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looking for oscilloscope what to get

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windtech

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OK her it goes. i am looking for a oscop to work on UF vhf and UHF radios. i would like to get a scope that will work with what I'm doing. I'm getting more advanced in the shop. i do most of my tuning with a dimm meter and other gadgets for radio. I'm looking for something that i can use in the shop and for other projects that I'm thinking of building.

but i don't know what to get.

sorry i forgot to mention i am self taught no schooling at all so this will be my first scope.

and can some one put in to caveman terms what this bandwidth thing is. because thats where i hit the brick wall on what to buy.
some call for 30 some call for 50 MHz. can i go higher or do i have to stay in the lower bandwidth. i don't know HELP PLEASE????????????:eek:
 
Bandwidth is a tough call because your need for a high bandwidth is sort of matter of chances. By this I mean that as a hobbyist or beginning tech, the chances that you really need 1MHz bandwidth is 100%. The chances that you really need 30MHz bandwidth is about 20%. The chances that you really need 100Mhz bandwidth drops to about 2% and the chances that you really need 300MHz bandwidth or more is about 0.01%. My reasons for these numbers are based on the knowledge that your average project includes lots of checking DC, audio or low frequency circuits, a fair bit of looking at low to medium speed digital I/O and clocks of microprocessors and not much else. Typical circuits might include:

- microprocessor clock: should have 30MHz bandwidth to just check if it is running ok. You need 100MHz bandwidth if you are diagnosing an oscillator problem at the clock, but I've never had to do this.
- op amps: for audio, 5 MHz bandwidth is plenty. For everything else, default to 20MHz or more.
- switching power supplies: these have lots of high frequency in them, so 50MHz is the minimum for these
- control circuits and microcontroller digital I/O. You can analyze these fine with 40Mhz or more bandwidth in most cases.
- video circuits: again, I would go with at least 40Mhz so that you can see all the distortions that are possible when dealing with 5 to 10MHz baseband signals.
- motors, speed controls, and other line power stuff: 5 Mhz bandwidth is enough

These are just a few examples, but it does go to show that what you should have depends on what you are doing.

When I started out as an RF tech, I thought I would be using the scope to look at HF and VHF carrier signals, but the more I worked in the business, the more I learned to rely on a spectrum analyzer instead of oscilloscope for such frequencies. A spectrum analyzer tells you a lot more about an RF circuit than a scope and a scope is tricky and difficult to apply to circuits with 100MHz and higher frequencies. Most scope users are sloppy about what to do about grounding the probe, or just ignore the ground clip completely. Such people should not be using a scope for looking at RF carriers and signals above 20MHz. At VHF frequencies, you can only trust your scope if the probe is connected with a very small ground clip (the little springy type that attaches in place of the probe hood works well) or with a coaxial connection.

You will find that some of the most popular scopes are used commercial grade ones with 100Mhz bandwidth. This is because 100MHz covers the vast majority of things you may come up against where a scope makes sense. My own experience is that I started out as a tech with 35MHz bandwidth. Most of the companies I worked with later on provided 100MHz bandwidth as the basic bench scope. I've had two analog scopes with 100MHz bandwidth and often fell back to using my first 35Mhz analog scope because it has a sharper display. I have worked with many digital scopes, usually 100MHz or 200Mhz bandwidth, but the one I use on my bench now is 50Mhz bandwidth and that is fine. I actually have one digital scope and one analog scope on the bench. The analog one is 275MHz bandwidth.

You get by far the most value for your money by getting a good used analog scope with 50MHz or more bandwidth for $100 or less. The best value for a digital scope for you is either a new Rigol DS1052E at $399 from dealers like Saelig in the US or a used Tektronix TDSxxx series digital scope for around $250 on ebay. Beware that scopes are complex and used ones need to come from a reliable seller and/or they need to be really inexpensive to compensate for the risk that there is something wrong with it (which, on ebay is often true).
 
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Well said.

But Bandwidth is generally defined as the the -3db point. It's the amplitude of the frequency measured is 0.707 (sqrt(2)/2) or about 70% what it was at the lower frequencies, thus certain scope can be USABLE at greater frequencies than the BW. The shape would be OK, but the amplitude would not.
Rise time is often thought to be 1/BW. Rise timeis the amount of time it takes the signal to go from 10% to 90% of the max amplitude.

Options such as cursors and FFT Fast Fourier Transform (changing signals from the time domain to the frequency domain) might be useful for you too.
 
OK well i did some checking on a few service manuals that i have for the MHz required for testing. on is a 30 and the other is a 50 MHz. I'm using it for the modulation testing i have a analyzer that wasn't cheap that one was a hit and a half at 4500. i use the analyzer allot. but i would like to get better on tunning the modulation of the radio with out having a bounce station to help me. having to rely on some one Else's ear Ive seen it done with the scope but i need to know if a 100 would be fine for all my testing. i don't want to buy three or four scopes for testing. can i do it with one?
 
Get the highest bandwidth you can afford. But don't forget to get appropriate probes, hopefully with all of the probe accessories.
 
I just acquired a GW INSTEK GOS-620 20MHz analog oscilloscope for just 60 SGD (45 USD) and it's exactly what I need for hobby projects. At home I build mostly audio circuits and simple RF circuit such as FM transmitters. So to me, a multimeter is far more useful to me than a scope (seriously!). Most of the time, I only use the scope once I finish building a circuit to visualize the waveforms and how the circuit works. For RF circuits, I believe a spectrum analyzer or frequency counter is much more useful than a scope. Sadly I don't have either of them.
 
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OK this is a new one " A spectrum analyzer" what does this do and why would i need one for RF?
not to sound stupid but ive never heard of this.
 
A spectrum analyzer shows all the signals in a given (selectable) bandwidth. For example, for a modulated AM signal, it would show the carrier frequency and any sideband frequencies due to the modulation. Thus they are very useful when working with those type of signals.

The signals are displayed on a screen with frequency horizontally and amplitude vertically.

They tend to be expensive for the higher frequencies above a few hundred MHz, but you can get software for a PC that will do a spectrum analysis for any captured waveforms, either from an oscilloscope connection or a PC oscilloscope module.
 
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