Exploring filtering functions in PS7 #
PicoScope 7 has two filter options built in and both function differently. They are the Bandwidth limit and DSP (low pass). You can learn about both in our short tutorials before we dive in a little deeper.
https://www.picoauto.com/library/picoscope-automotive-training/knowledge-base/bandwidth-limit
https://www.picoauto.com/library/picoscope-automotive-training/knowledge-base/low-pass-filter
There are two distinct differences between the Bandwidth limit and DSP. Bandwidth limit is a hardware filter (just think of it like attaching something between the scope channel and the probe that places a 20kHz filter in line). Bandwidth affects the operation of the input signal, while DSP does not directly (but it can affect measurements!).
DSP (digital signal processing) is a software filter applied in PS7.
Let’s look at where these are located in PS7.
So how do you know when each option is active?
The channel button will tell you!
Question: Are these two options available all of the time and to all automotive scopes?
Answer: No, they are not!
First off, with the Bandwidth limit, only the current and previous automotive 4×25 and 4x25A scopes have it. Note: the “x” in the 4×25 is either a 2 or 4 and denotes the number of channels.
Also, with the Bandwidth limit, you must have a scope plugged in…
Another important feature with Bandwidth (and sometimes I call it a non-forgiving filter) is that if it is applied and the .psdata file is saved with it applied, you cannot reopen the capture later and remove it. Hence why I call it non-forgiving!
However, with DSP (that I call a safe filter), you are able to apply and remove at any time. Remember, DSP is for “our eyes” and not the scope!
Another interesting feature of Bandwidth is that it is automatically applied to our family of BNC+ current probes when they are plugged in (don’t worry, it can be turned off).
Using this PS7 filtering knowledge, let’s look at an interesting case study on a 2019 Chrysler Pacifica with a 3.6L V6 engine that is running fine with no codes or complaints.
Will start with the initial capture of the CKP, both Cam sensors and a COP on cylinder #1.
In this case, I wanted to monitor the edge counts on the CKP sensor to verify that each 360 crank degrees was consistent (looking for any anomalies manually could be very time-consuming). I will clean up the capture by going to Views and hiding channels B, C and D. You cannot turn them off, as that would affect the capture since I’m triggering on channel D.
I will also close the notes and channel labels page.
Next, open Measurements (mine is pinned to the PS7 custom menu under the channels) and select Rising Edge count, making note of what channel you are applying it to. In my case, it is channel A.
Next, I’m going to bring in the cursors and place them at each missing tooth area.
I’m also going to lock the rulers so I can easily move them together.
You will notice that it reads 58 edges between the rulers.
Moving the rulers to the next 360 degrees, something odd happens.
It now reads 61 between this set. Is there a problem with the vehicle or the scope? As previously stated, the engine appears to run fine with no codes.
You can download this .psdata file at the bottom of the page and follow along.
I decided to clean up channel A using DSP.
I set my low pass filter at 10 kHz and performed my edge count test again.
Note that to update the edge count reading, you do have to move the cursors.
Still 58 on the first set.
And now 58 on the next set.
Using DSP in this case not only helps our eyes see the pattern better, but also helps our edge count case be accurate.
I hope this article and short case study help explain how DSP can be used. It is recommended to always start with no filtering, then add as needed.
2019 Pacifica 3.6 idle CKP_CMP 11 &12 w NO VVT at 20ms and 1MS.psdata