“You can tell the difference between a professional and an amateur by the tools they use,” or so the saying goes, but even pros have a
limited budget. So, you only buy the tools as you need them. Sometimes, you only need a tool for a short period of time or just for one
project. So, when you need a specialty tool, do you rent, borrow, or buy? If you need a full set of features, it might be cheaper to lease
the equipment. Alternatively, you might want to buy an inexpensive device if it satisfies your immediate requirements.
I’m more of a software person, but I work on embedded systems, so I do get to work with hardware. I was
working on a project that required the use of a logic analyzer. I had never used a logic analyzer before, so I had to
do a bit of research to understand what I would need before making the lease versus purchase decision. I’ll first briefly
introduce the project, so that you know the scope of the requirements. Then, I’ll consider leasing as an option. Finally,
I’ll give a brief overview of the products that I considered and, ultimately, the product that was purchased.
The project involves the conversion of three-phase power with variable voltage and frequency to 60 Hz three-phase power (the grid). A piece of the hardware involves all digital signals for input and output. I won’t go into the
details of the project, but the hardware consists of eight switches, six phase polarity detector circuits, and a current
zero crossing detector circuit for a total of 15 logic signals in the 0-5 volts range. Since I was developing the switching
algorithm, I needed to examine and record all of these signals to determine if the device was performing properly.
NUTS & VOLTS
Before looking for a logic analyzer, I needed to determine my data acquisition requirements. The 15 logic
signals, eight switches, and the current zero crossing signal never change faster than 50 kHz. Also, the six phase
signals change much more slowly — less than 200 Hz. I want to monitor these signals while the amplitude and phase
of the input power change. Thus, I need to be able to look at an entire switching sequence for several seconds.
Assuming a sample rate of 50 kHz for all 15 signals for four seconds, my worst case data buffer calculation is:
50,000 * 15 * 4.0 = 3,000,000 samples
I’m currently working with a fixed input amplitude and frequency, so my immediate needs are only for one full
60 Hz cycle or about 16. 7 milliseconds worth of data:
50,000 * 15 * 0.167 = 12,525 samples
The above estimates assume that all of the signals are sampled at the same rate — which might not be the case
— but the estimates define the lower and upper bounds for the data buffer size. So, my minimum requirements are a
buffer size of at least 12,525 samples and a sampling rate of at least 50 kHz. Additionally, I want to be able to get
digital copies of the images from the logic analyzer so I can include them in reports. I needed any software to run on
both Windows 2000 and XP Pro. My bench is already covered with equipment, so a small footprint is preferred.
Oh, and I’m not working on this project by myself, so it would be great if I could save the recorded data and
Email it to someone else so that person could look at the entire recorded stream without being limited to just screen
shots and without having to Email large files (tens of megabytes). I will need this piece of equipment for at least a
month, but six months is a more realistic time period. Also, I don’t want to spend a lot of time looking for and pricing
this piece of equipment because I want to use it as soon as possible. So, vendors with prices on their websites and
next day shipping are preferred. Lastly, I wanted to keep the total cost under $1,000.00 and ideally under $700.00.