Tech Doubt: Function Generator vs Audio Oscillator?

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
Hello, this weekend I've bought some equipment for my little lab, in order to make my projects easier...

Only bad thing is that maybe the machines I've gotten are a bit outdated, I think they should be ok for what I'm gonna do, basically try to repair car audio amplifiers... Anyway, I wanted to ask some questions.

1)The oscilloscope I bought is a TDS210, which is from 1997, it's digital, has start stop and memory, so I think it will be okay... Until now I had to use the dso150 with alligator clips... which was waaaaaaaaaay worse...

Even though ¿Do you think this will be enough for most audio work, or should I spend double in for example a Rigol DS1054Z?

2)Main question:

I bought an audio oscillator, and a function generator... The audio oscillator, it's named HC 0204, while the function generator is a PROMAX gf232.
The first one seems older...

The main differences are:
The audio oscillator only generates sine waves and TTL output , its output impedance is 600 ohms, and has less bandwidth (15Hz to 200kHz) but it's easier to choose one given frequency with the knob (for example 65 Hz) with +-1Hz error. It also has 40,20 and 10dB attenuators.

The other one does all types of waveforms (triangle, sine, pulse, etc), but its output impedance is 50 ohms, it's really difficult to select a chosen frequency with +-1Hz error, it only has 20dB attenuation but it has more bandwidth (0.2Hz to 2MHz).

I mainly wanted to be able to do frequency sweeps, to get the output response of the amps or any other thing... From what I saw, I can only do them by getting a 0-10V ramp signal in the VCO input of the promax GF232, so that it sweeps one full scale...

So I'm starting to think that the audio generator is not really useful for me... I don't see what it could do that the other doesn't, more than attenuating the signal, or letting me have two signals, but I can't even get the ramp signal I need to get the sweep out of the promax :/

I've seen there's a difference between the construction and workings of the internals of both equipment...
The audio oscillator uses RC network, while the generator modulates output with a sawtooth signal, anyway I couldn't appreciate much difference on the oscope with both equipment set to the same working frequency...

Could you please give me some advice?

Should I give back the audio oscillator and save money for some other thing, or is it really useful for something I'm not caring about?

Thanks a lot
 

dl324

Joined Mar 30, 2015
14,897
Welcome to AAC!
Do you think this will be enough for most audio work
A 60MHz bandwidth should be sufficient for audio work. For that bandwidth, I'm partial to the Tek 7D20 plug-in for Tek 7xxx mainframes. Though I still prefer the clean traces of analog scopes.

When I googled to find the specs, the first page I looked at was a recall notice:
https://www.tek.com/en/service/safety/tds210-tds220
These serial numbers were included:
1657638567757.png

Should I give back the audio oscillator and save money for some other thing, or is it really useful for something I'm not caring about?
If you're doing audio work, it might be convenient to have a sinewave with a frequency that's easy to set.

I have about a dozen function generators, but I'll still keep my HP204C. None of them cost very much.
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
Hello, thanks a lot for the welcome and the response.

A 60MHz bandwidth should be sufficient for audio work. For that bandwidth, I'm partial to the Tek 7D20 plug-in for Tek 7xxx mainframes. Though I still prefer the clean traces of analog scopes.
I was about to buy a hameg analog oscilloscope, which was even cheaper, but it hasn't the features of measuring, or start/stop, which I find really useful... I've seen video #601 from eevlog (why digital oscilloscopes appear noisy), and I guess that cleaner lines are good but not exactly better, and in a modern oscilloscope you can set it to average so that it does smoother lines, anyway I found it interesting that some people preffer "old" scopes anyway. I guess they are still suitable for most of the jobs, and this "noise" isn't really important in most applications.

If you're doing audio work, it might be convenient to have a sinewave with a frequency that's easy to set.

I have about a dozen function generators, but I'll still keep my HP204C. None of them cost very much.
I guess that the fact that it only outputs sines is good, but I think I can do the same with a car head unit and a phone app, i think it'll be even more exact, and I don't have to build an bnc to rca adapter... Anyway i'm not sure in which cases will I need such an exact frequency so I would like to know the opinnion of more experienced people... The other generator can be set easy with just a bit more error I guess, the LCD screen is faster for sure... I don't really know the cost of the oscillator, but I don't see them on sale anymore :/

When I googled to find the specs, the first page I looked at was a recall notice:
https://www.tek.com/en/service/safety/tds210-tds220
These serial numbers were included:
About the safety warning... Yes, I read something but didn't check it, I saw GND was correctly grounded, but didn't find more, thanks for the exact information... Finally my SN is B039296, and it doesn't say it's been modified anywhere... So I guess I have a problem... Thanks for the warning, I'll check what I can do.
 

dl324

Joined Mar 30, 2015
14,897
I found it interesting that some people preffer "old" scopes anyway.
When I first started studying electronics, the Tek 7xxx mainframes and 465 were state of the art and my school lab had some because one of the instructors worked at Tek in the summer. When I started buying equipment for my hobby, that was what I wanted.

