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By Oleg Mazurov  Fan regulator
This simple mod, originally described by tinhead on EEVBlog forum, is intended to make built-in cooing fan quieter. Typical problem with cooling fans is the noise they make and typical solution is to decrease voltage (and therefore current) to the fan to make it rotate slower. Hantek DSO5000-series oscilloscopes have 3 terminal 12V regulator dedicated for the fan (pointed at by an arrow on the title picture). To change the fan voltage I simply need to replace the regulator marked U7 on the power supply PCB.
Tinhead’s original mod was to replace 7812 (12V) regulator with 7805 (5V). It will work assuming good quality fan. Mine refused to start from 5V so I used MC7808 – a 8V regulator. Another good candidates were 7806 and 7809 but after first try I decided 8V is good enough – the fan is spinning well and the noise level is low.
The following picture shows power supply PCB unscrewed from the chassis and turned upside down. Again, an arrow shows the regulator. The PCB is single layer; replacing the regulator is a simple matter of removing solder with a solder wick, pulling the original regulator out and soldering on the replacement. If everything is at hand, the mod takes about 15 minutes to complete, including testing.
Enjoy!
Oleg.
 Bottom side of the PCB
By Oleg Mazurov  Hantek DSO5000 screen Some time ago I realized that I need to add digital oscilloscope to my set of instruments. DSO is handy for measurements and taking screenshots and this is what I have been doing a lot lately. After comparing specs of current models from several manufacturers I picked Hantek DSO5102B scope. The main reasons to choose this model were cost, screen size, and rich potential for hacking – in no particular order.
Hantek DSO5000-series scopes come in 3 bandwidth variants – DSO5062B(60MHz), DSO5102B(100MHz), and DSO5202B(200MHz). They are identical or nearly identical (early production scopes of different bandwidth had different value resistors soldered in analog front end but that’s the only difference), and making one from another is a simple matter of editing certain configuration files inside the scope. In addition to that, many other modifications can be made, including fan speed, low jitter ADC clock, low noise power supplies, and many others (see the EEVblog thread – the first link in the list at the end of this post). Newer benchtop DSO/MSO oscilloscopes from Hantek are based on the same hardware and there were successful attempts to make a MSO from this scope by adding a logic analyzer PCB.
As is often the case with Chinese products, the oscilloscope is also available under brand names Tekway, Voltcraft and some others with model numbers different from Hantek. The firmware has been originally developed by Tekway and uses Tekway model numbers – DST1062B(60MHz), DST1102B(100MHz), DST1202B(200MHz). Out of many brand names, Hantek seems to be the cheapest. Also, where I live (the US) Hantek can be bought locally. When I was shopping around, 60MHz models were more expensive than 100MHz so I ended up buying DSO5102B from Hong Kong for $388.88 shipped by UPS Global Express.
After receiving the instrument I checked the functionality. The scope worked well, the screen was large, and bugs were tolerable. I then proceeded to “modify” the device to 200MHz model; what follows is the detailed description of the steps I took to implement the mod outlined in EEVblog thread (Tinhead, you are the man!) – proceed at your own risk! The instrument has no “warranty void” stickers and I’m assuming that from the warranty standpoint opening the case is OK but I never bothered to actually check this assumption. In the worst case the instrument would have to be shipped to the seller/manufacturer for repair or replacement. Also, the procedure involves operating the device with power supply exposed – please be careful!
Continue reading Hantek DSO5000 series oscilloscope modifications. Part 1 – doubling the bandwidth of DSO5102B.
By Oleg Mazurov While making some measurements today I noticed a strange reflection in the signal generated by my Tektronix 7S12 TDR sampling plugin. At first, it looked like a bad connection at the output of the loop-through S-6 sampling head, however, after further testing it turned out to be more serious flaw. The following two screenshots show the reflection: the left one shows the step into terminated S-6 and the right one shows the same step with terminator removed to give me the reference for the distance. Apparently, the reflection occurs inside the head itself!
 Reflection with upper connector terminated by 50 ohms |
 Reflection with termination removed |
 S-6 sampling head disassembled
I pulled the head out and took it apart. It was rather easy – after removing 3 screws on the back the head internals slide forward from the case. The picture on the right shows the guts of the sampling head with preamplifier board removed. The square metal box next to the faceplate with two SMA females sticking out of the back is a hybrid sampling bridge and according to the distance to the reflection this is the place where I need to look.
Further disassemby is easy. First, 3 socket head screws need to be removed along with the board they are holding. After this is done, both male SMA connectors need to be unscrewed. Lastly, after unscrewing two nuts on the faceplate the bridge can be freed from all obstacles and taken apart.
