Decent Function Gen Under 200? - Page 1
Forum section: electronics / general

author: yanir
author_rank: Regular Contributor
timestamp: April 09, 2012, 05:55:52 pm
content: Hi all, I need to get a good starter function gen, I am working in audio frequencies right now, but I would like one that is good for general purpose.I am working on a project with a microphone and need to test out my filtering circuit. I need don't need very high speed, under 2mhz is ok. I would like sweep, and I would like good amplitude (1mV steps) control to simulate mic signals.I found a Tektronix CFG250 for $125 US on ebay, and from what I have read here it comes from an era of not so great products from Tek. but it seems suitable. Should I go for it?ThanksYanir

author: T4P
author_rank: Super Contributor
timestamp: April 09, 2012, 06:00:53 pm
content: Hantek DDS3x25 if you can live it being USB based .

author: yanir
author_rank: Regular Contributor
timestamp: April 09, 2012, 06:16:33 pm
content: Quote from: Dave.S on April 09, 2012, 06:00:53 pmI do prefer bench-top, but it seems pretty powerful for the money.

author: Unknown
author_rank: Unknown
timestamp: April 09, 2012, 06:30:39 pm
content: How is does the DDS 3x25 perform in the mV range? What is its minimum output amplitude and what is its amplitude resolution?I'm wondering if a function generator is the correct tool. Function generators are usually fairly high amplitude, the one I have here is 100 mVpp to 20 Vpp (35 mVrms to 7 Vrms) in open circuit, with mV resolution. Another older analog one is 16 mVrms to 8 Vrms. Good luck achieving mV resolution with an analog single turn pot. Sine wave distortion also sucks for the analog generator. Both are better on good modern DDS generators.A dedicated audio oscillator, like Tektronix SG502, might be more suitable. Distortion will be much lower than analog function gens, and its output goes down much lower. Note that you need an TM5xx/5xxx series mainframe to power it. Don't think it can do sweep, however.The Tek CFG250 is not a Tektronix design, just an Asian design with Tektronix branding. Documentation and quality were not up to Tektronix standards. Might as well buy something similar without paying for the Tektronix brand. If you want to look at Tektronix function gens, look at the FG5xx series (which also need TM5xx mainframes). These were real Tektronix designs with proper documentation. Also consider other good brands like HP and Wavetek.Not sure what to recommend for new products under $200, the somewhat popular GW-Instek SFG-100x series does not support sweep, and most DDS generators are > $200.

author: slateraptor
author_rank: Frequent Contributor
timestamp: April 09, 2012, 09:24:58 pm
content: Quote from: alm on April 09, 2012, 06:30:39 pmAgreed. CFG250 is usable, but it definitely doesn't strike par with the typical Tek standard.If you can score one for around $50, go for it.

author: yanir
author_rank: Regular Contributor
timestamp: April 09, 2012, 09:36:29 pm
content: So is an audio oscillator a better way to go for my task? Since I am mostly concerned with frequency filtering, if my amplitude control was crude it would still be an effective test as long as my parts are rated for that voltage right?I took a look at the SG502 and I wouldn't mind something like that but investing in the rack and power supply is a bit much.Some googling for audio oscillator didn't return many promising leads, is there a better way to search for that?Also (since I don't know too much about analog) can you produce a signal like a mic at a particular bias voltage? On the function gens they seem to only allow you do a voltage p-p, but what about something like 1.5v dc, with 1mV p-p on the line?One thought is to feed the 1mV signal into a 1.5vdc bias circuit to achieve that, is that correct?

author: Unknown
author_rank: Unknown
timestamp: April 09, 2012, 11:09:12 pm
content: Many function generators will allow you to add a DC offset, although very few will be able to generate a 1 mV signal without external attenuation. If the generator's output is floating, you can add an external DC source (eg. power supply) in series. As long as both your power supply and DUT is floating, you can do the same with a grounded generator.Voltage probably shouldn't matter for passive filters, unless you use high-K ceramic caps with a significant voltage coefficient. Not sure how critical spectral purity is for your measurement? If something like 1% distortion is fine, I would just go for a cheap analog function gen for < $100, or a GW-Instek SFG-1003 (basic 3 MHz DDS generator) if you don't care about sweeping. If you want to measure distortion, than a generator with 1% distortion is absolutely useless, since almost any audio circuit should outperform it. It may also be a problem for very sharp filters that may attenuate the fundamental frequency below the harmonics.

