Eric wrote:
>Ian:
>??????I totally agree with everything you say, especially the technical
>issues. As I have preached many times, the real problem(s) appear when
>we attempt to reduce complex technical phenomena to one-liners. We may
>have a genuine disagreement here, but it will be predominately
>semantic, and that we can easily fix.
>
Yes, I believe that too.
>??????The immediate problem is that I do not view two (or any multiple)
>tone testing as a means of testing power supplies. Or of testing
>ANYHTHING other than the raw IM of an amplifying device. We are
>obviously going to have to agree on what I mean by that last reference,
>and to clearly illustrate, let me give a clear and common example.
>
As you predicted, we seem to be talking about two different things.
If I have understood you correctly, you are looking for a way of testing
only the active device (tube, MOSFET etc). I agree that anything other
than a static two-tone test makes it almost impossible to isolate the
performance of the device from all the other influences - notably the
power supply.
I am proposing the speech test as a way of testing complete
transmitters, as put on the air - not only the active device, but the
entire combination of the device, the rest of the RF hardware, the power
supply and also the exciter.
Those two different purposes, device-level testing and system-level
testing, require different methods.
>??????I think I disagree with your prediction about non-correlation
>between results via multi-tone testing, and the other one ( I am not
>sure I like any of the titles we have applied to it so far.) Unless, of
>course the DUT is pathological in its non-amplifying basis (power
>supply sag, peak clipping, eg.) In the case of extreme power supply
>variation, which is probably more prevalent than we want to know, we
>are just confronting a well-known and dreaded monster: TIM (Transient
>IM). I have been in that one before, and don't ever want to go back.
>There are probably other second-order effects arising from non-constant
>supply voltages, and I don't deny that they exist.
Yes, TIM certainly is involved, but I don't think we need be afraid of
that. TIM only becomes really horrible when you try to study the
phenomenon itself. The proposed test has a much more limited objective -
it aims to measure the *effects* of TIM, in terms of spectrum occupancy.
With a long-enough sampling time on a repeating test signal, there can
be good statistical confidence that the analyser will sooner or later be
looking at the correct frequency when a TIM peak occurs. All we need is
to capture the highest peak IM level that is ever reached, at each
frequency.
In practice, this takes only a few minutes of sampling random speech.
The storage screen of my 141T shows the limits of the area that has been
'painted' at any time since the test began, and the roughly triangular
shape of the spectrum occupancy plot stabilises quite quickly.
(Resolution bandwidth, frequency drift and screen capture are something
else, though...)
>??????Here is another interesting question to ponder. Suppose that we
>agree on a system-level definition for IM (that's the title I'll use in
>this paragraph) and write a formal definition, which leads to a
>properly documented measuring procedure. In order to use this (this is
>where the final ultimate reduction of this complex issue to a one-liner
>is REALLY established) we now have to assign a numerical limit - say 50
>microzorkons per root-hectacycle. So, all of the faithful one-liner
>believers will very carefully monitor their station to produce less
>than 50 micr....... ?????of splatter. Then one day they discover that
>the zorkon meter just went to 50.6, so they reduce the drive level by
>just barely touching the drive control. ?Here is the real question:
>where is that number 50 going to come from? Who is going to set it and
>based on what data?
We seem to have drifted from standardized lab measurements into
adjustments made when actually transmitting. In amateur radio, those are
two completely different things.
The main value of standardized lab measurements is for product reviews,
comparing the performance of different equipment in the same test.
On-air management of IMD is done without any test equipment, other than
your receiver and your own ears. Results are highly impressionistic and
at best can be only semi-quantitative. My own objective is very simple:
I want other stations to find my signal "narrower" than most other
people's.
These two approaches are complementary, but separate.
>Tell GW4(???) that he is not finished yet - we still need him.
GW4FRX is probably reading this... I don't believe he has permanently
hung up the cape and the shiny suit.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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