 S-6 sampling bridge Luckily, taking the hybrid apart was not necessary this time. I discovered that one of SMA connectors (upper left on the picture) came loose so I simply tightened it up, assembled the head, placed it back into 7S12 and observed a nice step with good corners and no extra reflections. The whole project took less than an hour, most of the time spent taking pictures.
I’m a happy time-domain reflectormetrist again, writing this article hoping it will be helpful for someone. Next time your sampling head starts acting funny don’t run to eBay for the new one (and I checked – they are out of S-6 at the moment). Take it apart and look inside – you may be able to fix it, even though in my case it was more by luck than judgement.
 Correct step waveform
By Oleg Mazurov  Crossover distortion I was playing with some analog circuits and while reading this article made a test circuit to get better understanding on how an operational amplifier maintains linearity using negative feedback. While experimenting with it I produced enough content to make a short article so I did just that in hopes it will be interesting to somebody. I also made a video, which is available at the end of the article.
1. The problem
While driven by a sine wave, push-pull stage constructed out of PNP/NPN transistor pair exhibits so-called “crossover distortion” (see Wikipedia article linked above). The reason for this is a dead band – a region on a waveform between negative Vbe and positive Vbe where both transistors are closed, don’t conduct and consequently do nothing. The green oscilloscope trace on a title picture shows the distorted waveform. The modified circuit from Wikipedia article has been breadboarded to produce the waveforms. I also made an LT Spice model – it could be useful if you don’t have parts to build the circuit or want to study circuit details which are difficult to measure in real life. LT Spice is available for free from Linear Technology.
Couple notes about the circuit. For frequencies that I’m using none of the parts are critical. I used TL081 op amp an 2N3904 and 2N3906 transistors. Other will work and for LT Spice model I randomly grabbed Linear’s opamp and simulation results are surprisingly close to the real circuit. I will be using simulation for the rest of the article; real circuit can be seen in the video.
When we load a model into LT Spice, run the simulation and then probe voltages on the opamp output and push-pull output, we will see waveforms as on the following screenshot. The green waveform is the opamp output and the blue one is power stage. If kept within specified levels opamp is quite linear so there is no surprise its output faithfully reproduces the input sinewave. The opamp output drives the power stage and power stage tries to reproduce the sinewave but it can do nothing when input signal is within the deadband.
We know that the original negative feedback idea came to Harold Black when he tried to solve very similar problem – how to keep a circuit linear while using non-linear components. In our case, transistors on the real circuit are not matched and we can see that in addition to the crossover the negative half of the waveform has larger amplitude as well. Note that a simulated waveform is perfectly symmetrical and this is why any simulation result should be used with caution.
Let’s modify our circuit and move the right side of the feedback resistor to the output of the power stage. This will allow the opamp to sample the output and make an attempt to correct it. The result can be seen on the following screenshot. The opamp output is now sprints through a deadband as fast as its slew rate allows and as a result the output of the circuit is now much closer to a sinewave.
To understand what is happening let’s make another measurement (and yet another screenshot). Here, the red trace shows the inverting input of the opamp. It is a sum of generator signal and output signal which is in antiphase to the input so the red waveform really represents a difference between them. For the most part the difefrence is very small and we see a straight line. However, in the beginning of a deadband the difference between the input and the output becomes larger and the vertical spike representing this difference starts driving the opamp output correcting the non-linearity of the transistor.
Note the peak-to-peak amplitude of the signal. This measurement is quite difficult to make on a real circuit as 300uV is well below the sensitivity of most oscilloscopes. That’s where simulators shine – any signal can be easily visualized, no matter how small or fast. If you are curious, you can probe the circuit in other places observing other signals like voltages across resistors or currents in and out of transistor terminals.
By changing the model once again, the shortcoming of opamp can be seen. Here, I increased the frequency of the input signal to 100KHz and due to the finite slew rate the opamp is having difficulty following the signal. The distortion is back.
This is all. Play with the model, watch the video (link), have fun!
Oleg.
By Oleg Mazurov  Dummy load Jig Complete Here is a little jig I made to test and characterize the BLDC controller I wrote about a while back. It is a dummy load consisting of two coupled motors: one driven by a controller and another having its windings shorted either directly for maximum load or through series resistors when measured load is desired. Title picture shows finished jig (click on it to make it larger). The construction details follow.
