# Choosing a DSO, advice sought

As a tinkerer, hobbyist, wannabe-'maker' (as in the protagonists in the Make magazine), sometimes I feel the lack of better tools in some projects, although for most, I can make-do with my DMM for test/measurement. Have been considering buying a basic 2-channel DSO, because I am having some timing issues which I'd like to investigate. I have used a DSO back in school for a semester, as part of an Analog electronics elective, so I think I can manage to refresh and learn.

The latest project for which I felt motivated to buy a DSO, involves some RF based sensor network. The RF in question is the 315MHz/433MHz kinds, with the prototyping being done around a 80MHz Cortex-M3 MCU based board.

Other than that, at the moment, I do not foresee much analog projects. Given my nature of requirement, can someone confirm if a 50MHz 2-channel 1Gs DSO would be good enough ? I am not very keen on used CROs, since I believe there is a bit of gamble with used stuff, and chasing for returns/refunds is no fun, especially for online transactions.

If all you care about looking at is the output of your receiver (which is likely a slowish serial output like 2400 baud if it's anything like what I used on the Wicked Node) you don't need a DSO, just get an inexpensive USB logic analyzer like this one. If you have built your own RF front end (because you are a masochist for instance), you'll need an RF-rated oscilloscope, which will cost you a pretty penny (>$2k USD probably). Hope this helps. • thanks for answering @vicatcu. Wicked Node is wicked cool !!$2k USD (or more) is certainly something I don't have to spare on my hobby project, so that is out of question. just to elaborate, I want to observe the waveform on the DATA pins of this module, s.t. I could decode it on my MCU. Is this something I could hope to do with the 50MHz DSO ? – icarus74 Nov 6 '11 at 10:09
• Yea that will be no problem at all - it's going to look like a serial data stream (square wave) at most 8 kBaud on a 50 MHz scope will work just fine for that signal – vicatcu Nov 6 '11 at 14:45
• Okay great. BTW, does it make any sense to use this Logic Analyzer to determine the waveform s.t. the encoding logic can be determined ? I am pretty sure that the DATA out of that RF module is not Manchester/NRZ, but something proprietary. I believe it boils down to detecting the lengths HIGH/LOWs, and absolute waveform isn't essential. It that correct ? – icarus74 Nov 7 '11 at 17:04
• @icarus74 I think I actually have one of those logic analyzers :). Anyway, the DATA out is going to be a 0-5V signal that is directly consumable by a microcontroller UART. So yes, you should be able to decode it pretty easily. There's nothing proprietary about it - these modules basically output a "1" when they "lock" on the operational frequency, and a "0" when they don't, and constantly stream out serial data at the prescribed baud rate (whether or not there is an actual transmitter out there). – vicatcu Nov 18 '11 at 16:28
• thanks for confirming @vicatcu, although very silly of me to ask about the LA model, since you pointed to the same one in your original post. Must've been cross-eyed ! Good news is that I've received the same LA and after some experimentation, did manage to decode the signal timing. Still struggling to get triggers set up, but with manual hit-n-trial, I've got enough. Thanks a ton. – icarus74 Nov 19 '11 at 20:22

Don't expect to see much in that 315/433 MHz band if you try to scope it with a 50 MHz scope.

1 GS/sec is also subjective. Some scopes will do this per channel - others will share the 1 Gs over all channels.

Also, depending on the probes and connection methodology, it can be difficult / impossible to see high-speed signals accurately regardless of the scope.

For digital debugging, you may find more value-per-dollar if you look into logic analyzers - you get more channels and higher bandwidth at the expense of the flexibility that a scope can give you in the analog domain.

• thanks. Well, give a budget of about \$500, I think my options are pretty limited with DSO. 50MHz dual-channel with 1GS/s shared over the 2-channels is what I've seen. The kind of 315/433MHz comm investigation I am interested in is Manchester or NRZ encoded at max.9600bps. My impression was that I would be able to detect, to a fairly reasonable extent the waveforms to determine the high/low/transition timings. Is that a reasonable expectation ? – icarus74 Nov 5 '11 at 10:19
• Logic-Analyzers seem to be available from around less than 10MHz to around several hundred MHz, and expected priced pretty much as a factor of speed and no. of channels. Given my kind of work/hobby profile, what might be the least-expensive but reasonably-useful investment ? I understand that it may not be a very fair question. – icarus74 Nov 5 '11 at 10:22

A 50MHz scope will have its limitations, but I think the strategy of starting at the low end is the right one. It is good enough for most uses, and you can gain some experience that helps you if/when you need to move on to something better (which will be substantially more expensive).

I bought the cheapest DSO from Tektronix in 2007 (TDS1001B, 2 channel 40 MHz), and it is sufficient most of the time. Sometimes it would help if it had 4 channels, but I mostly could get by with two. Bandwidth has not been a problem so far. Sample memory can also be a limitation when looking at longer messages in digital communications, but I managed to work around that by looking at the message piecewise.

• thanks @starblue. could you explain (or point to) the technique of "looking at the message piecewise" ? I can't seem to wrap my head around it quite clearly, though I seem to have a vague idea. – icarus74 Nov 6 '11 at 10:11
• You can move the piece of signal that the scope captures w.r.t. the trigger. So if the messages is sent repeatedly in a loop you can sort of scroll through it. – starblue Nov 6 '11 at 10:21
• ah, so the key is message has to be repetitive. thanks for sharing this @starblue. – icarus74 Nov 6 '11 at 18:51