Alan: Coilcraft is direct sales only - no minimum quantity for internet orders and they are pretty good about free samples if you have a business name and address.
Jack K8ZOA On 4/14/2011 5:58 PM, Alan Bloom wrote: > Hi Jack, > > You're right that when testing a wide-band device like an amplifier you > have to filter out the generator harmonics in order to get a valid > third-order IMD test. But when testing a narrow-band device like a > receiver the harmonics are normally not an issue. > > Those Coilcraft low-pass filters look very nice, especially for he > price. Where can you buy them? > > Alan N1AL > > > On Thu, 2011-04-14 at 16:54 -0400, Jack Smith wrote: >> 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 >> > > ______________________________________________________________ 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
