Posted by
Alan Bloom on
Apr 15, 2011; 9:43pm
URL: http://elecraft.85.s1.nabble.com/XG3-device-tp6271930p6277757.html
Hi Geoff,
The case we're considering is where the test signal(s) are near the
receiver tuned frequency so they are well within the passband of the
input filters.
If the receiver's in-band dynamic range is significantly affected by
non-linear filters in the front end, then that is a serious design
error. The distortion should be dominated by the mixer, not the filter.
If the filter is distorting then it doesn't matter if the generator has
harmonics - you're hosed anyway.
For in-band receiver testing, generator harmonics should not matter.
The only exceptions would be for receivers that do not have filters
protecting the front end or perhaps the exceedingly rare case where a
generator harmonic happens to fall on a receiver image or spur frequency
(easily fixed by changing the test frequency slightly). But in almost
all cases the harmonics should not matter.
Alan N1AL
On Fri, 2011-04-15 at 21:30 +0100, Geoffrey Mackenzie-Kennedy wrote:
> Hi Alan,
>
> Sorry that I was not here to reply to your previous e-mail.
>
> With respect, filter non-linearity and the level of harmonics coming from
> the signal sources are related in terms of the amplitude of the IMD products
> seen at the output of the filter.
>
> If the receiver being tested is perfect and linear, obviously it would not
> matter if the IMD test sources contained harmonics because, for example, the
> fundamental of one source (F1) has nowhere to mix with the second harmonic
> of the second source (2F2) to produce the third order product 2F2-F1 -
> assuming there is no IMD contribution from the test equipment.
>
> On the other hand if the receiver's signal path passes through a non-linear
> element such as an input filter, then of course there is 'mixer' available
> inside the filter to produce the natural third order product 2F2 - F1 as
> would be expected when using clean sources. If the sources produce harmonics
> however, then a twin third order product also at 2F2 - F1 would also be
> generated, where 2F2 is the second harmonic generated by the F2 source. The
> composite level of these two third order product as seen at the filter's
> output could be significantly different from that of the natural product
> depending on how they add.
>
> Although the second harmonic of a test source should be greatly attenuated
> by a typical front end filter, an unwanted 'mixer' can exist near the input
> of the filter, for example in the form of an inductor's core, and would be
> exposed to the full impact of the source's harmonics.
>
> Geoff
> GM4ESD
>
>
> On Friday, April 15, 2011, at 18:45 +0100, Alan Bloom wrote:
>
> > Sure, but if the filters are non-linear that's a whole different problem
> > that has nothing to do with harmonics in the signal source.
> >
> > The question was whether signal generator harmonics are normally a
> > problem when measuring a receiver. I claim they are not because they
> > are removed by the filters in the receiver front end.
> >
> > When I was working in the R&D lab of the Hewlett Packard Signal Sources
> > division, one of our competitors came out with a new signal generator
> > with digital modulation. The modulation was generated at an IF which
> > was heterodyned up to microwave frequencies without any filtering to
> > remove the image. So there was a zero-dB spur X MHz away from the
> > desired signal. They got away with it because it was far enough away to
> > be filtered out by the receiver under test.
> >
> > Alan N1AL
> >
> >
> > On Fri, 2011-04-15 at 13:11 -0400, Jack Smith wrote:
> >> Only to a first approximation are filters linear.
> >>
> >> Usually the major source of non-linearity is in the inductors. Even
> >> powdered iron is non-linear to some degree. Ferrite is worse, of
> >> course. Air is pretty close to perfect.
> >>
> >> Capacitors are also non-linear at some level, as C is not constant
> >> with applied voltage. NP0/C0G caps are very good, but again not
> >> perfect.
> >>
> >> Filter generated intermodulation problems are real and measurable.
> >> Been there and done that and have the Tee-shirt.
> >>
> >> Jack K8ZOA
> >>
> >>
> >> On 4/15/2011 1:01 PM, Alan Bloom wrote:
> >> > Hi Geoff,
> >> >
> >> > I guess I don't understand. Filters are linear devices. How can they
> >> > cause IMD? If the signal generator's harmonics are removed by the
> >> > receiver filters then it is the same as if the sig gen were "clean",
> >> > no?
> >> >
> >> > 73,
> >> >
> >> > Alan N1AL
>
>
>
>
>
______________________________________________________________
Elecraft mailing list
Home:
http://mailman.qth.net/mailman/listinfo/elecraftHelp:
http://mailman.qth.net/mmfaq.htmPost: mailto:
[hidden email]
This list hosted by:
http://www.qsl.netPlease help support this email list:
http://www.qsl.net/donate.html