In article , Don Pearce
wrote:
Just made me think back to my "Ultimate tuner" I built back in the late
seventies when FM stations were over 2MHz apart. I built it with a very
wide, linear phase IF filter to prevent exactly this kind of distortion.
I don't have it any more, but if I did, it would no longer work because
of in-band interference.
Alas, the transmission may already be processed/filtered, so even without
the filtering in the RX you might get some distortion. Hard to determine
the details, though, as it is difficult to determine quite what is being
done by some of the boxes in the TX chain. Particular problem here is that
with some units like the Orban ones it isn't at all clear exactly what they
are up to.
Actually, now I think of it, the phase response of the IF filter is also
important here. Square, steep sided filters would seem to be the best
choice, but they tend to have a huge group delay wiggle as you approach
the corners, so you do need to compromise.
Yes, indeed. I first twigged all this some decades ago. My colleague at
Armstrong, Ted Rule, had been working on a new tuner to accompany the 700
range amps I was developing. At that time RF was his end of the work. But
over a period of time it became apparent that there were a set of 'target
specs' where he could meet one or two at any time, but not all
symltaneously. Things like distortion for various modulations, capture
ratio, adjacent channel rejection, etc.
After he'd left I was asked to look at tuners. Found that you could 'tweak'
the IF/demodulator to suit any specific spec, but generally by degrading
something else. Sat down and did some analysis.
One result is as you describe. In practice the combination of RF, IF, and
demodulator tend to create a group delay value that varies across the
required IF band, with sharper edged filters giving more gross waggles in
the phase responses, These upset the distortion properties. Turns out that
this is all a fundamental set of trade-offs which stem from the properties
of FM signals when real filters and finite bandwidths are part of the
equation. Unavoidable in reality, therefore.
You can therefore tweak a set to give amazingly low apparent distortion
for, say, 30 percent mod mono 300Hz, but at the expense of, say, the few
kHz R-only distortion going sky-high, or ruining the adjacent channel
rejection, or...
I also twigged that the bench FM generator had no output filtering, so
would have no such effects applied at source and was sending a signal of
arbitrarily wide bandwidth to the tuner. Unlike a real-world TX.
Since experimenting then I have become quite cautious of reviews where a
tuner gives a remarkably good value for some specs on magazine bench tests.
Not surprised if such a tuner might have been 'lined up' for them, or -
perhaps worse - if the production tuners have rather poorer performance
that the magazine values indicates.
Knowing this, if you look at reviews where they give a range of values you
can soon spot the trade-offs involved affecting the results.
Also impressed by some reviews where the comments made about the
'subjective' results were clearly based on the measurements, and not what
would have really been happening if the published values were doing what
simple theory and experience led me to expect would be going on. :-)
In addition to which, in many cases with narrow IFs even tiny FM offsets in
tuning have a marked effect on all the figures. And in some tuners the AFC
pulls the tuning to a place which gives worse results than if hand-tuned.
So it is just as well that most people don't notice these levels of
distortion. ;-
Hence my decision to do the above article. Really to warn people to take
the idea that FM has distortions as low as the magazine values with some
salt. For modest signals the distortion can indeed be low enough to deliver
good results. But that isn't all the story - particularly when listening to
stations which are level compressed to max modulation.
Plan to tackle multipath and impulse interference as well in due course.
Compare the effects on FM with those on DAB and DTTV.
Alas, never did finish off Ted's tuner. This used helical resonators for
the IF and was quite a nifty design. But never made it to manufacture as
the company decided to shut up shop before it reached that point. Shame as
his ideas were quite good even if he'd fallen into a trap which I suspect
has caught many people.
Slainte,
Jim
--
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FM Bandwidth Blues
...
Hybrid Mode
