Limited scope bandwidth and higher frequencies

[Sceadwian]

Using a scope with a limited bandwidth, say 20mhz, what would be the best option for viewing waveforms at higher frequencies?

I'm a hobbyist and budget is more important than features, but I would like to view waveforms in the 72mhz and 900mhz ranges (R/C band and unlicensed band)

Freeware and opensource data analasys tools are pretty prevalent and can turn captured data into excelent spectral graphs for frequency viewing so I'm pondering getting a 10 or 20mhz DSO and creating a basic external analog heterodyning circuit to frequency shift the target frequency into the range the DSO can use.

I'm currently using a 500khz analog scope I picked up for 30 bucks in working order and find it to be very useful for understand DC and audio AC circuits but useless for RF, I'd like to hear people viewpoints on what to buy next as far as signal analasys goes on a limited budget, and using a more limited scope to view higher frequencies (though not bandwidth)

Even my most ambitious projects don't involved a bandwidth over 10mhz and even past 1mhz or so I'd likley rely on a 'pre packaged' sollution, but I would like to use a DSO to better understand and charactorize the kinds of noise and harmonics a circuit that works in those ranges creates.

 

[Marks256]

Hmm, a new scope? I am not sure. I would surf ebay. I think they are fairly priced well...

 

[philba]

Depending on the scope, you won't see much above 1/2 of the rated frequency. Though, HP and Tek scopes usually go up to the rated frequency (and sometimes beyond). The cheaper scopes will give you mush. I doubt even the best 20 mhz scope will do much even for 72 mhz, not to mention 900. Your probes need to be rated for the higher frequencies as well.

If you are trying to determine if you are getting transmissions, you probably can just use a swr meter. easy enough to build. If you want to see the transmission envelope, you could use a simple detector and look at the output on the scope. Are you building RF transmitters? I'd check out what the hams do. I think arrl has lots of stuff. they seem to get by with out high frequency scopes.

 

[Papabravo]

Get your wallet ready for a 1 GHz. scope. There is a reason why they are so expensive. I'm pretty sure that your efforts to get there on the cheap will be spectacularly unsuccessful. That does not mean you should not try. If you're determined to go this way then by all means keep us informed of your progress.

 

[Sceadwian]

I didn't mean to directly deal with the modulating frequencies, I know 72 not to mention 900mhz is well out of the range of a 20mhz scope, but what about using a custom heterodyning pre-filter? I'm not interested in the 72mhz spectrum past 1mhz. Or the 900mhz spectrum past 5mhz The bandwidth used is therefore well within the range of even a 20msps scope, if it's heterodyned down that is. I'm on the cheap, and even considering the phase noise, frequency drift and other uncertanties of a heterodyned signal I figure it'd be workable.

I'm looking for more input before I decide what's best for me, right now I'm thinking along the lines of a 20-100msps DSO if I can find something afordable and decent.

 

[Papabravo]

Are you telling us that you think such a thing exisits, or are you telling us that you want to build such a device "on the cheap" so to speak?

 

[Sceadwian]

Virtually every RF receiver use a superhetereodyne receiving scheme. It is too expensive and pointless to deal with the modulation frequency at it's native value. If you're only dealing with a bandwidth of 1mhz why buy a 900mhz scope to view a phone call? Inject a quality 900.5 mhz oscillator into the pre-filter and just view your 1mhz byproduct.

 

[RadioRon]

Virtually every RF receiver use a superhetereodyne receiving scheme. QUOTE]

This is bit off your point, but to be precise, most mobile phones produced in the last couple of years have used direct conversion topologies for receiving. Mobile phones account for an awful lot of the world's RF receivers, and I wouldn't be surprised if most RFID receivers are direct conversion as well. Not sure about TVs and stereos, but I wouldn't be suprised if they are moving over to DC receivers too. And lots of base stations have been moving to software defined structures, sometimes with a direct conversion front end. The technology commonly used in radio has been changing quickly in the last few years, thanks mainly to the mobile phone.

 

[Nigel Goodwin]

Mobile phones account for an awful lot of the world's RF receivers, and I wouldn't be surprised if most RFID receivers are direct conversion as well. Not sure about TVs and stereos, but I wouldn't be suprised if they are moving over to DC receivers too.

No, they use superhets, DC is far too limiting for a decent receiver - much too poor selectivity!. FM radio uses a 10.7MHz IF, and TV round about 38MHz.

 

[RadioRon]

Back to the point of the original post, here's some insights from the world of radio design. None of the hundreds of radio engineers that I've worked with use oscilloscopes above 100 MHz. To understand what a VHF/UHF/microwave radio signal is doing, we use a variety of analyzers. The basic one is a spectrum analyzer. To view modulation we use general purpose modulation analyzers or "transmitter analyzers". You know, these analyzers do pretty much what you are proposing, that is, they convert the modulated carrier down to a lower frequency, make measurements on the signal and then extract the modulation from it and make even more measurements. So obviously there is much precedent for the general idea.

Rather than use a scope, which is not very useful at higher frequencies and not really essential either, radio engineers will often interpret the frequency domain picture you get from a spectrum analyzer. For example, instead of looking for clipping of a signal on a scope, the radio engineer will look at the amplitude of harmonics of his carrier, which result from clipping.

Modulation analyzers tell us things like "error vector magnitude" or "hum and noise ratio" or "carrier imbalance" for example. With these measurements and the images from a spectrum analyzer, we can tell pretty much everything we need without using a scope.

 

[kchriste]

An RF probe in conjunction with a voltmeter is a cheap alternative for tracing signals though transmitter circuits.
You can also use a multiband receiver combined with a step attenuator to sniff at RF circuits for go/no-go tests in place of the expensive communications monitor.