacing) and check the performance level is within 6db of the reference level set at centre frequency.

[Paul_Sengupta]

If the -6dB points are at + or - 8kHz then surely testing at + or - 3kHz will definitely be well within 6dB of the reference?

[The Westmorland Flyer]

Paul is correct, of course, the ±3kHz performance will also be within -6dB.

I'm not sure why Flying_john keeps wilfully ignoring the requirement for a ±8kHz bandwidth at 25kHz to accommodate offset carrier. As a qualified air traffic engineer I maintain a considerable number of ground-based 25kHz air band radios and they all, without exception, have Rx bandwidths in the region of ±8kHz. Indeed we are required to check this figure is being achieved every six months.

Quoting a paper by the Defense Information Systems Agency, in Arizona, USA has no bearing on what happens in Europe. America has neither offset carrier operation nor 8.33kHz. I note also that the referenced document merely says to check the ±3kHz figure is no worse than -6dB - it makes no reference to the actual bandwidth, which could be and indeed will be much greater. In short, the document is not an equipment specification, it is a test specification.

[Flying_john]

Ah yes - I see my error now. The Icom 200 210 were designed for the US market where there was no requirement for offset carrier reception and thats why (I guess ) their test schedule talks about varying the frequency +-3khz whereas I see in the European/UK test spec it says;
A test signal (see clause 6.1.3) at 127,5 MHz at a RF level necessary to achieve a 12 dB SINAD ............................
........................ signal frequency shall be varied ±8,5 kHz for 25 kHz receivers and ±2,8 kHz for 8,33 kHz receivers from the nominal carrier frequency of the receiver................

....................... For receivers intended for use with 3, 4 and 5 offset channels a 6 dB point of ±11 kHz is recommended.

So might the original spec in the OP be refering to the USA equipment perhaps, that just got carried over to the European radios ?

Edited to add: Spec in Original post says:-
Measurements made in accordance with RTCA DO-186B for [u]USA version[/u].

[The Westmorland Flyer]

In the beginning, when 8.33kHz was a new thing, the architects of this brave new world decreed that everything that was on 25kHz would move to 8.33kHz. There would be no 25kHz channels. That would have been simple enough, if still somewhat irksome in an environment that still refers to stations by their frequency rather than by channel name/number. Icom, amongst others made the assumption that this remained the case.

In practice there were many services that the 8.33kHz architects failed to take into account which couldn't operate on 8.33kHz channel spacing, offset carrier stations being one such. It took a while but eventually we ended up with the dog's breakfast that is our current hybrid 8.33/25kHz architecture.

The result is a vastly more complex (and therefore costly) radio, with dual IF filters and a load of logic/software to adjust the radio between 8.33kHz and 25kHz mode. For those of us with long memories, the situation is akin to those awful dual standard 405/625 line television receivers of the 1960s.

In Icom's case I would imagine that the new radio specifications were driven from their largest market, which is the USA, where most of these 25kHz requirements would have been either unknown or misunderstood.