author: MikeK
author_rank: Super Contributor
timestamp: April 10, 2012, 04:03:18 am
content: I recently got an Instek SFG-1003 for $153. I don't know enough to comment, but I've used it a bunch. I should have gotten the "voltage display" option, though, as without it you don't know the amplitude or offset.

author: Vredstein
author_rank: Contributor
timestamp: April 10, 2012, 05:22:26 am
content: Quote from: MikeK on April 10, 2012, 04:03:18 amI've ordered one of these also. Could you connect a DMM in either across or in series to monitor voltage without affecting the signal?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 10, 2012, 06:03:37 am
content: Quote from: alm on April 09, 2012, 06:30:39 pm12bit = 4096 values in 7Vpp range (3.5Vpp terminated), hence = 1.7mV resolution for 7Vpp Hi-Z range, and half of that if terminated.edit:out of curiosity, i captured mV resolution range for Hantek3x25 @ 40KHz max audio freq, allow me to post it here...please note i've modded the 3x25 output to produce 3X++ (3.429X to be exact to 3 decimal places) of what its original can do (ie 24Vpp max from 7Vpp original range) and the noisy environment probably comes from the modded circuitry i havent tried to clean up or investigate the original output, as i usually work in bigger volt range.1) 100mV amplitude setting, digitization start to become visible2) 10mV amplitude setting (digitization... digitized sine)3) 5mV amplitude setting4) 4mV or smallerback calculating the minimum output/resolution, in the 4th pic to the best of my eye can see its 8mVpp, since its 3.429 amplified, the original output should be from dividing = 2.3mVpp, pretty close to theoritical 1.7mV resolution, BUT... there's another issue that i just become aware while capturing this. it seems DDS (or particularly this 3x25 fpga based AWG) suffer digitization problem/distortion, so i guess DDS will have this digitization problem in every sine wave it generates in 1.7mV scale no matter how big or small the signal is. or perharps this is how the DAC/DDS chip front end works.edit: or perharps "user error" i didnt set/tune the DAC clock to appropriate value, from the picture, DAC is clocking at 400KHz (200MHz max for 3x25)

author: Unknown
author_rank: Unknown
timestamp: April 10, 2012, 06:39:32 am
content: I wouldn't pay much extra for the voltage display option, might as well use a scope to measure it.Mecha: I haven't studied the design. Is that 12 bits of vertical resolution due to the DAC, and does this mean that you effectively have a 1 bit DAC at 1.7 mV amplitude, or is there a programmable gain amp / attenuator on the output with 12 bits of dynamic range, offering decent performane and SNR?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 10, 2012, 06:47:51 am
content: Quote from: alm on April 10, 2012, 06:39:32 ami'm not sure i understand about attenuator. i believe 3x25 doesnt have any programmable gain amp or attenuator, 12bit is just spitted out from fpga to DAC, and then DAC to output opamp to produce 7Vpp swing, and thats it. please refer to above edited attachment, it might answer your question, and might not.