The Tek 7xxx mainframes are very versatile. In addition to multiple amplifier and timebase options, there are plugins for DSO, frequency counter, DVM, logic analyzers, TDR, spectrum analyzers, curve tracer, etc.

I like to use the 7D20 in a 7603 mainframe for its large CRT.
I guess they are still suitable for most of the jobs, and this "noise" isn't really important in most applications.
Jim Williams was one of the giants in the industry who preferred the sharp traces of analog scopes.
Thanks for the warning, I'll check what I can do.
It might have already been modified because the recall was in 1998. Not sure if you could get it fixed now because Tek is just a shadow of its former self.
 

crutschow

Joined Mar 14, 2008
30,419
The main reason to use an audio oscillator is if you want to do distortion measurements, since it likely has lower distortion than the function generator.
Otherwise the function generator should do everything you want.

If you don't have a ramp generator to provide the frequency sweep, you can build one with 555 or op amp circuit.
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
When I first started studying electronics, the Tek 7xxx mainframes and 465 were state of the art and my school lab had some because one of the instructors worked at Tek in the summer. When I started buying equipment for my hobby, that was what I wanted.

The Tek 7xxx mainframes are very versatile. In addition to multiple amplifier and timebase options, there are plugins for DSO, frequency counter, DVM, logic analyzers, TDR, spectrum analyzers, curve tracer, etc.

I like to use the 7D20 in a 7603 mainframe for its large CRT.
Jim Williams was one of the giants in the industry who preferred the sharp traces of analog scopes.
It might have already been modified because the recall was in 1998. Not sure if you could get it fixed now because Tek is just a shadow of its former self.
In the post it says there should be a sticker stating it has been modified, I guess if I can't ensure it I'll have to give it back... And I don't think I'll be saving the audio oscillator neither, feels too much money for me just for the sake of chosing one frequency "easily" ://

I guess I'll have to go for a newer option for the scope, and I'll keep the function generator...

Thanks a lot
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
The main reason to use an audio oscillator is if you want to do distortion measurements, since it likely has lower distortion than the function generator.
Otherwise the function generator should do everything you want.

If you don't have a ramp generator to provide the frequency sweep, you can build one with 555 or op amp circuit.
Thanks, how are these measurements made?

Because I guess I don't have the neccesary equipment to do them, since the signals on the oscilloscope seem too similar to distinguish any difference by sight
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
You could find out what the defect was and repair it yourself. It said incorrect usage could cause a ground fault.
I still don't really like messing with line powered devices, since I don't have the equipment neccesary for that, and I think a scope is too much of a deal for me to be able to fix it right now... Since I just payed for it I preffer to get the money back if it has some fault
 

crutschow

Joined Mar 14, 2008
30,419
how are these measurements made?
You use the ramp to sweep the function generator frequency, with the ramp also used to generate the horizontal Oscilloscope sweep (XY mode).

You might look up some tutorials or YouTube videos on how to do an audio frequency response sweep with an oscilloscope.
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
You use the ramp to sweep the function generator frequency, with the ramp also used to generate the horizontal Oscilloscope sweep (XY mode).

You might look up some tutorials or YouTube videos on how to do an audio frequency response sweep with an oscilloscope.
Thanks for the explanation, but these is to get frequency response, right?

I wanted to ask how to measure the THD, I don't really know how, and I don't think I'll need the audio oscillator because I guess I don't have the equipment neccesary to measure THD neither
 

crutschow

Joined Mar 14, 2008
30,419
I wanted to ask how to measure the THD,
One way is to apply a low distortion sinewave to the amp input, and use an instrument that will do an FFT (Fast Fourier Transform) on the output signal to determine the distortion products (which some digital oscilloscopes can do).

There are also dedicated instruments that measure THD but they are likely expensive.

Again there are tutorials and YouTube videos on how to do that.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
4,920
To fix a car audio amplifier you do not need a frequency sweep. Simply input 1kHz, then 20Hz then 20kHz sinewaves and note the output levels. You can see high distortion on the sinewaves.
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
One way is to apply a low distortion sinewave to the amp input, and use an instrument that will do an FFT (Fast Fourier Transform) on the output signal to determine the distortion products (which some digital oscilloscopes can do).

There are also dedicated instruments that measure THD but they are likely expensive.

