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Final Touches for my 2.5GHz 8-digit Frequency Counter

I've been busy adding the finishing touches to my new Chinese-made frequency counter. It uses one of those cheap modules purchased over the Internet. To complete the counter, I designed and printed a compact enclosure, and then set about the final assembly. A new translation of the operating manual completed the job.

Introduction

I have designed and built a number of different frequency counters over the past twenty years. However, I don't have any frequency counters which can count frequencies above 500MHz. That became apparent recently when, several times, I’ve needed to measure some UHF frequencies. I was forced to build a small prescaler module as a temporary solution. That wasn’t an ideal solution for the longer term, so I began exploring options to fix this.

Available Options

After weighing up several published designs and a number of designs from various magazines and websites, I took a closer look at the low cost frequency counter modules now widely available from the usual Chinese sources on the Internet. These fall into three main types; Simple 5 digit LED frequency counter modules operating up to about 50MHz, 8 digit LED counter modules operating up to 2.5GHz, and complete 8-digit frequency counters with one or two inputs complete in a case with integrated AC power supplies.

The 8-digit frequency counter modules seemed to fit my requirements quite well, and the prices are so low, it is nearly impossible to see how I could make one myself for the same cost. While there are many suppliers, they all seem to offer the same product, albeit with a choice of red, green or blue LED displays. I decided to buy the red LED version, and within a few weeks, it was in my hands.


While the frequency counter module met all my technical and price requirements, and I even had a suitable 12V 300mA power supply for it, I needed some kind of enclosure for the counter module. I couldn't leave the BNC input connector and the DC power connector just floating around at the end of the somewhat thin connecting cables.

Designing and Building an Enclosure

There are several ways to mount the counter module and connectors. The best method would be to make a metal box since this would provide shielding for the counter. I don’t have a suitable workshop or tools for that, so I explored the alternatives. The most obvious solution is to buy a simple metal box and mount the module and connectors in it. That’s certainly the easiet approach, but all of the boxes that I could find seemed to be much too large. They would take up far too much bench space. Besides, they looked ugly!

I decided that the best approach was to design a suitable 3D printed box. While the counter module will not be shielded in this box, I did not feel that it would be a significant problem in use. Also, I could custom design a much more compact solution, hopefully one that was more attractive than one of those simple square metal boxes.

My design was completed as usual with DesignSpark Mechanical. This is a reasonably easy to use free 3D design package which I really enjoy using. I experimented initially with some folded paper shapes before coming up with the final basic hexagonal design. This shape kept the overall enclosure size to an absolute minimum while also allowing the counter to sit on the workbench, either on its base or its back, as well as in its usual place on top of some other test equipment. 


Since my printer is limited to 150mm in each axis, that set the maximum possible enclosure size. Once the basic shape was determined, it became clear that the best way to print it was in a vertical “tube”, with separately printed end caps and buttons. The BNC connector is fitted at the left hand side of the front panel.



 



The counter module’s pushbuttons are quite small, and the new case required the switches to be each fitted with a short extension. These were just pushed into place on top of the existing pushbuttons.

Figure 4 : Two small printed caps are inserted through the front panel onto the existing PCB switches once the module is mounted in the case

Standard PLA filament was used to make the enclosure, printed in 0.2mm layers. The industry-standard STL format files for the main case, the end caps and the two button caps are all available for download (You will find them at the end of this webpage, below).

Final Assembly

Once all of the parts for the case were printed, the counter module was slid into one end and mounted into place in the main case using four 6mm long 3mm diameter cap-head screws. These must be purchased separately because these are not supplied with the module.


The module is supplied complete with two short wire leads, each with a PCB connector to match the connectors on the module. The DC cable was shortened to suit, soldered onto the DC socket, and pushed into place. Although the DC connector is a tight fit in the case, a drop of hot-glue was added to firmly hold the DC connector in place.

The front panel artwork was printed on a colour laser printer, covered with clear contact plastic film, and glued into place. Once this has been fitted, the BNC connector can then be mounted. The short connecting wires on one of the provided cables should be cut to a suitable length, and then soldered in place on the fitted connector. This has to be done carefully because too much heat from the soldering iron can melt the case!


The switch caps were then inserting through the holes in the front of the counter box and pressed into place. Finally, the end caps were pushed on. A drop of glue can be added if necessary to hold these firmly in place.

Counter Labels

The various front and back labels were designed and printed on paper using a colour laser printer. The various holes were then cut out with a sharp hobby knife, and each label then covered with self-adhesive transparent plastic. With the exception of the main display opening, the clear plastic film covering all of the other holes was trimmed off and the labels then glued to the enclosure.

Operation

The PDF manual from the suppliers left a great deal to the imagination, in part because it was all in Chinese. Not being able to find a good English version online, I translated the Chinese text into English, and redid the flow diagrams for good measure. A copy of this manual complete with the new flowcharts can also be downloaded below.

There are two buttons on the right edge of the counter. The upper button is the “SET” key. This allows programming of an offset frequency, for example when the counter is used as a frequency display in a receiver or transceiver, and the selection of the counter input channel. A digital filtering mode, LED display brightness and least significant digit blanking functions can also be selected with this button.  The lower “” button is used to make subsequent selections. For most situations, these two buttons can be ignored.

Final Comments

While there are two input channels in the counter module, one using a discrete component RF amplifier for frequencies up to about 50MHz and the other channel having an IC prescaler for frequencies up to 2.5GHz, the inputs for these channels are simply combined onto the counter’s single PCB RF connector. This is a little "agricultural", but it does avoid the need for additional circuits and parts to switch between the inputs. It comes at the cost of reduced sensitivity and a somewhat uncertain input impedance.

I briefly looked at separating the input channels to allow the use of two input connectors. This would improve the input matching and sensitivity. However, the location of one of the channels under the permanently soldered LED displays made that impractical, so the module was left unmodified. It seems to work well enough for my purposes, so that was probably a very good choice.

I was able to adjust the counter’s calibration preset (See the manual below for more details) to match my GPS-based frequency reference, making it accurate within 1Hz at 20MHz. That was done after I first turned the counter on and I let it stabilize to room temperature for about 10 minutes.

In practice, the counter has proven to be sensitive, accurate, and easy to use. I have no complaints, especially given the excellent price. In particular, I like the convenient size and shape of my new 3D printed counter enclosure.

I hope you give it a try too.

Downloads:

Counter Box 3D Files: This ZIP file includes all of the industry-standard STL files for the main case, the end caps and the pushbutton extensions

ENGLISH version of the operating manual: This PDF file is my translation of the original Chinese manual and includes translated diagrams and some additional information about  sensitivity test results

Panel Labels This JPG file contains the artwork for all of the labels used on the counter. There are everal different colours available in this file

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© 2016 Andrew Woodfield ZL2PD
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