Posted by
Alan Bloom on
Apr 14, 2011; 6:26pm
URL: http://elecraft.85.s1.nabble.com/XG3-device-tp6271930p6273915.html
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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