How do you use the spectrum analyzer?

[Euler's_Identity]

I'm having a bear of a time with it.  Does anyone out there use it successfully?  Is there any sort of tutorial explaining how to use it?

For instance, span is the display span, and center is the center of the display, correct? 

I'm trying to see a JFET's linearity by using the spectrum analyzer to see a frequency domain representation of the fundamental and the harmonics.. 

[yyao]

Hi there,

I found the Multisim help file for the spectrum analyzer to be quite useful. To get it, open up  your spectrum analyzer and  press F1. There's an entire content tree of information including examples on how to use the spectrum analyzer. One thing that I always find is that, the default settings for vertical range is too large for many real life RF signals. Depending on the signal that you are measuring, you may want to change the reference amplitude and range to see your signal better.

[Euler's_Identity]

Hi yyao,

I've found

---------------- begin paste -------------

Spectrum Analyzer

Spectrum Analyzer

The Spectrum Analyzer is capable of measuring a signal's power and frequency components, and helps determine the existence of harmonics in the signal.

One area that has an interest in spectrum measurement is communications. For example, cellular radio systems must be checked for harmonics of the carrier signal that might interfere with other RF systems.

Usually the frame of reference in signal analysis is time. In that case, an oscilloscope is used to show the instantaneous value as a function of time. Sometimes a sine waveform is expected but the signal, rather than being purely sinusoidal, has a harmonic on it. As a result, it is not possible to measure the waveform’s level.

However, the Spectrum Analyzer displays its measurements in the frequency domain rather than the time domain. If the same signal (with harmonics) is displayed on a Spectrum Analyzer, the amplititude of its fundamental frequency displays, as well as its frequency components (harmonics).

Time domain measurements such as rise and fall times, pulse width, repetition rates, and delays, cannot be easily obtained in frequency domain measurements. Therefore, both techniques are important.

--------- end paste --------

and

-------- begin paste -------

Spectrum Analyzer

Spectrum Analyzer

   Note  If the simplified version option is selected, the Spectrum Analyzer is hidden. Refer to the Simplified Version section for information.

The Spectrum Analyzer is used to measure amplitude versus frequency. It performs a similar function in the frequency domain as an oscilloscope performs in the time domain. It operates by sweeping through a range of frequencies. The amplitude of the signal at the input of the receiver is plotted against the frequency of the signal. This instrument is capable of measuring a signal's power at various frequencies, and helps determine the existence of the frequency components’ signal.

The Spectrum Analyzer is part of the RF Design module. Refer to the RF Instruments section for more information.

   Note  If you are not familiar with connecting and adjusting the settings of the instruments, refer to the Adding an Instrument to a Circuit and Using the Instrument sections before using this instrument.

--------- end paste ----------

Neither of these seems to take me to an explanation of how to use the spectrum analyzer. So could you please show me how to get to some explanation of how to use it?

Also, is there some frequency-range minimum I should be aware of?  I'm getting the impression that the spectrum analyzer is only meant for very high frequency-range signals.  Yet, and I should have mentioned this, I'm looking at audio range signals.  It seems that nothing I did with respect to the spectrum analyzer settings would allow me to see a low frequency fundamental spur and its harmonics, but I need to see those harmonics (as I see it) to determine the amount of distortion after a signal is passed through a JFET.

For all I know, my JFET model isn't correct (a 2n5639), as all I'm going by is the time-domain signal which looks ok.  Maybe the above explains what I need a little more.

I'm attaching a circuit.  The idea is to attach the spectrum analyzer to the drain so that I can view the harmonic content as I vary the DC (via the battery) to the gate.  Notice that what I've done is I've set up a R-R voltage divider across the AC source which currently (with the JFET off) produces 0.5vin from the drain to ground.

  

[Euler's_Identity]

Plus, there's one other thing.

When using the included JFETs (at least the ones I've picked) from the Master database, I've found that the simulation engine is always spitting out a slew of

"------ Checking SPICE netlist for vctrld_atten - 2009-02-10 11:07:17 ------
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'betatce', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'vtotc', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'isr', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'n', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'nr', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'xti', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'alpha', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'vk', this parameter will be ignored.
SPICE Netlist Warning in schematic RefDes 'q1', element 'j2n3458__jfet_n__1':  Unknown model parameter 'm', this parameter will be ignored.
======= SPICE Netlist check completed, 0 error(s), 9 warning(s) ======= "

ignored parameters.  That's odd, for these are parameters that are coming from the Master database.  My model doesn't produce these ignored parameters.  Is something not right?  As I see things, you can only ignore so much definition before the model doesn't work correctly enough to be useful.

[yyao]

When the help file comes up, look at the Contents tab. Expand the tree that is labeled "Spectrum Analyzer". You should see more subtopics about the spectrum analyzer such as "Frequency Range", "Amplitude Range" and "Examples". It explains in depth each option and action that exists in the spectrum analyzer.

[Euler's_Identity]

Aha!  (Please still address my JFET issues though.)

I found the tree!

And yes, there is a frequency limitation.  It looks like I'm going to need to bump up the frequency a tad.  I'll try 3KHz instead.

I'll let you know how it goes.

[yyao]

The JFET model that you are looking at contains parameters that Multisim does not support yet as of 10.1. However, we are working on support for more advanced models and the 2N3458 should be supported with the next major release of Multisim.

Sorry for the inconvenience.

[Euler's_Identity]

Ok, though I haven't fired it up yet, the tree information seems to be helping.  However, what are the "start" and "stop" button for?  How exactly do they work?

[yyao]

They allow you to change the range of the spectrum analyzer without having to stop and restart the simulation. Remember to press the Enter button when changing your frequency range.

[Euler's_Identity]

Ok, I've modified the circuit.  Clearly I'm getting distortion, even though the time domain signal (the scope) looks ok.

But now I know the harmonic levels!  Thanks yyao!

I've attached the new circuit.  Notice that, using the switch, you can see the before and after spectrum analysis. 

But I still don't know about the JFET part model.  I built that model by copying the symbol but substituting On-Semi's model and then tweaking the pin out definition in the manner

(alphabetical order and rotate)

d-g-s

s-d-g

g-s-d

until things seemed right.  It was a matter of "syncing up" the symbol with the voltages on the meter.  However, with a JFET it's tricky, in that the source and the drain are generally interchangeable as I understand it.

A JFET's like a doughnut and a doughnut hole or maybe a dumbell and center ring.  The materials are P and N.  The idea is to vary the reverse bias to expand or contract the depletion region between the P and N materials.  Too much reverse biasing and the channel (the dumbell or doughnut hole) is pinched off, raising the resistance of the channel to essentially open.  Too little reverse bias and the channel (the dumbell or doughnut hole) resistance decreases to essentially short, permitting excessive current flow.  The trick is to keep the reverse bias in the middle some where.  In most JFETs (as I understand it), the channel is symmetrical at the top and bottom; therefore, standing alone, there is no difference between the source and the drain.  But I'd still like my model to work correctly.  I'd think that my biasing network would be the key to maintaining the symmetry, where it'd be all about the biasing network having as little influence as possible on the JFET's channel.  Right?

Radically changing the subject briefly, what's the "security copy" of a circuit file about?  I'm thinking it's not the one that should be uploaded to this forum, but I don't know or know why.