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mapleLC
Help Choose a Used Waveform / Function Generator Upgrade
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on: June 17, , 08:48:14 am »
I have a crappy BK Precision function generator. It does what I need, more or less, but its crappy. Feels crappy to use. Dials are lighter than 80s American car steering, and worst, the frequency drifts if you leave it running overnight on a task etc.
I dont need the latest, I just want a really nice piece that feels good to use and I know will last.
Since I don't know this type of piece that well, I would like to know what everyone considers a really good classic unit. Please note, I don't want anything older than perhaps late 80s, just to contextualize what sort of era I am willing to go back to.
I need a general purpose unit that goes all the way down in frequency, I see units that start at 10mhz etc, I dont want that. It would also be nice to read the voltage output of the signal, not just db or other scaling.
nctnico
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #1 on: June 17, , 09:35:34 am »
Why used? Nowadays you can buy very nice low cost function generators like Feeltech FY or the UNI-T UTG932E. Any used function generator is either crap or way more expensive compared to the units from Feeltech and Uni-t. Don't make the mistake thinking that older (>10 years) units are somehow better; they aren't. Technology has advanced a lot and nowadays you can build a very good function generator from cheap common parts that don't run extremely hot or are under stress.
« Last Edit: June 17, , 09:39:58 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
mapleLC
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #2 on: June 17, , 09:42:15 am »
Why used? Nowadays you can buy very nice low cost function generators from Feeltech FY or the UNI-T UTG932E. Any used function generator is either crap or way more expensive compared to the units from Feeltech and Uni-t.
Because I hate XhinaJunk, planned obsolescence, and disposable products with no real history, large scale support, schematics or maintainability.
A cute display is, cute.
If we had more serious companies, we'd have products which had the right characteristics, perhaps THEN introduce a cute display.
The caveat is that if you spend a Gorillion Dollars, you can get a new nice piece of way overpriced kit like a Keysight. But we get Rigol.
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nctnico
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #3 on: June 17, , 10:32:57 am »
And yet cost wise the Feeltech FY or the UNI-T UTG932E are the best choices if all you need is a simple, general purpose function generator. You can buy the Uni-t from various local suppliers in case you are worried about warranty & support. In the end it is your money; spend it wisely.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
tautech
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #4 on: June 17, , 10:33:56 am »
I have a crappy BK Precision function generator. It does what I need, more or less, but its crappy. Feels crappy to use. Dials are lighter than 80s American car steering, and worst, the frequency drifts if you leave it running overnight on a task etc.
I dont need the latest, I just want a really nice piece that feels good to use and I know will last.
Since I don't know this type of piece that well, I would like to know what everyone considers a really good classic unit. Please note, I don't want anything older than perhaps late 80s, just to contextualize what sort of era I am willing to go back to.
I need a general purpose unit that goes all the way down in frequency, I see units that start at 10mhz etc, I dont want that. It would also be nice to read the voltage output of the signal, not just db or other scaling.
How high frequency do you need ?
What scope might you be pairing it with......there are serious reasons for these questions.
Avid Rabid Hobbyist.
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mapleLC
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #5 on: June 17, , 11:50:15 am »
And yet cost wise the Feeltech FY or the UNI-T UTG932E are the best choices if all you need is a simple, general purpose function generator. You can buy the Uni-t from various local suppliers in case you are worried about warranty & support. In the end it is your money; spend it wisely.
And like I said, thats not what I want. If I wanted that, I wouldnt open up a discussion, I would open a browser window from Amazon and see what china junk their abusive search "algorithms" will favor today.
« Last Edit: June 17, , 11:56:12 am by mapleLC »
mapleLC
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #6 on: June 17, , 11:55:17 am »
How high frequency do you need ?
What scope might you be pairing it with......there are serious reasons for these questions.
High is not as important as low, if I go past audible then it's to test a cap or match a frequency I wouldn't normally encounter. That said, if whatever I end up with had good range at high frequency, it inoculates me from that possible need later on.
I have both modern digital and old analog scopes, so I should be able to pair with most anything.
rob77
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #7 on: June 17, , 12:10:17 pm »
+1 for the FY , it's in a crappy plastic box with lack of gravity in it (it's very lightweight ) , but id does exactly what one could expect from a signal gen.
if you looking for a "cool looking" boat anchor then it's not for you of course
mapleLC
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #8 on: June 17, , 12:39:56 pm »
+1 for the FY , it's in a crappy plastic box with lack of gravity in it (it's very lightweight ) , but id does exactly what one could expect from a signal gen.
if you looking for a "cool looking" boat anchor then it's not for you of course
I cant wrap my head around it. If I use a unit like that once or twice I never want to touch it again. The tactile feel is so poor, the unit is so light and flimsy, and I know the build quality is crap and has tons of components that are designed to fail. I guess its the same mentality you need to enjoy these superhero movies that are just identical bad movies with different characters.
