Time for something different – and perhaps the start of several new articles containing teardowns. In our first instalment we examine the Tektronix CFC250 100 MHz frequency counter circa 1994: Not the most spectacular of designs, but it has worked well right until the present day. The update speed of the display wasn’t lightning fast, however for the time it would have been quite reasonable. Here is a short video I shot last year comparing it against a small frequency counter kit:
However after staring at this thing every day on my desk for a couple of years it has now become impossible to overcome the temptation to have a look inside. Therefore the reason for this article. You can click on the images to see the full-size version. So let’s go back to 1988 and check out the CFC250…
A quick look around the outside. The casing is reminiscent of the Escort brand of test equipment from the era, and (I suspect that) they OEM’d the CFC250 for Tektronix. (Interestingly enough Agilent bought the assets of Escort in 2008). Moving forward, the external images of the CFC250 starting with the front:
… and the rear. The AC transformer is tapped out to accept four different mains voltages, which you can select with the slide switches:
Opening up the unit involves removing screws from the base. The first ones were only for the feet, so they could stay put:
It was the screw on the right of the foot that was the key to entry. After removing them from each side and the other pair on the rear-bottom, the top casing pulls off easily…
… leaving us with the internals for all to see:
Although the LED display is a fair giveaway to the age of the CFC250, a quick look around the PCB confirms it… and the display is ultimately controlled by an LSI Systems LS7031 “Six decade MOS up counter” (data sheet.pdf). It is matched to some DS75492N MOS-to-LED hex digit driver ICs (data sheet.pdf) and some other logic ICs. It is interesting to compare the number of parts required to drive the LEDs compared to a contemporary microcontroller and something like the TM1640 used in this module.
Now for the LED display board:
Nothing too out of the ordinary. A closer look at the rear panel shows some very neat AC mains wiring:
Now for some more close-ups. Here we can see the use of the MM5369 17-stage oscillator/divider (data sheet.pdf). I haven’t seen one of these for a while, the last time we used them was for a 60Hz timebase. However in this case it would be used to create an accurate timebase within which the CFC250 would count the number of incoming pulses:
The removal of two more screws allows removal of the main PCB from the base of the cabinet, which reveals as such:
There is also an opaque plastic sheet cut to fit, helping insulate the PCB from the rest of the world:
The PCB is single-sided and very easy to follow. I wonder if it was laid out by hand?
It reminds me of some old kits from the past decade. Moving forward, there is a metal shield around the PCB area of signal input and low-pass filter:
A quick desolder of three points allows removal of the shield, and reveals the following:
At the top-left of the above image reveals a resistor in a somewhat elevated position, as shown below:
If anyone can explain this one, please leave a comment below.
What impressed me the most during this teardown was the simple way in that the unit was designed – all through-hole parts, mechanical connections either soldered or nuts and bolts, and all components labelled. I can imagine that during the lifespan of the CFC250 it would have been relatively simple to repair. Such is the price of progress. And yes, it worked after putting it all back together again.
In the meanwhile, full-sized original images are available on flickr. I hope you found this article of interest. Coming soon we will have some more older-technology items to examine and some new tutorials as well.
Today we examine a kit that is simple to construct and an interesting educational tool – the Sparkfun Frequency Counter kit. This is a revised design from a kit originally released by nuxie1 (the same people who brought us the original function generator kit). As a frequency counter, it can effectively measure within the range of 1 to a claimed 6.5 MHz. Unfortunately the update speed and perhaps accuracy is limited by the speed of the microcontroller the kit is based upon – the Atmel ATmega328. Arduino fans will recognise this as the heart of many of their projects.
Interestingly enough the kit itself is a cut-down version of an Arduino Duemilanove-standard board, without the USB and power regulation hardware. The ATmega328 has the Arduino bootloader and the software (“sketch”) is open source (as is the whole kit) and easily modifiable. This means you can tinker away with your frequency counter and also use your kit as a barebones Arduino board with LCD display. More about this later.
This becomes more obvious when looking at the PCB:
It was a little disappointing to not find any power regulator or DC socket – you need to provide your own 5V supply. However Sparkfun have been “clever” enough to include a cable with JST plug and socket to allow you to feed the frequency counter from their function generator kit. In other words, buy both. Frankly they might as well just have produced a function generator with frequency counter kit all on one PCB. Anyhow, let’s get building.
The kit comes in a nice reusable stiff red cardboard box. One could probably mount the kit in this box if they felt like it. The components included are just enough to get by. The LCD is a standard 16 x 2 character HD44780-compatible display. (More on these here). It has a black on green colour scheme. You could always substitute your own if you wanted a different colour scheme:
An IC socket is not included. You will need to install one if you intend to reprogram the microcontroller with another Arduino board.
Assembly was quick and painless. I couldn’t find any actual step-by-step instructions on the internet (Sparkfun could learn a lot from adafruit in this regard) however the component values are printed on the PCB silk-screen; furthermore no mention of LCD connection, but the main PCB can serve as a ‘backpack’ and therefore the pins line up.
To make experimenting with this kit easier I soldered in some header pins to the LCD and matching socket to the main PCB; as well as adding pins for an FTDI cable (5V) to allow reprogramming direct from the Arduino IDE:
So there are in fact two ways to reprogram the microcontroller – either pull it out and insert into another Arduino board, or do it in-place with a 5V FTDI cable. Either way should be accessible for most enthusiasts. At this point one can put the screen and LCD together and have a test run. Find a nice smooth 5V DC power source (from an existing Arduino is fine), or perhaps plug it into USB via a 5V FTDI cable – and fire it up:
Well, that’s a start. The backlight is on and someone is home. The next step is to get some sort of idea of the measurement range, and compare the accuracy of the completed kit against that of a more professional frequency counter. For this exercise you can observer the kit and my Tek CFC-250 frequency counter measuring the same function generator output:
As you can see the update speed isn’t that lively, and there are some discrepancies as the frequencies move upward into the kHz range. Perhaps this would be an example of the limitations caused by the CPU speed. Next on the to-do list was to make the suggested connection between the function generator kit and the frequency counter. This is quite simple, you can solder the included JST socket into the function generator board, and solder the wires of the lead included with the frequency counter as such:
When doing so, be sure to take notice about which PCB hole is connected to which hole, the colours of the wire don’t match the assumed description on the function generator PCB. Furthermore, the voltage applied via the WAVE pin (the frequency source) should not fall outside of 0~+5V.
As mentioned earlier, this kit is basically a minimalist Arduino board, and this gives the user some scope with regards to modification of the software/sketch. Furthermore, the kit has been released under a Creative Commons by-sa license. So you can download the schematic, Arduino sketch and EAGLE files and create your own versions or updates. If doing so, don’t forget to attribute when necessary.
Overall, this was anther interesting and easy kit to assemble. It is ideal for beginners as there isn’t that much soldering, they end up with something relatively useful, and if you have a standard Arduino Uno or similar board you can upgrade the firmware yourself.
However as a standalone frequency counter, perhaps not the best choice. Think of this kit as an educational tool – involving soldering, Arduino programming and learning how frequency counters work. In this regard, the kit is well suited.
You can purchase the kit directly from Little Bird Electronics. As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our Google Group.
High resolution images are available on flickr.
[Note - The kit was purchased by myself personally and reviewed without notifying the manufacturer or retailer]
Otherwise, have fun, be good to each other – and make something!