The problem is that you are using a MEMS digital accelerometer, and what you are reading is the SCK (serial clock) pin of the serial interface. In order to function, that sensor needs to be interfaced with a microcontroller, that sets it for the sampling frequency, the range and so forth.
So you don't have to expect a square wave with 100Hz frequency, but a fast (depending on the bus bitrate) spike, corresponding to a transmission. Expanding the spike, if the scope is fast enough, you should then see the clock square wave inside the spike.
Moreover, if you don't set the SPI interface correctly, the uC will not generate the clock (the sensor operates in slave mode), and you won't read any value.
If you want to see a 100Hz signal, you could probe the Int pin, which sends an interrupt to the microcontroller every time a measure is available. Then, if you handle the interrupt from the microcontroller properly, you wil see the pulse corresponding to the transmission every 10 ms (100Hz).
But make sure that you're not using motion detection; in that case, only when an acceleration is measured, it will generate the interrupt.
To read the data at the SPI port, the simplest thing is to configure the communication with the sensor; otherwise, it won't send data at all. Then, check if the microcontroller is getting the interrupts and if it's reading the data the sensor gives; you can use a timer to add a timestamp to values and check the frequency they come.
(still WIP)
NOTE: A lot of this is now outdated, as Tektronix have released some interesting scopes lately (2015).
This started out as a comment, but I'm expanding it to an answer:
Basically, Tektronix is not competitive in the digital oscilloscope market any more.
Your comparison is fundamentally flawed too. You are comparing Tektronix's bottom-of-the range scope to Rigol's middle-of-the-range model.
The actual Rigol scope that best matches that Tektronix scope is the DS1102E.
- Both have tiny, crappy, QVGA (320*240) screens
- Neither have intensity grading.
- The Rigol still has a lot more sample memory, 1 Mpoints vs. 2.5 Kpoints.
Note that the Rigol scope listed above is only US$400!
Really, if you're shopping for a US$500-US$3000 DSO, the only two players on the market even worth bothering to look at (at least at the current time) are Rigol and Agilent. They are the only two people on the market that offer intensity grading (Rigol call it "Ultravision" and Agilent call it "InfiniiVision").
This is a technique that actually measures the time the input waveform spends at each ADC value per X-axis time-step, and actually varies the intensity of the drawn scope trace to reflect the period of time the input spent at that voltage. This produces a display that actually somewhat resembles a traditional cathode-ray oscilloscope. It is absolutely a excellent feature, and I, at least personally wouldn't even consider a DSO that lacked it at this point.
Basically, Tektronix are just not producing DSOs worth looking at. They did have some good DSOs in the early 2000s: they produced a nice, primitive DSO, garnered a significant chunk of market share, and basically then sat there resting on their laurels and stopped innovating. This is supported by the teardowns I've seen of some of their late-model scopes, which were using rather ancient silicon for their processing.
Note that this is changing, but only for Tektronix's higher end. They're doing some really cool stuff with their MSO devices (mixed-signal oscilloscopes). They basically combine a spectrum analyzer and a DSO, and for RF work, they look excellent. They're also $50K+.
Then, Agilent came along and basically completely wiped the floor with them in short order, with their much deeper memory scopes, and introduced intensity grading.
Now, Rigol have subsequently come out with a competitive mid-range scope line that makes them also worth considering, together with Agilent.
As far as I can tell, Tektronix's superb reputation should only really be applied to cathode-ray oscilloscopes (I have several, all Tektronix). They really didn't take the transition to digital, and its high innovation rate well at all.
If I were buying a scope now, I would look for:
Absolutely essential at ANY price-point:
- Greater then 100 KPts memory.
- 640*480 or larger screen. This is why I never bought one of the cheaper Rigol scopes
Absolutely essential a >~$1K price-point:
Nice to have:
- High waveforms/second
- This ranges from merely nice to totally essential, depending on what you are using the scope for. If you're glitch-hunting, you pretty much have to have high waveforms/second rates for decent coverage. The Tektronix scopes are an order of magnitude lower in waveforms/second then the Rigol and Agilent scopes (though the latest ($$$) Agilents are even better).
- Protocol decoding, at least as an option
Best Answer
Included mostly for interest, as this is not a beginner project.
There's a way, but it's not all that simple. Build a sampling front end for an oscilloscope.
The above circuit will allow periodic signals with 1GHz bandwidth to be measured on a 10MHz or better analog oscilloscope. It's no good for one-shot signals.