EEVblog Electronics Community Forum

Products => Test Equipment => Topic started by: araugh on March 04, 2013, 10:40:15 pm

Title: Signal Generator advice
Post by: araugh on March 04, 2013, 10:40:15 pm
Hey there folks, long time lurker.  I have a situation that requires a very specific set of requirements from a signal generator.  I'm looking to generate 2 sine waves, 10v peak to peak, at frequencies between 1hz and 200khz.  The difficulty is, they have to be offset so that one = sin and the other = cos.  Basically so the X/Y will be a "perfect" circle.  With what we do we've never really had a need that required having a signal generator around that we couldn't accomplish by other means, so I frankly don't have a ton of experience shopping for one, but this scenario is a showstopper without a good, accurate pair of signals.  My budget is preferably in the 3-800USD range, but I can stretch that to about 1200USD if needed.

Any advice on what to go towards or avoid?  I can look at datasheet after datasheet but none of them seem to tell me if I can do a cos offset.  Is this such a standard feature on 2 channel generators it's not even mentioned, or is it rare enough that I'm better off finding a way of offsetting the sync between 2 small cheap generators?

Thanks for any advice you folks are willing to give.
Title: Re: Signal Generator advice
Post by: c4757p on March 04, 2013, 10:49:25 pm
No two-channel function generator that I have ever seen is incapable of this, it's just a 90° phase angle.
Title: Re: Signal Generator advice
Post by: GK on March 04, 2013, 10:59:00 pm
A State Variable Oscillator produces quadrature signals.

See my THD analyser project ("Generator" schematic) for one example of many:

http://www.users.on.net/~glenk/thd/thd.htm (http://www.users.on.net/~glenk/thd/thd.htm)

Your other option is DDS.


EDIT: oops, sory, wrong forum section; you're looking to buy a commercial unit, not build something.
Title: Re: Signal Generator advice
Post by: LaurenceW on March 04, 2013, 11:17:49 pm
An easy one, I reckon! I am looking at a perfect circle on my 'scope right now - anything from a few Hz to 200Khz my sig gen is producing two independent outputs of sine waves, locked to the same source frequency, and 90 degrees (or anything else I wanted to specify) apart. Sin and Cos!

Twelve hundred bucks, you say? OK, that will be $300 of the Rigol DG1022, and $900 for my expert advice ^-^

oh, ok - SMALL caveat - looks like Channel 2 only goes up to 6V PK-PK. I will reduce my fee to $800 accordingly...

Laurence
Title: Re: Signal Generator advice
Post by: araugh on March 05, 2013, 12:55:55 am
great!  It sounds like the rigol DG4062 is the right way to go for this then.  Thanks so much for the assistance folks, great group of people here.
Title: Re: Signal Generator advice
Post by: Wuerstchenhund on March 05, 2013, 08:34:34 am
great!  It sounds like the rigol DG4062 is the right way to go for this then.  Thanks so much for the assistance folks, great group of people here.

The Rigol is fine for hobby use but I'd just be careful if this is for business use (i.e. it needs to earn you money and you need some kind of reliability), as in this case I'd rather buy something from one of the established brands like Agilent.
Title: Re: Signal Generator advice
Post by: nctnico on March 05, 2013, 09:14:08 am
So you recommend buying a Lecroy Siglent SDG1000 series?   ;)
Unless you buy really high end gear everything comes from China and is more or less rebadged equipment also sold under other brand names. Quality of the casing, features and some component choices may be lesser but its still decent gear.
Title: Re: Signal Generator advice
Post by: araugh on March 05, 2013, 02:51:49 pm
Fair enough, but given the the Agilent that I'd need to do this is 2500USD, that makes the purchase a bit less palatable.  This is for a business, but it's not for day to day use.  If it becomes a problem we'll upgrade.  Normally I'm the "buy once, cry once." sort, but given it's limited use in this case I'm willing to risk it.

Thanks!
Title: Re: Signal Generator advice
Post by: Marco on March 05, 2013, 03:50:55 pm
Is the spectral purity for the Rigol sufficient?
Title: Re: Signal Generator advice
Post by: AndyC_772 on March 05, 2013, 09:32:30 pm
Don't knock the DG4062, it's a perfectly decent piece of kit IMHO. I have one and see no reason why it shouldn't be suitable, I'm looking right now at a perfect circle on my scope in X-Y mode with an amplitude of +/- 10V in each axis, and a frequency of 200 kHz.

All you need to do is set up the DG4062 like this:

(http://www.andrewcawte.webspace.virginmedia.com/Rigol29380.JPEG)

... and the result is this:

(http://www.andrewcawte.webspace.virginmedia.com/TEK00026.png)
Title: Re: Signal Generator advice
Post by: Telstar on March 05, 2013, 10:03:28 pm
A State Variable Oscillator produces quadrature signals.

See my THD analyser project ("Generator" schematic) for one example of many:

http://www.users.on.net/~glenk/thd/thd.htm (http://www.users.on.net/~glenk/thd/thd.htm)

Excellent work!
Looking forward the completion of your project.
Title: Re: Signal Generator advice
Post by: Wuerstchenhund on March 05, 2013, 10:36:03 pm
So you recommend buying a Lecroy Siglent SDG1000 series?   ;)

Certainly not. It's a toy, nothing more.

Quote
Unless you buy really high end gear everything comes from China and is more or less rebadged equipment also sold under other brand names. Quality of the casing, features and some component choices may be lesser but its still decent gear.

Some gear from the big names may come from China but it's generally produced after their specifications and quality standards, and despite the manufacturing location there is a large organization standing behind the product. That is not equally true for Rigol, Siglent and other China-designed devices, which is primarily designed for low cost, and support is regularly poor to non-existent.

