# How to view f spectrum?

D

#### DaveC

Jan 1, 1970
0
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,

K

#### Kevin Aylward

Jan 1, 1970
0
DaveC said:
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?
It requires the spectrum analyzer.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

http://www.anasoft.co.uk/replicators/index.html

M

#### mike

Jan 1, 1970
0
Kevin said:
It requires the spectrum analyzer.

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

--
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX TEK Sampling Sweep Plugin and RM564 Tek 2465$800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

D

#### Don Taylor

Jan 1, 1970
0
DaveC said:
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?

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.

E

#### Ed Price

Jan 1, 1970
0
Kevin Aylward said:
It requires the spectrum analyzer.

Kevin Aylward

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

B

#### Bob Masta

Jan 1, 1970
0
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,
--

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

Shameless plug: If your system can run real-mode DOS
(Win9x or earlier) and has an ISA-bus Sound Blaster
(or one of a few others, including a simple home-brew
that uses your printer port), then you may want to
check out my Daqarta package. Win32 version is
in development.

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

D

#### dB

Jan 1, 1970
0
Kevin Aylward said:
It requires the spectrum analyzer.

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.

K

#### Keith R. Williams

Jan 1, 1970
0
It requires the spectrum analyzer.

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

D

#### DaveC

Jan 1, 1970
0
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.

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,

D

#### Don Taylor

Jan 1, 1970
0
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?

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.
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!

D

#### DaveC

Jan 1, 1970
0
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.
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.

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."
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 plainly labelled on the sig gen.
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.

Thanks. I look forward to the phosphorous pyrotechnics!

E

#### Ed Price

Jan 1, 1970
0
DaveC said:
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.

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

C

#### CWatters

Jan 1, 1970
0
Audio frequencies, yes. No active components; LC only.

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

S

#### Spajky®

Jan 1, 1970
0
parallel circuit, sometimes called a "tank" circuit.

Is there another "name" also for serial resonant LC circuit or is just
called serial "tank" circ.?

-- Regards, MERRY CHRISTMAS & HAPPY NEW YEAR, SPAJKY Â®
& visit my site @ http://www.spajky.vze.com
Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!"
E-mail AntiSpam: remove ##

D

#### ddwyer

Jan 1, 1970
0
Kevin said:
It requires the spectrum analyzer.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

http://www.anasoft.co.uk/replicators/index.html
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 .

D

#### DaveC

Jan 1, 1970
0
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.

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

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

Thanks,

E

#### Ed Price

Jan 1, 1970
0
DaveC said:
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

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

Thanks,

Dave:

Your terminology is a bit fuzzy; I don't know what you are describing when

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

D

#### DaveC

Jan 1, 1970
0
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.

Thanks, Ed. I'll try your suggestions.

Happy Holidays,

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