third-order IMD test. But when testing a narrow-band device like a
price. Where can you buy them?
> Alan:
>
> One are where a square wave RF source can get you into trouble is 2nd
> and 3rd order intermodulation measurements.
>
> Mini-Circuits Ap Note 00008 discusses the reason why.
>
http://www.minicircuits.com/pages/pdfs/AN00008.pdf>
> I usually use an outboard low pass filter on my HP 8657A signal
> generators when making IMD measurements of high performance amplifiers,
> and it's not a bad idea even with HP 8640B generators which are 60 dB+
> down on the harmonics.
>
> As far as a low pass filter goes, CoilCraft has some very inexpensive
> ($3 range) 7th order elliptical low pass filter modules, the P7LP
> series, with cutoff frequencies from 300 KHz to 500 MHz.
>
http://www.coilcraft.com/pdfs/lcfilt.pdf These are perfectly fine for
> output filtering of a single generator, but should not be used with
> multiple signals as they can themselves be intermod generators. I use
> them here for various tasks including signal generator clean up.
>
> If anyone is interested, I have a few extra printed circuit boards with
> provisions for PCB-mount BNC connectors and a P7LP filter and can make
> them available at a reasonable price (PCB only, you supply the
> connectors and filter modules). Usual double sided, silk screen, solder
> masked boards. I have installed a socket strip in one as an experiment
> so that one might just plug in the appropriate filter, but that's not a
> great idea as the filter modules are not all that mechanically robust
> and the extra lead inductance from the socket degrades stop band
> performance at higher frequencies. I can send a photo of the board to
> anyone interested.
>
> Jack K8ZOA
>
>
>
>
>
>
> On 4/14/2011 2:26 PM, Alan Bloom wrote:
> > Question 1: The frequency is based on the period of the square wave, in
> > other words, the frequency of the fundamental. If you program a
> > frequency greater than the maximum 200 MHz, software automatically
> > selects the proper fundamental so that the harmonic comes out at the
> > desired frequency.
> >
> > Question 2: I wasn't one of the XG3 designers, but my understanding is
> > that the amplitude is that of the fundamental (the sine-wave portion)
> > and does not include the harmonics. When you are operating on a
> > harmonic, the amplitude is uncalibrated. Although theoretically the
> > third harmonic is 1/3 the amplitude (-9.5 dB) and the fifth is 1/5 the
> > amplitude (-14 dB) it is not very accurate because of the finite
> > rise/fall times of the square wave.
> >
> > For most things you use a signal generator for, the harmonics don't hurt
> > anything - they are filtered out by the receiver or other device under
> > test. Even expensive professional signal generators typically only
> > specify 30 or 40 dB harmonic suppression.
> >
> > Alan N1AL
> >
> >
> > On Thu, 2011-04-14 at 04:14 -0400, John Ragle wrote:
> >> The more I have glanced through the specs for this device, the more
> >> puzzled I have become. I grew up on signal generators that put out a
> >> (good approximation to a) sine wave. This device is described as an RF
> >> "square wave" generator, which I take to mean that the output wave shape
> >> is a trapezoid with very fast rise and fall times. As everyone knows,
> >> the harmonic content of an ideal square wave falls off as 1/N. In other
> >> words, this device is a harmonic generator, par excellence, very rich in
> >> harmonic content. To get a sine wave, one has to run it through a
> >> bandpass filter. In this case, the bandpass filter is the receiver to
> >> which you connect it.
> >>
> >> So there are two questions.
> >>
> >> 1. What is the "frequency?" This term usually refers to a sinusoid, in
> >> which case the answer is trivial. Presumably the "frequency" meant here
> >> is the fundamental component of the "square" wave, but the true answer
> >> depends on the shape of the on/off switching, which presumably depends
> >> on the "frequency" one has dialed into the device. Is one getting more
> >> or less a 1/N dependence on amplitude at all frequencies?
> >>
> >> 2. When one refers to the calibrated "levels" of output, is one
> >> referring to the level of the square wave or of its fundamental
> >> sinusoidal component? Clearly, these are related...but which is meant?
> >> What about the high frequency end of the operating range, where the
> >> spectrum may well depart substantially from 1/N (because the trapezoid's
> >> edges are less "sharp").
> >>
> >> Neither of these questions has anything to do with the "phase noise"
> >> specification.
> >>
> >> John Ragle -- W1ZI
> >> ______________________________________________________________
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