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suitable scope to use for RF work

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jason695:
hi anyone,

i hope this doesnt seem to broad or vague a question.

can anyone answer my question about a suitable dso if any.

i am trying to do some RF measurement. it will all be done on unmodulated sine waves. i was looking ultimately to measure a 100 mhz sine wave. however, most of my work will be done at about 30mhz.

the only problem is significant parts of my study is being done on single transistor oscillators designed and constructed by me. these are simple sine wave oscillators e.g. colpitts, hartley etc. im not using premade square wave clocks etc.

in this situation obviously there'll be more significant harmonics than from signal generators or premade i.c. but, more importantly i think, there'll might be distortion as in "squashed sine waves" to some extent.

obviously the DS1052e is only 50mhz banwidth but, it is 1ghz sample rate so although attenuated it can theoretical measure 100mhz sinewaves. i know about nyquist etc but, would such signals be heavily aliased & what about limited bandwidth of 50mhz.

can anyone tell me if the ds1052e would be suitable with all these effects present.

thanks everyone
jason

alm:
Sounds like you need a spectrum analyzer, not a scope. A scope is only useful to establish the presence of an RF signal. It will lack the bandwidth to show much of the harmonics, and FFT is a lousy replacement for a real spectrum analyzer due to very limited vertical resolution.

gregariz:
I regularly use both specans and scopes for this type of work. The scope is a useful tool to look at things like clipping and waveform shapes that give one an idea of circuit function and design problems. I would suggest you find a 100 or 150MHz scope if your intention is to look at the higher frequency. Regardless of which scope you buy it will have a higher frequency rolloff that will result in any higher frequency components either being missed or distorted.

alm:
For 30MHz signals a 100-150MHz scope may be OK. For 100MHz, I would recommend a 500MHz scope if you want to measure the shape as the signal, and not use it as a clumsy frequency counter / power meter. Note that the scope bandwidth is the point at which signals will be 30% down in amplitude. It will have a hard time displaying anything but an attenuated sine at its rated bandwidth.  Any signal except perfectly sinusoidal signals will have a larger bandwidth than just the fundamental frequency.

vk6zgo:
Obviously,a Spectrum Analyser would be nice,but it may be outside your budget.
 
As far as Oscilloscopes go,I would suggest you look around for an analog 'scope,as you may be able to get a 400MHz one reasonably cheaply.
Unless your sinewave is severely mangled,the level of any harmonics above the 3rd, probably won't be too high so 400MHz BW would be fairly useful.
If you already have a DS1052e,then try it & see what happens.

Other than a Spectrum Analyser,there are other & sometimes more useful ways than looking at waveform distortion to check for harmonics of a sine wave,such as:-

(1)
Absorption Wavemeter ( basically a tuneable detector--you tune it for the suspected harmonic).,
These are very old technology now,you might find one on EBay,or you could make one!
Commercially made ones have a calibration chart,so you can set the fundamental as 0dB,& read off the amplitude of harmonics in dBs down.

(2) VHF/UHF receiver---obviously,these won't give you an accurate measurement,due to AGC,etc,but they will indicate the presence of a harmonic.

This is all presupposing a limited budget.If you can spend around $1600 you can pick up a brand new Chinese Spectrum Analyser which should make things quite easy.

VK6ZGO
.

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