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Author Topic: Oscilloscope bandwith requirement  (Read 997 times)
AF6G
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« on: December 24, 2009, 03:07:58 PM »

I am trying to get an idea of the minimum bandwidth (BW) requirements for a scope to be used as an HF station (waveform) monitor.

I have read on some sites that, generally, the BW should be four times the highest frequency to be observed. For an HF station the upper limit is 29.7 MHz, so using this rule 120 MHz would appear to be the minimum to consider.

I then looked at a Kenwood SM-220 station monitor owner's manual for some guidance on BW to frequency ratio. On page 12 of the manual, the scope's bandwidth specification is stated as 2 Hz to 10 MHz. It does specify that the waveform observation limit is 10MHz, however, this implies that a 10 MHz scope is suitable to observe (up to) a 10 MHz waveform.

Given this, I am not sure what the minimum bandwidth requirement is for an analog or digital scope to be used to monitor my signals up to 29.3 mHz.

Can someone enlighten me on this or point me to a definitive, true source of information?

Thanks!
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WX7G
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« Reply #1 on: December 24, 2009, 03:36:02 PM »

Go the the tektronix website for papers on this.

For an analog oscilloscope it depends on how much measurement error you can tolerate and what the nature of the waveform is. If your are viewing a sinewave and a 3 dB error is acceptable a 30 MHz oscilloscope will work for you.

If you are viewing a digital waveform and accurate risetime measurement is required an oscillscope having a risetime of 1/4 the waveform risetime is a good rule-of-thumb.

For what you are doing a 50 MHz o'scope should work well.
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KA5N
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« Reply #2 on: December 24, 2009, 04:36:18 PM »

A station waveform monitor is used to look at the signal envelope, not individual cycles.  So to look at a 10 meter signal does not require a wide bandwidth.  Also many station monitors couple the signal directly to the CRT plates and are not limited by amplifier bandwidth (there is no amplfier).
Now if you are trying to look at a high speed digital wave train you will need some sort of storage scope in order to study the individual signal pulses.
Make sure you really need wide bandwidth before chasing an expensive scope when you really only need a much cheaper station monitor.
Allen
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WX7G
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« Reply #3 on: December 25, 2009, 08:04:32 AM »

KA5N you make an excellent point on not needing high bandwidth to view the signal envelope.
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AA4PB
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« Reply #4 on: December 25, 2009, 08:15:30 AM »

He also makes a good point that station monitors often couple the signal directly to the CRT without using a bandwidth limited amplifier. When monitoring a 100W transmitter there is plenty of signal to drive the CRT. The sweep frequency doesn't need to be fast enough to look at individual RF cycles but only fast enough to see the highest audio frequency (3KHz generally).
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KE3WD
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« Reply #5 on: December 25, 2009, 11:28:58 AM »

Any scope that has direct hookup to the CRT and generates a sweep freq of double your audio freq can be used as a station monitor.  

That covers a large territory in available choices.  

One can even use recurrent sweep scopes for this purpose, ala the old heathkits and such.  

That is, if you are only interested in seeing your trapezoidal modulation pattern.  

For troubleshooting purposes, it is good to have a 100MHz, Dual Trace, Triggered Sweep scope, minimum.
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AF6G
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« Reply #6 on: December 25, 2009, 11:40:21 AM »

Thank you to everyone who posted a reply! You all have helped me to understand what I need in a scope for the given application.

Allow me to to pose another question on the issue of coupling.

Station monitors obviously have transmission line I/O connectors on them and RF sampling circuitry built in. As I will be using a standard scope, what are some suggested ways to couple an RF sample to the scope's input?

A few turns of wire around the transmission line come to mind.
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K8KAS
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« Reply #7 on: December 25, 2009, 12:37:08 PM »

I think we used to use the 10 times the RF waveform for a base. That is, a 7 Mhz signal would require a 70 Mhz scope. The software development engineers always wanted 200 to 500 Mhz scopes for lab work. They wre always over the top when it came to toy's in the lab.

Denny K8KAS
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KE3WD
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« Reply #8 on: December 26, 2009, 07:05:33 AM »

Take a look at the input schematic for the Heath monitor scopes in which they used two capacitors as voltage dividers at RF.
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N7DM
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« Reply #9 on: December 29, 2009, 01:27:38 PM »

If you are only going to 'monitor' things in the Shack, and not care especially about [somewhat] accurately measuring wave shape amplitudes, bandwidth is almost unnecessary. Where bandwidth *IS* necessary is in any kind of PULSE waveshape study, where the shape of the pulse is due to MANY harmonics...extending a long way up there in frequency.

Most probes for that kind of work allow for 'adjusting' the cap in the tip of the probe for 'true-ness' of a square wave terminal on the front of the 'scope.

dm
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K7PEH
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« Reply #10 on: January 08, 2010, 08:00:18 PM »

When I was in high-school (around 1963, 1964) I built a little oscilloscope for monitoring the output of my transmitter.  I did this from the suggested CRT driving circuits that were in the ARRL handbook of the day.  No amplifiers.  Direct feed to the plate controls of the CRT.  However, the best part of this little project was that I could not afford a real oscilloscope CRT so I begged the local marine electronics shop (in North Bend, Oregon) and they gave me an old used radar CRT with about a 6 inch diameter CRT.  The radar CRT had a longer persistence then a regular oscilloscope so it made for some interesting but often totally useless waveform displays.

Bandwidth?  I don't ever remember even thinking about the problem of bandwidth.
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WX7G
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« Reply #11 on: January 10, 2010, 01:32:50 PM »

K8KAS, 500 MHz doesn't cut it for most digital work these days. Even the 4 GHz oscilloscopes are too slow for much of the work. 12 GHz real time digital 'scopes with 12 GHz active diff probes are the way to go for very fast CMOS and other buses. $$$.
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