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 >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