What is funny is how the folks suggesting these crap products are wondering why I'd want an older piece of gear.
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rob77
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #9 on: June 17, , 01:05:31 pm »
+1 for the FY , it's in a crappy plastic box with lack of gravity in it (it's very lightweight ) , but id does exactly what one could expect from a signal gen.
if you looking for a "cool looking" boat anchor then it's not for you of course
I cant wrap my head around it. If I use a unit like that once or twice I never want to touch it again. The tactile feel is so poor, the unit is so light and flimsy, and I know the build quality is crap and has tons of components that are designed to fail. I guess its the same mentality you need to enjoy these superhero movies that are just identical bad movies with different characters.
What is funny is how the folks suggesting these crap products are wondering why I'd want an older piece of gear.
it depends what's your use case...
if you need a signal with the correct waveform, frequency and dc offset for your testing or measurement then you use whatever is able to do it for you while it fits your budget.
your topic is "Function Generator Upgrade" , so people were expecting that you need a fuction gen, not a collectors item
mapleLC
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #10 on: June 17, , 01:13:23 pm »
+1 for the FY , it's in a crappy plastic box with lack of gravity in it (it's very lightweight ) , but id does exactly what one could expect from a signal gen.
if you looking for a "cool looking" boat anchor then it's not for you of course
I cant wrap my head around it. If I use a unit like that once or twice I never want to touch it again. The tactile feel is so poor, the unit is so light and flimsy, and I know the build quality is crap and has tons of components that are designed to fail. I guess its the same mentality you need to enjoy these superhero movies that are just identical bad movies with different characters.
What is funny is how the folks suggesting these crap products are wondering why I'd want an older piece of gear.
it depends what's your use case...
if you need a signal with the correct waveform, frequency and dc offset for your testing or measurement then you use whatever is able to do it for you while it fits your budget.
your topic is "Function Generator Upgrade" , so people were expecting that you need a fuction gen, not a collectors item
The title also says "Used"
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Stray Electron
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #11 on: June 17, , 01:33:30 pm »
Why don't you look for a used HP, Tektronix or other name brand FG on that over-priced auction site? That stuff is built to last! And there's usually complete service manuals, parts lists, etc available for those brands. I always look and see what kind of documentation is available before I buy any brand or particular model.
I hate flimsy disposable TE too and most of the Chinabuilt stuff that I've seen doesn't come even close to meeting the advertised specs.
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, mapleLC
mapleLC
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #12 on: June 17, , 01:43:21 pm »
Why don't you look for a used HP, Tektronix or other name brand FG on that over-priced auction site? That stuff is built to last! And there's usually complete service manuals, parts lists, etc available for those brands. I always look and see what kind of documentation is available before I buy any brand or particular model.
I hate flimsy disposable TE too and most of the Chinabuilt stuff that I've seen doesn't come even close to meeting the advertised specs.
Mainly because I don't know the different models in detail, and I thought perhaps the community had a few favorites like they do scopes.
JohnG
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #13 on: June 17, , 01:52:14 pm »
FWIW, I have gone through a slew of various old signal generators, both function generators and pulse generators. HP, Wavetek, General Radio, TM series Tektronix, etc. Every since one either had or would develop problems. Distortion, noisy pots and switches, bad caps, failed outputs, drift, you name it. Some can be repaired easily, and some you end up taking irreplaceable switches, pots, etc apart in hopes of getting a stable output.
Got a Siglent generator and even though it has its issues, it works reliably and continuously. I have some boat anchors that I would not sell, but I'm done with old signal generators. I want to use it, not repair it.
John
"Reality is that which, when you quit believing in it, doesn't go away." Philip K. Dick (RIP).
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Stray Electron
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #14 on: June 17, , 02:13:06 pm »
I don't use a FG very often so I'm not familiar with all of the different models either. I would just look and see what models are available for sale and then look up those models and see what people on the net and here on Eevblog have to say about them including any repair problems, etc and also what documentation is available, then buy whatever model you think will suit your needs.
