| Electronics > Repair |
| Calibrating and repairing a function generator versus safety |
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| McMonster:
I've just watched video #217 and I've decided it's time to repair/calibrate my Wavetek function generator, but I'm concerned about safety of this operation. There are informations about this scattered through different videos and a separate one on isolation transformers and I'm going to watch those again, but are there any other practical guides to basic rules of safety when repairing or calibrating bench equipment? My health first, not destroying anything if possible. More specifically, I have Wavetek 172B function generator which is working, but on higher frequencies (say above 1 MHz and it can go up to 13 MHz) all waveforms start to smooth out until they look almost like a sine wave at 13 MHz. I have the service manual and I intend to start with checking the voltages. The most dangerous place is 250 V in the display section. The manual includes all the schematics, location of pots and test points, detailed service procedures etc. My gear include two scopes, two meters (one decent and one cheap, 50$ one), common sense and a plastic calibration tool. Any hints abouth both safety and possible ways to repair the generator? |
| sub:
It sounds like you are seeing the effects of the output filter. Are you certain that this is indeed a fault? If you have been making 13MHz square waves for the last decade on the device then something would certainly be amiss, but the frequency response in the link below suggests that the harmonic content at higher frequencies might be rather diminished: http://www.teknetelectronics.com/DataSheet/WAVETEK/WAVET_172B105856.pdf |
| rf-loop:
1. Output is specified to 50ohm load. 2. Square wave rise and fall time is 15 - 20ns. (specs value <20ns) (it also means, read first user manual and specifications before you thinking it need repair or adjustment. Most time peoples first use equipment and reading manual is last point - or more worse: sometimes peoples make some "this is bad" rewievs before they know how stuffs work in principle and what is specified) . but :) :) also this is natural ) One cycle have 2 edges. Worst case 40ns + this time what need between 0% to 10% and 90% to 100%.. If you have sine where cycle is 40ns what is frequency. 1/.00000004 (25MHz) But normally risetime is specified so that it is time between 10% and 90% points. (sometimes but more rare 20% to 80%) If you have square there is more time just becouse it is measured as 10 and 90%. It can think something like 2 tr + 0.85 tr. (not exactly but just as practical imagine) It means 40ns + 17ns = 57ns. 57ns cycle time sinewave is around 17.5MHz. With this risetime maximum frequency is. 17.5MHz 20ns risetime square looks nearly as some amount distorted sine in practice and level also may be dropped around 3dB. (just also with gaussian BW shape oscoilloscopes there is equation 0.35/tr = freq.) If with this risetime still rise frequency signal level drops more than 3dB and there can not really see any square. So you can easy imagine that 13MHz "square" looks not square at all. It looks very soft corner and with eyes it looks more sinewave as square wave. So, this is nominal for this generator. And it all reads very clearly in specifications. |
| McMonster:
Thanks for the info, I'll analyze it later and do some more checking with DS1052E (only tested it with analog scope before). I read the manual before, but I was not completely sure if it really needs calibration because I have nearly no experience in the matter of signal generators and high frequencies, I've only worked with less than 500 kHz before. All I knew for sure is that this unit spent a few years in poor storage conditions, was sold as untested (got it for less than half the price of a good one) and waveforms looked very bad for my liking. I guess I have a lot to learn. |
| alm:
I'm impressed that rf-loop can determine that the generator is within specifications without even looking at it. That would save a bundle on calibration costs! Based on the specs, it sound likely, but it's hard to tell if that's all that's going on based on just a description. It should be easy to measure rise time with a DSO. Ideal connection would indeed be a coax cable with 50 ohm terminator on the scope side, but given the slow rise time, I don't think it's that critical. Keep in mind that the amplitude will be double the indicated amplitude without termination. Just sticking a probe in the output with a BNC-to-probe tip adapter, or even just a regular ground lead, should give a fair representation of the signal. Then set up the scope to measure rise and fall times (10% to 90% if you have to specify the levels), and it should give something close to the 20ns. Note that aberrations like overshoot can screw up the rise time measurements. There's no harm in doing a performance verification (calibration procedure without doing any adjustments). Best is to use an isolation transformer between mains and the DUT (function generator), especially when working on the primary side of its transformer. Keep your fingers away from the high voltage bits. Be careful that in the past it was common to route mains connections over the main PCB near the low voltage section. The supply to the display is likely fairly low current and much less dangerous than mains. If you feel nervous about reaching a test point, clipping the probe on while the power is off and turning the power on with your hands away from the circuit is another option. Your interaction with the high voltage parts is likely limited to measuring the voltage. Be careful not to let a probe slip, accidentally shorting something can damage the DUT. |
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