How to view f spectrum?

I want to view a frequency spectrum, displayed such that I can see the center f of a resonant circuit.

What do I need to do this? I have a sweep generator and oscilloscope. Is this sufficient to the task? Or does this require a spectrum analyzer?

Thanks,

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It requires the spectrum analyzer.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

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Reply to
Kevin Aylward

I must have misinterpreted the question. If you have a ramp out to drive the X of your oscilloscope, you should be able to see the response of the resonant circuit on the Y-axis. If the frequency is too high for the BW of your scope, you'll need a detector.

Spectrum analyzer has this built in, even better with a tracking generator, but shouldn't be necessary for this task. mike

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mike

So what do you see if your circuit doesn't have any active components, (so you should only see an attenuated version of input at the output) you hook the output of your sweep generator to the input of your circuit you hook the ramp signal output of your sweep generator to your scope X input you hook the output of your circuit to your scope Y input you set the timebase of the scope to let your ramp complete nicely or get it into XY mode, depending on what your scope has.

Roughly what this is doing is generating a lissajous pattern on the scope, the ramp/sweep frequency being far lower than the frequency that you are sweeping. If your scope is up to it this might do what you are looking for.

Start out simple and see if you can make this work.

Reply to
Don Taylor

To do the job slickly, yes. But bare-bones, no.

OK, let's assume the "resonant circuit" you ask about is a simple LC parallel circuit, sometimes called a "tank" circuit. You connect the signal source, which has a 50 ohm source impedance, to the LC network. Remember that, at resonance, a parallel LC circuit will exhibit a high impedance.

If you have a sweep generator that has a DC output proportional to frequency, then just connect the analog voltage to the oscilloscope horizontal deflection. Connect a voltage probe across the LC network, and connect that to the vertical deflection. As you sweep the frequency range, you will see a peak at the resonant frequency.

No signal gen analog voltage capability? Why not just turn the generator frequency control by hand until you see a voltage peak, then just read the sig gen dial?

If you were talking about a more complicated "resonant circuit" like possibly an IF strip, you could again monitor the strip output while you slew the sig gen frequency, automatically or manually. Max output will be at resonance (neglecting such things as stagger-tuned elements).

Now, if you question is more of an operational than design nature, then you may be trying to say that you want to view this segment of spectrum to observe signals passing through a resonant filter. Then, I'd agree with Kevin; you need an SA.

Ed

Reply to
Ed Price

If you are working at audio frequencies, you may be able to use spectrum analyzer software with your computer's sound card.

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Bob Masta

If you lightly couple both the generator and the 'scope to the circuit through low value capacitors and sync the 'scope from the generator you should be able to get useful results.

Reply to
dB

Depending on the requirements, it can also be done with a digitizer and software to do the Forier transform of the captured waveform.

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Keith R. Williams

On Mon, 22 Dec 2003 0:19:13 -0800, Don Taylor wrote (in message ):

Audio frequencies, yes. No active components; LC only.

Scope (Tek 2465 DMS) has Z-axis input at the rear panel, but no X-axis input. Does this kill the possibility of simple display of spectrum?

Thanks,

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Hi,

Are you sure that the 2465 doesn't have an XY-mode as most multi-beam scopes do? Usually 'X' on ch-1 and 'Y' on ch-2.

Cheers - Joe

Reply to
Joe McElvenney

Excellent, just about what I was guessing from your original posting.

1: You don't need the Z-axis input on this. That is for driving the brightness in relation to your input signals, don't need that. What you want is an external X input, usually the scope generates this by itself, with the time base controlling how fast that goes. Sometimes it isn't obvious how to use one of your signals to drive X. 2: You have a couple of input channels in your 2465, that will do fine.

See if you can grope around the net or find an example or some kind individual to get you a button-by-button description of how to draw lissajous patterns on your scope. THese are the "loopy" patterns where if you have a 3 Hz sine wave and a 2 Hz sine wave it will make up a pattern with 3 lobes in one direction and 2 lobes in the other. Asking for specific instructions for your scope and this display should get you at least one good helpful answer somewhere. It has been too long for me to be able to tell you how to do this step by step, and I wasn't on the 2465's.

For your nice audio no-active-components want-to-see-frequency-response you are looking for a ramp or sawtooth output from your sweep generator. That is going to drive the horizontal part of your display. And your substantially higher audio signal that is sweeping upwards is going to drive your vertical part of the display. Actually it is going to be bouncing up and down wildly but it will be scribbling on top of itself. The amplitude of the frequency coming out of your circuit is going to look like a bright green vertical line, the height of that line is the frequency response you are wanting to see as you sweep across your filter range.

I hope this helps. Toss me mail if it works out or if you can't figure out how to do this and I'll try to help.

Email address is valid, been "dont" on the net since before there was spam!

Reply to
Don Taylor

On Mon, 22 Dec 2003 18:40:44 -0800, Don Taylor wrote (in message ):

OK. Found it in the User's Guide: "Full counterclockwise rotation of the SEC/DIV switch selects the X-Y display feature. In X-Y, the "CH2 OR X" input drives the horizontal deflection system."

That is plainly labelled on the sig gen.

Thanks. I look forward to the phosphorous pyrotechnics!

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output)

circuit

X

scope,

for.

input.

I think you should look a bit more closely at the time-base knob of your

2465. Mine sure has an X-axis front-panel capability.

Ed

Reply to
Ed Price

There are some programs you can get that might do this using a PC sound card.

Reply to
CWatters

A spectrum analyser may be an overkill. As this is an audio application the simplest approach would be to drive a variable frequency into the network and manually sweep the frequency, to observe the amplitude at the other side of the network by means of an oscilloscope or ac voltmeter. Use the scope to estimate the frequency and amplitude. If the measurement is to be repetitive then use the X output from the baxk of the scope to drive the frequency .

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ddwyer

On Tue, 23 Dec 2003 2:44:25 -0800, Ed Price wrote (in message ):

I got it all hooked up: ramp output from sig gen connected to X (Ch 2) scope input. Sig sweep output drives RC circuit and is connected to Y (Ch 1) scope input.

Not clear that this is the waveform I was expecting; no clear peaks, quite broad spectrum with high amplitude.

One thing is weird; the ramp frequency is only 15 hz. Shouldn't this be the same frequency as the sweep?

Thanks,

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scope

scope

the

Dave:

Your terminology is a bit fuzzy; I don't know what you are describing when you talk about a "broad spectrum."

Not all signal sweepers work alike. Some will scan from frequency A to frequency B, and then halt. Some will stop at B, then reset to A. You appear to have the sweep rate set to "continuous", so the sweeper starts it's sweep and ramp at the same time. At the end of the sweep, the sweeper resets to frequency A, resets the ramp voltage accordingly, and begins a new sweep. Unfortunately, there's often no definition of the time needed to reset. Also, some sweeper have a little pause or hesitation as they begin their sweep. The sweep frequency may be quite a bit slower than the sweep rate during the sweep.

Finally, the sharpness of the peak (at resonance) will be affected greatly by the Q of the circuit. You may be slipping past the peak, without noticing it, because it's so narrow. Try a slower sweep rate, or a narrower frequency span width.

Ed

Reply to
Ed Price

On Sat, 27 Dec 2003 1:06:40 -0800, Ed Price wrote (in message ):

Thanks, Ed. I'll try your suggestions.

Happy Holidays,

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