Again there are tutorials and YouTube videos on how to do that.
Thanks, for the answers, i'll check those :)
 

MrChips

Joined Oct 2, 2009
26,784
TDS210 would be fine for audio work. You really don't need START/STOP which comes in handy for one-shot digital events. Even an older 20MHz analog oscilloscope would do the job.

If I were primarily trouble shooting and repairing audio equipment I would mostly be looking at DC voltages and searching/listening for audio. I wouldn't be too concerned about distortion providing that you cannot hear it.

A DMM (digital multi-meter) , inexpensive oscilloscope and a music source would be my basic test equipment.

One item that really comes in handy if you are doing a lot of this type of repair is an electronic circuit breaker.
Most often, the output transistors are shorting and they blow the fuses. An electronic circuit breaker allows you to set the maximum current and it trips when this is exceeded. It saves having to replace fuses too many times.
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
TDS210 would be fine for audio work. You really don't need START/STOP which comes in handy for one-shot digital events. Even an older 20MHz analog oscilloscope would do the job.

If I were primarily trouble shooting and repairing audio equipment I would mostly be looking at DC voltages and searching/listening for audio. I wouldn't be too concerned about distortion providing that you cannot hear it.

A DMM (digital multi-meter) , inexpensive oscilloscope and a music source would be my basic test equipment.

One item that really comes in handy if you are doing a lot of this type of repair is an electronic circuit breaker.
Most often, the output transistors are shorting and they blow the fuses. An electronic circuit breaker allows you to set the maximum current and it trips when this is exceeded. It saves having to replace fuses too many times.
Hello, thanks for your response!!

Yes, for sure, I don't really need the "pro" equipment, so that's why I don't want to expend nosense in things like the audio oscillator, with the function generator is more than enough for the most basic I could get to do... Anyway, good tools make the work way easier.

I think I already said it but I managed to learn and do the first troubleshooting with a DSO150 with cocodrile clips ^^' But the bank scope is for sure a new level...

I think the music source will be a better tool than the audio oscillator, since i'm going to be dealing with car audio, it makes sense that I can use the same type of source I'd be using in a car I guess... For the moment I've been using my phone with a function generator app and a jack to RCA cable...

The DMM I have is actually also a DCAC current clamp, I really like it, it's the UNIT-204+ the only thing i'm missing is maybe some rubber case, to make it even more robust.

I also have a variable voltage supply (Peaktech 6226) which can limit current, I'm usually limiting to 3A-5A for basic troubleshooting, if I see something goes wrong you can easily notice thanks to it rising, otherwise I'd have to use the current clamp with some turns, which would make it less comfy...

The only thing i don't like about the PSU , is that I'm not clear if its completely isolated from mains, since I saw a little spark when connecting both the negative of the power supply and the oscilloscope...
 

gade4479

Joined Jul 12, 2022
1
If your primary concern is audio work, I suggest a 24 bit audio box, like UMC202HD to use with Rew or Arta. It generates the stimulus as well. And eventually a super low distortion 1KHz or so generator, and a matching notch filter. It will allow spectrum analysis details the scopes are not going into.
 

Thread Starter

MarcosKDC

Joined Sep 10, 2021
67
If your primary concern is audio work, I suggest a 24 bit audio box, like UMC202HD to use with Rew or Arta. It generates the stimulus as well. And eventually a super low distortion 1KHz or so generator, and a matching notch filter. It will allow spectrum analysis details the scopes are not going into.
Thanks a lot!! I understand getting the real audio response is the next step after I get a good understanding of the circuits involved in the amps :))
I've been told about REW but didn't really know what should I use as a good audio output from the PC to match my equipment I guess this is good solution if don't want to go through a radio :)
 

tautech

Joined Oct 8, 2019
278
I still don't really like messing with line powered devices, since I don't have the equipment neccesary for that, and I think a scope is too much of a deal for me to be able to fix it right now... Since I just payed for it I preffer to get the money back if it has some fault
Of course you do, everyone has a DMM and a soldering iron !

The recall of Tek TDS200 series was to correct a too thin PCB trace that acted as a fuse if a ground loop was made so that if the trace blew/vaporized the input BNC's were no longer mains ground referenced therefore use of the scope in such a state could present a shock hazard to the user.
PCB's were replaced for the increased load arising from a ground loop so to blow or trip a fuse/breaker and not fuse the trace.
With a soldering iron you can easily do the mod yourself just by adding a cable of say 1-1.5mm2 from the power inlet mains earth to the PCB ground plane near the input BNC's.

If the connection is already open between mains ground and input BNC's it needs fixing anyways, but this is where you need the DMM for the continuity check.
 
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