I have HP equipment that was built in the s that is still working fine. Unless it's a particular model that is known to be problematic or one that doesn't work and that parts are unobtainium, I wouldn't hesitate to buy that brand. Tektronix is also good but their caps are big problem in older equipment. Personally I LIKE the Tektronix TM-500 equipment and even though it's old and the caps have failures, most of the design and construction is simple and easy to work on. So if if any of those FGs will serve your purpose, grab one and one of the TM-500 main frames. DO test the output transistors in the main frame before you put a plugin in it. Those are prone to shorting when they get old and they will short out and fry many plugins.
It's your choice as to whether you want to buy something that's "Used" and therefore guaranteed (by Ebay) to work, or buy a "for parts" unit and repair it if necessary. Just about anywhere else that you buy from there wouldn't be any guarantee, other than the seller's word.
Stray Electron
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #15 on: June 17, , 02:17:49 pm »
FWIW, I have gone through a slew of various old signal generators, both function generators and pulse generators. HP, Wavetek,
EVERY piece of old Wavetek that I've even gotten in my hands (probably 2 dozen) has been bad! I don't think that there is any service info available for most of it either.
You've been warned!
HalFoster
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #16 on: June 17, , 02:19:59 pm »
First, it would help to give a little more to go on... what will you be using it for? Frequency range? Do you need the waveforms *other* than a sine wave? Are you wanting it for casual use or for more precision applications? Depending on the answers you may well get by with a toy like the Feeltec or it might take a lab grade instrument - or more than one.
To throw out a recommendation without knowing your requirements, it would be, for an used instrument, an HP A, B or on the upper end a HP A. Or, if going with something new, it is very hard to go wrong with Siglent for a attractive price/performance standpoint.
Hal
--- If it isn't broken... Fix it until it is ---
HalFoster
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #17 on: June 17, , 02:26:36 pm »
+1 for Stray Electron's recommendation: be very careful when looking at used equipment - some brands are very poor quality or prone to issues. - You are very much doing the right thing by running your questions by others before buying - we all have made mistakes and (most of us, anyway) don't want to see others make them if they don't have to.
Hal
--- If it isn't broken... Fix it until it is ---
rdsi
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #18 on: June 17, , 02:43:42 pm »
Since I don't know this type of piece that well, I would like to know what everyone considers a really good classic unit.
A great classic is the HP A. These can be had for a couple hundred dallars (US). I have one sitting next to my Siglent SDGX but I only ever fire it up for nostalgic reasons.
MarkL
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #19 on: June 17, , 02:49:56 pm »
If you prefer something more vintage flavored, take a look at the HP A. It's basically an analog function/pulse generator that is digitally controlled, and can be connected to a GPIB controller if you're into that. Output is up to 50MHz +/-8V into 50R and +/-16V into high impedance. The user interface is simple and should satisfy your "feels good to use" requirement.
Look for option 001 which gets you a settable N-burst mode. And I'd strongly recommend not buying one that is "As-is" with the intent to fix it, because they do have unobtanium parts.
I have a A for about 20 years and also a Tek FG504. I would stay away from the FG504. My FG504 is currently working, but has spent more time in the repair pile than any other TE with an assortment of silicon failures and mechanical problems.
ozkarah
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #20 on: June 17, , 03:24:09 pm »
Take a look at Siglent SDGX. Reasonable price with upgrade/hack option up to 120 MHz.
I am very happy with its performance.
nctnico
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #21 on: June 17, , 03:32:48 pm »
Since I don't know this type of piece that well, I would like to know what everyone considers a really good classic unit.
A great classic is the HP A. These can be had for a couple hundred dallars (US). I have one sitting next to my Siglent SDGX but I only ever fire it up for nostalgic reasons.
I can imagine. The HP A and related models are horrible to use.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
nctnico
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #22 on: June 17, , 03:56:51 pm »
+1 for the FY , it's in a crappy plastic box with lack of gravity in it (it's very lightweight ) , but id does exactly what one could expect from a signal gen.
if you looking for a "cool looking" boat anchor then it's not for you of course
I cant wrap my head around it. If I use a unit like that once or twice I never want to touch it again. The tactile feel is so poor, the unit is so light and flimsy, and I know the build quality is crap and has tons of components that are designed to fail. I guess its the same mentality you need to enjoy these superhero movies that are just identical bad movies with different characters.
What is funny is how the folks suggesting these crap products are wondering why I'd want an older piece of gear.
'We' are wondering why you would want an older piece of test gear because 'we' have the experience that older test gear is severely outdated, has limited features, is prone to failures and more often than not loud & heavy. There really is no upside to buying an older general purpose function generator. Your perceived 'quality' just isn't there. It is like wanting a model T-Ford instead of a modern day car made in Korea.
