Buing tips about Used Function Generator  - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 14, 2011, 03:35:57 pm
content: There is plenty of offers on ebay and pricing vary too.I need an frequency generator up to 11 or 20Mhz, plus Duty cycle.The questions are many:1) is it worth getting a very old one from a major brand?2) is it worth getting fresh model from unbranded channel?3) is the 20Vpp a standard speaking about output?4)does it worth getting a old programmable instead of an analogue?5) is there specialized people about function generators calibration or repairs?6) does it worth trying to repair a broken one if you are not that expert on them ?

author: robrenz
author_rank: Super Contributor
timestamp: December 14, 2011, 04:13:04 pm
content: If it is a DDS FG remember the max Hz specs are for a sine wave. All the none sine waves will be significantly less Hz so you may need a higher spec to get the square wave Hz you need.

author: kaz911
author_rank: Super Contributor
timestamp: December 15, 2011, 01:39:28 am
content: Quote from: Kiriakos-GR on December 14, 2011, 03:35:57 pmRight now it seems like there are a lot of new FG's coming to the market at substantially lower prices.The Frequency sets the price (more or less)I'm waiting for the Rigol DG4000 series. It does not have a super sample memory (16k points). But price range is from $800 for 60MHz dual channel 14 bit FG to about $1600 for the 160MHz.http://int.rigol.com/prodserv/DG4000/If that does not work - i'll start searching again :-)20Vpp is not standard. Some are 5v, some are 10v, some are 20v, some are 20v P2P OC but only 10V on circuit.

author: george graves
author_rank: Super Contributor
timestamp: December 15, 2011, 02:13:38 am
content: Hope this isn't seen as a thread jack, but how would one go about testing a used FG? Throw some different loads on it's output and see how it performs?

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 15, 2011, 02:12:15 pm
content: Regarding output voltage: 20vpp is indeed quite typical, but only in high impedance mode. In 50Ohm mode, most generators reach only 10vpp.And about PWM mode: be aware that most generators are crippled regarding the duty cycle range. E.g. older model from Agilent support only 20% to 80% duty cycle - even for low frequencies. Some Rigols etc. only support 50% above a certain frequency range. That makes the PWM/DC feature kinda pointless for certain applications.The next question would be if you need arbitrary mode and/or remote control via Visa/SCPI. Both can be very handy, but ARB mode is very limited on most affordable generators (e.g. 1MPoint would be state of the art, 16k would be acceptable, but everything below that is kinda useless). Also while e.g. Rigol generators support Visa/SCPI, the command set is somewhat crippled if you compare it to an Agilent 33521A (which is the best affordable arbitrary generator anyway).Last but not least, the question is how much money you want to spend.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 15, 2011, 03:03:51 pm
content: Quote from: 0xdeadbeef on December 15, 2011, 02:12:15 pmIt is worth pointing out that there is a common misconception that function generators have "high Z" and "50 ohm" modes. The output impedance of almost all function generators is always(*) 50 ohm. When driving a 50 ohm load, the voltage will be half of what a high impedance load sees. The impedance setting is for display only it doesn't change the output impedance of the generator.(*) Some function generators will have 75 ohm (for CATV) or 600 ohm (for low frequency audio). Usually these will be completely separate output jacks from the 50 ohm output, or possibly a real mechanical switch. This is different than the 'high z / 50 ohm' mode found on many digitally controlled function generators and AWGs.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 15, 2011, 03:21:56 pm
content: Quote from: ejeffrey on December 15, 2011, 03:03:51 pmIt is not a misconception that they have different output modes, as they have. The misconception would be to expect that these modes have a physical meaning beyond changing the output amplitude tosimulatea different load.

author: grenert
author_rank: Frequent Contributor
timestamp: December 15, 2011, 04:02:47 pm
content: Quote from: 0xdeadbeef on December 15, 2011, 03:21:56 pmI don't think the amplitude changes at all, just the display of output amplitude. Sort of like if you had a standard weight, and it had two values on it, depending on whether you were at sea level on Earth, or on the moon

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 15, 2011, 05:41:05 pm
content: Quote from: grenert on December 15, 2011, 04:02:47 pmAt least the generators I know (Agilent) switch to half the voltage when switching to 50Ohm and double the voltage when switching to "High Z". Or to be more accurate: they keep the output voltage but half/double the displayed voltage.However when you load e.g. a 5V Vpp arbitrary function it will be output as 5V only in "High Z" mode. If you load it in "50 Ohm" mode, it will be output as 10V Vpp. Agilent claims that this is supposed to be this way, but I find it a little weird to say the least.

author: robrenz
author_rank: Super Contributor
timestamp: December 15, 2011, 05:48:56 pm
content: The Rigol 1022 allows you to enter the actual load you are driving in ohms. When it is correct the actual output will match the setting or displayed voltage. If you dont have a 50 ohm load and you dont set the load value the actual output will not match the selected voltage.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: December 15, 2011, 06:45:28 pm
content: Well currently I have in my hands one PHILIPS (Fluke) PM 5134,1mHZ-20MHz50ohn & 600ohm output selectable by switch10-90% Duty cycle, but it actually does 10.10 - 88.00%Looks robust enough, but I got the specific as a loan, and it looks that in needs calibration.Basically inside haves lots of PCB and it is a bit heavy, but to manually adjust a setting at the max accuracy it takes ages.From all your opinions about the specifications, it looks that I have a missile in my hands with bad steering wheel.

author: don.r
author_rank: Frequent Contributor
timestamp: December 15, 2011, 07:21:29 pm
content: The VERY old ones (from the 70s/early 80s) are very simple units and thus easy to repair. Many have schematics or block diagrams available online with full calibration and debugging procedures (particularly Wavetek models). Stability and precision are usually what's lacking in the older models but good ones can last many decades.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: December 15, 2011, 08:03:49 pm
content: We have some PM 5131 and 5132 (which are obviously prequels to the 5134) at work, but nobody actually uses them as they are considered old crap. In most of them some switches are broken and/or the output is erratic. Besides, they never really were precise. Probably not even in the early eighties or whenever they were built. As far as I recall (it's probably more than 10 years ago that I actually tried to use one of these things), they also had a lot of jitter. Dunno if the 5134 is a lot better, but I somewhat doubt it.