BK 1856D Frequency Counter review / teardown - Page 1
Forum section: electronics / testgear

author: alank2
author_rank: Super Contributor
timestamp: August 17, 2016, 08:43:47 pm
content: I've been wanting one of these frequency counters for a long time. I'd been looking it over at Zoro because they have some test equipment and sometimes offer at 25% or 30% off coupon, but even with that it would have been around $325 or so. As luck would have it someone put what looked like a nice one up on eBay and accepted my offer of $175 shipped. I've got to say that I do like the unit so much that I wouldn't have felt bad about buying a new one.This is a reciprocal style of counter which is a feature I wanted because it can offer a full display of digits even for slower frequencies. My older counter will only display "1000" for a 1kHz signal, but a reciprocal counter can display "1000.00000".The heart of any counter is its clock. The 1856D has a TCXO that is pretty stable once warmed up, but like a lot of counters it takes a while to get it there. The manual talks about 30 minutes, but it reached it peak at 37.5 minutes during my test. I ran an 18 hour test and after warmup, it held to within +/- 3.5e-7 which is 0.35 ppm. You can choose to use the internal TCXO or use an external 10 MHz reference signal with a switch on the back of the unit. The BNC on the back is used to input the reference signal if set to external, or it will also output the internal TCXO if set to internal. Connecting something up to this while using the internal TCXO will load the internal oscillator just enough to push readings higher by 3.5e-8 which isn't much, but you can notice IF you do the extra digit hack!I fed my rubidium reference 10 MHz output to the input of the counter and set the longers 10s gate time. I then captured readings for 18 hours to see how much the internal clock wanders.It took some work to calibrate the internal clock this well. I used a scope to trigger on the Rb signal coming from my reference and adjusted the internal TCXO's adjustment screw until the signal was moving just enough leftward as to balance out the loading from viewing the internal clock on the rear BNC connector. The little screw in the TCXO needed to be adjusted very finely and took some back and forth tries to get this close.The extra digit hack is a way to get one more digit displayed even when using the internal clock. The manual covers how you will get 8 digits in the 10s mode when using the internal clock and 9 digits when using an external reference clock. This is probably their way of not having a customer complain at them when the final digit moves from the internal clock, but the internal clock is actually pretty decent. Normally when you flip the switch on the back between internal and external, a wire from this switch is opened or shorted that goes to the CN6 connector of the main board. If it is shorted, you get the extra digit on the display. You can simply unplug it from the mainboard and use a small jumper to short the connector. The pins are little smaller so I had to pinch my jumper just a little for it to make a good connection. Once done, you will always get the extra digit. My theory is that the jumper only affects the extra digit on the display - I looked at the signal on the rear clock BNC connector to see if it acted differently whether CN6 is open or shorted to make sure the microcontroller wasn't somehow responsible for choosing to drive the rear BNC or set it for reference input, but the signal looks the same whether CN6 is open or shorted so I think the switch is responsible for that.The front panel features include a real power switch (not soft power), function selections buttons, a hold button, gate speed button, and finally buttons to adjust the signal A input (X1/X10 attenuation, AC/DC coupling, and a low pass filter). You can select frequency A, frequency C, period A, total, and RPM. I saw someone gripe about a misspelling of period as priod, but I wonder if it is simply a very bad abbreviation. One nice thing about the configuration part of the display is that you have light up letters on a black background so they are easy to see. Input C is the high speed input for 80 MHz to 3.5 GHz and it has a N style connector and says 3V max, 50 ohm impedance. Input A is up to 100 Mhz, 250V max, 1M impedance. I was able to put 120 MHz into Input A using my SDG2082X and it didn't overflow. I wish they had put in independent lights for the second labels, but I am just being picky. You will see "M/n" and "Hz" light up for MHz and "M/n" and "SEC" for nanoseconds. It has a green 9 digit seven segment display. It has indicators for just about everything, even RX and TX for RS232 port use.The other port on the rear panel is a RS232 DB9. It is peculiar in that it demands that DTR is enabled-high and RTS is disabled-low. I think it counts on those two signals for voltages it may need. You'll need to use it with a terminal software that allows you to set the states of these signals, or you can use their handy application that not only lets you control the counter, but has a nice capture to excel feature you can set for very fast (44 rdgs/s), 2sec, 5sec, 10sec, 20sec, 30sec, or 1min. It will launch excel and begin filling in values which is great because you can look through them while it is capturing them. Just make sure you don't open another dialog in that Excel session because you may make Excel unavailable when it tries to insert the next value! It is a handy application though and what I used to measure the internal TCXO above against my Rb clock. I'm not sure the application with work on x64 though, I get an error and was unable to run it unless I was using a 32 bit system.The inside seems finished nicely and it was nice to see a full metal sheild when you take the cover off from the top after easily removing 4 underside bolts. One more screw out of the top shield and you can remove it as well. It looks like there is an Atmel AT89C52 running the show and the mainboard looks nice. They have shielded the input stages. The power supply is near the rear of the mainboard and doesn't look too complicated. You can see on mine that I have the CN6 wire disconnected and have a jumper in CN6 for the extra digit hack mentioned above.