I have owned my fair share of old general purpose function generators (*) and I would have replaced them in a heartbeat if units like the Uni-t I mentioned earlier where around at that time. To me wanting an older unit makes zero sense.
* and other old test equipment as well
« Last Edit: June 17, , 04:15:10 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
bdunham7
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #23 on: June 17, , 05:13:50 pm »
You've written 10 millihertz in your post--do you actually use frequencies that low and lower? Or did you mean 10MHz (megahertz)?
The problem with what you are asking for is that if you want stable low-frequency signals, the way to get them is with an Arbitrary Waveform Generator (AWG) using Direct Digital Synthesis (DDS) and a decent TXCO clock. This is a more modern design and while there are older model AWGs available from Tek and HP, they are typically more limited in performance and still not very cheap for what they are. You can just search eBay for "Tektronix AWG" and "HP AWG" to see many examples.
Any earlier analog generators are going to be crap by comparison--even the aforementioned HP A--when it comes to frequency accuracy and stability.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
mapleLC
Re: Help Choose a Used Waveform / Function Generator Upgrade
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Reply #24 on: June 17, , 09:03:30 pm »
You've written 10 millihertz in your post--do you actually use frequencies that low and lower? Or did you mean 10MHz (megahertz)?
The problem with what you are asking for is that if you want stable low-frequency signals, the way to get them is with an Arbitrary Waveform Generator (AWG) using Direct Digital Synthesis (DDS) and a decent TXCO clock. This is a more modern design and while there are older model AWGs available from Tek and HP, they are typically more limited in performance and still not very cheap for what they are. You can just search eBay for "Tektronix AWG" and "HP AWG" to see many examples.
Any earlier analog generators are going to be crap by comparison--even the aforementioned HP A--when it comes to frequency accuracy and stability.
That was a typo.
I think I want the Rohde & Schwarz ADS dual arbitrary waveform generator. It doesn't have the higher frequency ability, but I like the build quality.
What do you think as a mix of more modern design and build quality?
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Low Cost Function Generator Amplifier DIY
14 Comments
Introduction
A majority of function generators are only capable of driving a couple of hundred milliamps, which is fine for most applications. If you want more output current, you can shell out $400 dollars for a professional signal generator amplifier, or you can do what I did and hack one together for under $40.
A signal generator is an indispensable tool for developing and testing electronic designs. You may find yourself wishing yours could output more current. You could test your power supply design by feeding in a noisy supply voltage, or you could see how it will handle a specific amount of input ripple. If you do this sort of thing regularly, you may want to invest in professional equipment. But, if you are on a budget, or only need this sort of thing occasionally, then keep reading.
Backstory
When I first started learning electronics in grade school, I had dreams of being able to build anything I wanted. After purchasing components in single quantity from Digi-Key for my first few projects, I learned a disheartening lesson.
It almost always costs more to make something yourself than to buy a finished product.
It was then I wrote Tim’s Golden Rule of Building Electronics:
“I shall not build what can be bought unless mine shall be better or cheaper.”
So, before embarking on this project, I checked to see if there were any low-cost units on the market. The least expensive option I could find was the Siglent SPA at just under $400. This unit would work for most cases, but only has a max output current of 1.1Amps, which just wasn’t enough for me.
Figure 1 – Siglent SPA
Signal Generator Amplifier DIY
Unable to find a low-cost option, I resolved myself to designing my own signal generator amplifier.
I hoped that I could design the amplifier around a high-power OP Amp. Searching for the highest output OP Amp on Digi-Key revealed the OPA541 and OPA549.
The OPA541 can handle +/- 35V rails, whereas the OP549 can only handle +/- 30V rails.
Since More Voltage = More Better, I went with OPA541.
I felt good about this selection, and I felt even better when I heard on their podcast that the guys at Macrofab were designing a power supply using this same OP Amp. Now, I just needed to whip up a schematic and layout a PCB (with a monster heatsink) to handle the OPA541.
Wait, Tim! Don’t forget about your golden rule!
Before embarking on my design, I decided to see if there were any breakout boards available for the OPA541 (preferably with a heatsink). I couldn’t find anything from the usual suspects (Adafrut, Sparkfun, etc.), but I did find something on Aliexpress.
Like most things on Aliexpress, it looked too-good-to-be-true. I found an OPA541 breakout board with free shipping for $35. The OPA541 alone costs almost $22 from Digi-Key in single quantity. So, I ordered one plus a few SMA to BNC cables for $3 each.
