Using CRO as a spectrum analyser

[Learner]

Hi guys,
I was wondering if someone could explain how to use a sweep oscillator with a CRO to display in frequency domain?

I have a Hitachi V660 CRO that has an ext input with a switch says Source or X and there is 2 button in the horizontal mode labled X and Y, I assume I would be able use this with a hp3581 to display in freq spectrum?

**broken link removed**

What I don't understand is that at the back of the hp3581 it has output for both X and Y and I only have 1 EXT input on the CRO, how does this work?

Any help would be greatly appreciated!!!

Thanks in advance!

 

[Learner]

Hi guys,
Did some more reading on the net and appreantly I would need a z axis modulation input on the CRO to make this work, its seems to be a fairly standard function of a CRO. I don't have a manual for my Hitachi V660 and can not find any where on the front panal that says anything about z axis, would this be the Ext input on my CRO?

Any help would be much appreciated!!!!

Thanks!

 

[Nigel Goodwin]

Hi guys,
Did some more reading on the net and appreantly I would need a z axis modulation input on the CRO to make this work, its seems to be a fairly standard function of a CRO. I don't have a manual for my Hitachi V660 and can not find any where on the front panal that says anything about z axis, would this be the Ext input on my CRO?

The Z input is for brightness, I don't see as you would need it for your spectrum analyser.

You said your HP3581 has two outputs, labelled X and Y, these connect (fairly obviously) to the X and Y inputs of your scope. The Y input is the normal vertical input channel, either CH1 or CH2, whichever you like (depending on the scope).

The X input is a horizontal input, and you need to turn your timebase off - the X input may be an extra input on the front, on the back, or it may use CH2 as the X input.

I've got a Grundig scope here at work, it's a dual-timebase scope and has a switch marked 'A, Alt, B, X/Y' - to disable the timebase you simply select X/Y with that switch, CH1 remains the Y input and CH2 becomes the X input. These are all clearly marked on the front.

The socket labelled 'Ext Input' is most probably an external sync input, and won't be needed.

I have an Hitachi scope at home, I can't remember the model number, I'll have a look tonight and see how that works.

BTW - Hitachi UK were very helpful with my scope, I rang them and they happily sold me a service manual for the scope, and a knob that was missing. Presumably you could do the same where you are?, I always like to have service manuals if I can.

 

[Learner]

Hi Nigel,

Thanks for your response!!!

According to this article here....

http://www.350rx7.com/sweeper/SweepText.htm

The oscilloscope needn’t be a high bandwidth model, as long as it has an external horizontal input and a Z-axis or intensity modulation input, which most do.

By the way, does anyone know what the concept is behind displaying signal in frequency domain using the CRO?

 

[audioguru]

Hi Learner,
According to the author, the z-axis modulation is used to display a bright "marker" spot on the trace. You can calibrate this marker so that you will know an exact frequency of the spectrum that is being traced. The z-axis is also probably used to blank the re-trace of the sweep, when the trace quickly returns to the left side.

The concept of this "spectrum analyser" sweeper project is explained very well in the article.

 

[Nigel Goodwin]

My Hitachi scope is a V-302B, it has a mode switch which you set to "CH2 X/Y" for X/Y mode - this gives CH2 as Y, and CH1 as X. Again, it's clearly labeled on the front.

As for the 'concept', the scope is used purely as a display device, very little of the scope is actually used, and what is used could be simplified considerably.

Simple X/Y scope designs used to be quite common as projects (because they are so simple), I seem to recall they were used for setting up amateur radio transmitters?.

 

[Dean Huster]

One way of using the XY scope or scope function for amateur radio use is for setting AM modulation. As I recall, the audio signal was put in on the X axis and the RF on the Y axis. The resulting display was a trapezoid. At 100% modulation, the trapezoid would run all the way out to a point (a triangle) and with overmodulation, a line would extend out on the X axis.

As Nigel mentioned, in spectrum analyzer work, the "scope" or "CRO" part of the system is a simple and minor portion used for the display only. The rest of the system consists of extremely complex electronically tuneable filters.

In the case of a sweep oscillator, the output is the sweep ramp (X) and the input to the scope is whatever you want, usually the output of the circuit under test, such as an IF strip output. The Z axis input (intensity modulation) of the scope is normally used as mentioned for a marker frequency so that you can tell where an exact frequency point is on the display. Alternatively, they often inserted one or more "birdie" markers that were little pips on the vertical display for this function.

A scope with a bandwidth greater than the greatest frequency to be observed is required for the above setup. Otherwise, a low-bandwidth scope can be used if a detector is inserted at the output of the circuit under test.

Dean

 

[Learner]

Thank you so much everyone, you guys are AWSOME!!!!!!




Must say I feel very lucky to have come accross you lot, hope you guys don't mind if I hang around for some more guidance.

regards,
Scott