[Elecraft] XG3 device...

[Jack Smith]

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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