Figure 2 – The OPA541 cost almost $22 in singles
Figure 3 – Inexpensive SMA to BNC cables
A few weeks later, the unit arrived and looked to be as advertised.
As expected, the amplifier came with zero documentation. The circuit looked simple, so I knew I could reverse engineer it if necessary. Instead, I decided to power it up and see what happened. It was immediately apparent that it used capacitive AC coupling because it only amplified AC signals while ignoring any DC offset applied.
Having nothing to lose, I sent a message to the seller on Aliexpress asking for a schematic. I got back a one-word reply “.”
I sent him my address, and he sent me a link and a password to a Chinese file-sharing site which yielded a PDF schematic of the device. Awesome!
*Click to enlarge
I noticed values for many components were not correct, so I marked up the schematic to show the actual values. The schematic is a little messy by my standards, but it was easy to see how it works. It is a two-stage amplifier. Both stages are set up as non-inverting with the first having a gain of 3 and the second a gain of 11 for a total gain of 33.
If you want to learn more, please visit our website Power Quality Analyzer.
*Click to enlarge
The most insane thing I discovered was that the first stage OP Amp is an OPA445, a high voltage OP Amp that costs over $10 in single quantity!
This, plus the OP541 (which costs $21), means I got $31 in chips alone for $35. Assuming these parts are legit, that’s a good deal in my book. Even if the OP Amps are counterfeit, the PCB, heatsink, and connectors are still worth $35 when considering that my alternative was to design and make my own from scratch.
Figure 4 - OP Amp for OPA541 Module
Below are side-by-side comparisons of the parts from China and ones purchased directly from Digi-Key. They don’t look identical, so I’m not sure if the parts from China are genuine.
There are two variants of the OPA541AP. One has a G3 suffix. Perhaps this explains the difference between the packages.
If anyone knows more about these ICs, please feel free to write in the comments.
To allow the device to amplify DC, I replaced C4 and C5 with 0 Ohm resistors. See below where I removed C4 enabling me to solder a 0 Ohm resistor in its place.
Other Changes
I changed the overall gain to 10 to simplify the mental math required.
To change the gains, I did the following:
- R2 changed to 10k. Since R1 was already 10k, this set the first stage gain to 2. [1+10k/10k = 2]
- R4 changed to 2.55k, and R7 changed to 10.2k which set the second stage gain to 5. [1+10.2k/2.55k = 5]
- Upgraded the main Sanyo brand capacitors with Panasonic 63V rated caps because the original caps were only rated for 35 volts despite the schematic calling for a 50-volt rating.
Final Schematic
Below is the final schematic including all of my modifications.
*Click to enlarge
Testing
With the modifications complete, it was time to test the performance.
I connected the amplifier to our Rigol DP832 and configured the DP832 to provide +/-30 volts as shown in the diagram below.
For the first test, I fed in a constant DC signal voltage of 2.5 volts. As expected, the amplifier output a constant voltage of 25 volts thanks to our 10x gain. We fed the output to our BK Precision programmable load and set it to pull 2.9 Amps, which is close to the maximum of 3 Amps for our Rigol DP832 Power Supply. We were able to source over 72 Watts to the programmable load! Sweet!
Our power supply was running close to its maximum output of 3 Amps and supplying 87.7 Watts. Since it was providing 87.7 Watts and our load is pulling 72.3 Watts, the amplifier would have been dissipating the difference between those two values, or 15.4 Watts.
The thermal image (and the burn on my hand from touching the OPA541) confirms the amp was getting hot.
It got hot but was still operating below its 125˚C limit as shown in the datasheet snippet below:
To minimize heat dissipation, we have to remember to set our power supply voltage just a few volts above our desired max voltage output from the amplifier. Doing so will reduce the voltage differential and hence reduce the power dissipated by the amp.
Next, I connected two 12v automotive light bulbs in series to act as a load and connect our differential Oscilloscope probe across the load.
I then connected the function generator and set up a 1kHz sine wave set to 2.5 volts peak to peak.
The Oscilloscope shows a ~25-volt peak sine wave at 1kHz as expected.
Below is a video showing the same setup but at 0.5Hz instead.
Conclusion
Overall, I’m quite pleased with my $40 investment. A few weeks of waiting followed by a few minutes of soldering yielded a nice addition to the test bench. It will come in handy for testing future electronic designs.
Learn More About DMC's Embedded Development and Embedded Programming Services or Contact Us to start developing a solution that works.
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