Two brackets made of 2″ aluminum angle profile hold 50-size brushless outrunner Chinese motors rated at 100A. The brackets are bolted to 0.5″ polycarbonate base. The motor shafts are coupled with a flex coupler. Finally, the contact plates are bolted next to each motor – this way if I burn a motor, changing will be easy. The high-current wires are soldered to the female contacts. I’m using double wires to increase current capacity of the wire and also to allow observing half of the flowing current with my little 50A current probe.
I tested the load with my prototype BLDC controller and was very pleased with results. The testing is documented in the short video – check it out.
By Oleg Mazurov  Access to 109V test point In this short article I want to share a trick that I learned today while checking the power supply of my trusty Tektronix 7104 oscilloscope. Step A2 of calibration section of the manual calls for measuring/adjusting of pre-regulated 109V voltage on TP1326 test point. Typically, test point access for this step requires removing power supply cover which takes time and exposes high voltages. The test setup presented on a title picture( click on it to make it bigger) shows how to access this test point leaving power supply cover in place.
The power supply cover is at ground potential so don’t try to reach the test point with non-insulated probe. I used Tektronix Klip Chip IC probe threaded through nearby ventilation hole to grab the test point post. The post is clearly seen through a larger hole, through which an adjustment potentiometer R1293 is usually accessed. A flashlight is handy.
Next picture shows the closeup of the test connection. A test probe is supported by a “Third Hand” thingy – I wanted to observe voltage fluctuations in the course of two hours to make sure it stays within limits.
I’m hoping this trick will be useful for somebody dealing with similar power supply.
Oleg.
By Oleg Mazurov  Modified USB Isolator David Peters shared his modifications to my USB Isolator board. The board can be seen on a title picture (click on it to make it bigger), the design files can be downloaded and below is the list of modifications:
- Isolated VBUS supply has been added
- DC/DC converter can be enabled or disabled by jumper to choose external power supply
- 7805 linear regulator can be used instead of DC-DC converter
- Optional additional capacitors for better filtering
- Enumeration reset possible with jumper
- LEDs for input and output power visualization
- Isolation between areas is 6mm providing ~600V protection
By Oleg Mazurov  Linear power supply Several days ago I received an e-mail from Larry Owens, my fellow Coloradoan. Larry is Hi-Fi enthusiast; he built USB isolator kit and made very elegant and clever modification to it – a linear power supply. Larry’s design fits on existing board footprint.
Here is a quote from Larry’s e-mail:
Thought you might like to see what a little creativity can yield — did not fancy a SMPS’s noise, but wanted to be able to handle various battery sources. Since the off-board supply would be rather quiet, just selected a plain old 7805C (found in drawer, able to dissipate plenty of heat w/ 12v+ supplies and full 500mA USB loads). Thought a power-up LED would be nice too… …the challenge was to find existing pads for all components without having to use any cut/jumps… do consider that outcome lucky at the least…
Take a look at the title picture (click on it to make it bigger). TO-220 next to the barrel connector is old trusty 7805 linear regulator. Small bypass capacitor is soldered between output and ground pins of it. Input and output pads are occupied with electrolytic caps, 100 ohm ( @100 MHz ) ferrite bead sits on inductor pads. Finally, a small LED is mounted in place of 3300pF cap with current-limiting resistor across LT1376 pins 7 and 8.
This is it – simple, elegant and very useful. Thanks again, Larry for sharing the design and very nice and clear picture!
Oleg.
By Oleg Mazurov  Soldering ADuM4160
Long overdue USB Isolator Assembly Guide is finished. It describes building Analog Devices ADuM4160-based USB Isolator kits. The guide contains component identification, step-by-step building instructions and soldering hints. Those who has their isolators already built will find useful powering and tweaking information at the end of the guide.
If you see any errors or have any questions, please let me know!
Enjoy!
Oleg.
By Oleg Mazurov  Magnetic probe amplifier connected to external trigger input I made (hopefully) last iteration of magnetic probe amplifier board. Schematic remains the same. Layout, however, is slightly different. First, I made it more narrow to better fit Tektronix 7000-series time base plugins external trigger input, as can be seen on the title picture. Second, the amplified probe output is made via SMx type connector – PCB-mounted SMA and SMB all have the same footprint. I used straight SMB since I have a surplus of Tektronix P6041 cables. The board layout permits soldering right-angle connector here as well. This arrangement is much handier than previous one.
Since publishing initial design I haven’t seen much interest in it, so instead of ordering a bunch of PCBs I made this board available on BatcPCB Marketplace. Schematic and board layout in Eagle 5.x format are also available. I built one board and haven’t found any errors on copper or silkscreen – if you find any, please let me know.
Oleg.
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