For a hobby, a lower price in exchange for lower quality (esp. firmware) and no real support network may be acceptable, for business use it usually isn't.
Title: Re: Signal Generator advice
Post by: EEVblog on March 06, 2013, 12:49:47 am
The Rigol is fine for hobby use but I'd just be careful if this is for business use (i.e. it needs to earn you money and you need some kind of reliability), as in this case I'd rather buy something from one of the established brands like Agilent.

That  would be the same Agilent whos low end scope is actually a rebadged Rigol?  ;)

Agilent actually (unwittingly) helped create what Rigol is today buy not only re-badging Rigol's gear 5-6 years back, but also helping them with manufacturing quality and I believe some R&D as well. It ended up being of much more benefit to Rigol (who is now a major competitor, and also still an OEM) than it was to Agilent

Dave.
Title: Re: Signal Generator advice
Post by: AndyC_772 on March 06, 2013, 08:12:53 am
despite the manufacturing location there is a large organization standing behind the product. That is not equally true for Rigol, Siglent and other China-designed devices, which is primarily designed for low cost, and support is regularly poor to non-existent.

There's a big difference between the toy and hobby grade stuff from Rigol and their serious professional tools. Take a look at the teardown video that mikeselectricstuff did of the DG4062; apart from embarrassing themselves by scrubbing the numbers off a couple of chips, Rigol did nothing wrong with the design and construction at all.
Title: Re: Signal Generator advice
Post by: Wuerstchenhund on March 06, 2013, 04:42:53 pm
There's a big difference between the toy and hobby grade stuff from Rigol and their serious professional tools.

So what are their 'professional tools' then?

Quote
Take a look at the teardown video that mikeselectricstuff did of the DG4062; apart from embarrassing themselves by scrubbing the numbers off a couple of chips, Rigol did nothing wrong with the design and construction at all.

The hardware quality may be fine (but that was already true for the DG1000 Series and other Rigol stuff) but there is much more than just that which makes the difference between a professional tool and something that's only good for hobbyists. Stable firmware for example (not really a strength of Rigol). Support is another thing (Rigol support in the US is said to be ok, here in Europe it's somewhat non-existent). Warranty is also often a problem (no international warranty, and again see the lack of proper support in Europe).
Title: Re: Signal Generator advice
Post by: AndyC_772 on March 07, 2013, 06:49:55 am
You write as though you're aware of a specific firmware bug in the DG4062 that should be stopping me from using it. What bug are you referring to?

I'm aware of just one, the duty cycle measurement which forms part of the frequency counter feature doesn't work properly. Mildly irritating? Sure. Significant effect on the usefulness of the instrument for every other purpose? Of course not.

My 4062 came from Telonic Instruments who have provided excellent support.
Title: Re: Signal Generator advice
Post by: Wuerstchenhund on March 07, 2013, 12:32:51 pm
You write as though you're aware of a specific firmware bug in the DG4062 that should be stopping me from using it. What bug are you referring to?

I'm not aware of a specific bug in the DG4062 (and I never said I was, that's your mis-interpretation). However, looking at other Rigol products, their firmware support is still lacking.

Quote
My 4062 came from Telonic Instruments who have provided excellent support.

So how many support cases have you opened with them?

Anyways, they are a reseller, and while it's of course great if they do support the products they sell, there is only so much a reseller can do (and he might not even be around any more in two or three years time). They simply can't replace proper manufacturer support which is required for many things.

I agree that all this may be a non-issue for hobbyists, but as soon as you have to rely on your equipment to make money then these things become very important and are worth spending a bit more for a device which is adequately supported and reliable. I probably wouldn't mind buying Rigol for my home lab (it's just that other options looked much better so far), but I wouldn't touch it with a barge pole for my professional work. It's just not good enough.
Title: Re: Signal Generator advice
Post by: Wuerstchenhund on March 07, 2013, 12:44:30 pm
That  would be the same Agilent whos low end scope is actually a rebadged Rigol?  ;)

I didn't say I would by anything from Agilent. The Rigol clones would most certainly be off my list, as would be LeCroys WaveAce/WaveStation products (relabelled Siglent/Atten).

Quote
Agilent actually (unwittingly) helped create what Rigol is today buy not only re-badging Rigol's gear 5-6 years back, but also helping them with manufacturing quality and I believe some R&D as well. It ended up being of much more benefit to Rigol (who is now a major competitor, and also still an OEM) than it was to Agilent

That may be the case (it's at least better than what LeCroy did/does with Atten/Siglent gear), and the hardware quality of current Rigol kit is apparently very good. However, they still haven't realized what 'support' really means, or that software plays a similarly crucial part to a product than hardware does.

I'm sure if they stop ignoring the world's largest economy (EU), provide proper support facilities there, fix the software issues and stop bitchering about warranty for Rigol products bought from other parts of the world then they might be taken seriously by professionals. Until then they will remain a brand for hobbyists.
Title: Re: Signal Generator advice
Post by: araugh on March 07, 2013, 01:26:16 pm
So am I a hobbyist because I bought a Rigol, or did I buy a Rigol because I'm a hobbyist?
Title: Re: Signal Generator advice
Post by: saturation on March 07, 2013, 05:57:48 pm
A simple RC circuit provides a phase shift between current and voltage.  You can rig that up for less than $1 using any single channel signal source you have.  The key problem is if you require a strict equal output impedance from both sources.

http://www.physics.ucla.edu/demoweb/demomanual/electricity_and_magnetism/ac_circuits/the_90d_phase_shift_of_a_capacitor.html (http://www.physics.ucla.edu/demoweb/demomanual/electricity_and_magnetism/ac_circuits/the_90d_phase_shift_of_a_capacitor.html)