[CQ-Contest] Fwd: [FCG] riding the RF gain...
i4jmy at iol.it
Fri Dec 7 19:54:41 EST 2001
The main reason why tuneable preselector where a practice until early 80'
was that amplifiers, I mean the active devices, had rather a bad overall IMD
compare with present ones and couldn't absolutely work without such a
Now things are much better and more than few receivers can easily withstand
a 40m european situation managing abnormal big broadcast signals and amateur
ones without the need of a preselector, like it couldn't happen 20 years
This of course doesn't mean that receiver immunity to distortion and IMD
wouldn't be better narrowing the input window with a preselector.
In my opininion anyway, a lot more has to be made with a number of modern
No preselector can clean the transmitted IMD products when one of that cheap
boxes is a few KHz apart from you.
----- Original Message -----
From: <owner-cq-contest at contesting.com>
To: <tree at kkn.net>
Cc: <cq-contest at contesting.com>
Sent: Friday, December 07, 2001 2:51 PM
Subject: Re: [CQ-Contest] Fwd: [FCG] riding the RF gain...
> Tree, this is a topic that W8JI and I have discussed in this forum in the
> past.. There were bad trade offs made when amateur transceivers went to up
> converting, band pass front ends that are 500 kc+ wide... Given todays
> cheap processing power, there is no reason, cost or technology, to not
> multiple Hi-Q, RF stages with distributed gain that track the tuning
> the band... Yeasu has gone part way there on the MK-V which has a tuneable
> LC filter that can be inserted before the First RF amplifier/mixer stage,
> but it is not particularily sharp and requires the operator to manually
> peak it as he moves up and down the band... However, it also has the
> ability to unpeak the immediate frequency which can have benefits at times
> so the Yeasu engineers did do some thinking outside of the box... We have
> driven the technology of receivers forward with our demands for better
> contest receiver performance, but there is lots of room for improvement
> > Sorry for the confusion generated by my post.
> > When I said "in band" I was thinking "in passband" as opposed
> > to signals within the amateur band.
> > Actually, I consider the "in band" IMD performance of most
> > receivers today to be really disappointing. I think that when
> > you have lots of noise, this distortion makes it really hard
> > to copy signals near the noise level. It might also explain
> > why some receivers sound bad when there is a big pileup.
> > The IMD graphs show two signals mixing - but that isn't real
> > life all of the time!
> > Backing off the RF gain quickly attenuates these products by
> > another 20 or 25 db.
> > You can see a graph of this IMD distortion at this URL:
> > http://www.arrl.org/tis/info/pdf/k2.pdf on page 19 under the
> > title "In-Band Receiver IMD Graphs". The K2 actually looks
> > about 10 db better than my TS850 and some other radios I have
> > looked at (including the MP). Perhaps this helps explain why
> >some people think this receiver responds to pileups better.
> > I did some work with AC6T to try and get better results with
> > the 850, but we weren't able to come up with anything after
> > about a month of work.
> > You can make your own measurements if you are interested. You
> > will need two XTAL oscillators about a few hundred Hz apart
> > with pretty much the same output. You can download software
> > to use with a soundcard to turn your PC into a spectrum analyzer.
> > Then, you can play with your AGC and RF gain settings to see the
> > difference.
> > Ask anyone who has a direct conversion receiver to bring it
> > over - and you will be amazed at the difference!!
> > 73 Tree N6TR
> > tree at contesting.com
> > --
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