> 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
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