author: ciccio
author_rank: Frequent Contributor
timestamp: April 10, 2012, 10:07:14 am
content: I'm not sure that a FG will serve you well for audio work.An audio oscillator will be better suited, because of low distortion (Distortion was terrible on old "analog" FGs) and you absolutely need a dB calibrated switched attenuator, plus variable (up to zero) output control.I bought about 2 years ago this one:http://www.circuitspecialists.com/gag-810.html(in effect I paid it more than 200 US$ here in Italy) that is a clone of an older TRIO (Kenwood) unit I have since 1980, and that too was a clone of some other unit..These are actually AC units, and they do not have an offset control, nor a sweep function.Some Chines companies sell other clones, very similar in appearance, and sometimes they can be bought for about 100 US$.EDIT: This is a new clone :http://www.ebay.com/itm/Audio-Low-Frequency-Signal-Generator-10Hz-1MHz-RAG101-/170465373462?pt=BI_Signal_Sources&hash=item27b0872d16and this is a used one:http://www.ebay.com/itm/Vintage-Leader-Audio-Generator-LAG-120B-/130677090992?pt=LH_DefaultDomain_0&hash=item1e6cf62ab0Sometimes an old HP audio oscillator appears on E-bay, in working conditions for about 100 US$.If you want a cheap (in fact free) but very useful tool for audio work, download Visual Analyzer from this linkhttp://www.sillanumsoft.org/prod01.htmand buy a good external USB audio card.Sorry, but I cannot be of more help because I don't know the country you live in.. Please add a Flag to your profile.Best regards

author: Unknown
author_rank: Unknown
timestamp: April 10, 2012, 10:55:20 am
content: Quote from: Mechatrommer on April 10, 2012, 06:47:51 amYes, your edits answer my question. The output amplitude is only determined by the DAC, which means that the dynamic range / distortion is degraded at anything except the max. amplitude. If you or someone else ever continues with the idea of an external amplifier, than I would also add the ability to attenuate the signal. This is how most 'real' DDS function gens work: the DAC outputs a fixed amplitude, and the gain of the output stage is adjusted to set the amplitude. I'm also surprised that the quantization steps make it through the filter, did you bypass the filter to improve bandwidth or does it lack the output filter necessary for proper DDS operation?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 10, 2012, 11:22:02 am
content: Quote from: alm on April 10, 2012, 10:55:20 amit doesnt have any filter at all! as i've said in 3x25 thread, it has nothing but only the main heart of dds. now i get what you mean by attenuator and prog. gain amp, but still as any digitalized system, the bottom line is how much bits it has afaik. you may change the dynamic range by using atten or prog.gain, but 12bits "tonal range" is still 12bits. you max the prog. gain, you'll decrease resolution, i mean for each bit will consist larger voltage range, except as you've mentioned, if there's robust filter upfront to absorb this quantization effect.

author: MikeK
author_rank: Super Contributor
timestamp: April 11, 2012, 01:33:43 am
content: Quote from: Vredstein on April 10, 2012, 05:22:26 amI just tried it with the FG signal to my scope and a DMM across the output and couldn't see any change. I ran it from 1k to 100k sine. The reading on my DMM went way down as I upped the frequency, but that's probably because it's a crappy non-RMS meter.

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 11, 2012, 01:56:34 am
content: Quote from: MikeK on April 10, 2012, 04:03:18 amYou're not missing a thing. I have the "voltage display" model (SFG-1013). It does not actually measure the output voltage, it is basically just a visual indication of the amplitude control. The voltage displayed is only accurate for a square wave, peak-to-peak, into a 50-ohm load.

author: Vredstein
author_rank: Contributor
timestamp: April 11, 2012, 01:58:34 am
content: Quote from: MikeK on April 11, 2012, 01:33:43 amMy unit should arrive tomorrow and I'll do the same with a couple RMS DMM's.

author: yanir
author_rank: Regular Contributor
timestamp: April 11, 2012, 01:16:59 pm
content: I decided to take the easy road and switch to a i2s mems Mic. I'll get a function gen one day, but for now it's easier to stick to digital and let the experts handle the filtering.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 11, 2012, 03:41:57 pm
content: Quote from: yanir on April 11, 2012, 01:16:59 pm"Arbitrary Wave Generator" such as 3x25 can also be used to generate/simulate digital data/comm. with Goltek Controller, i'm able to generate on the output, ie 1 ch only, 2ch incl synch if you can "find tune" it. its also possible with multiple channel (3x25 provides 10ch digital out port iirc), but i havent tried to develop "Goltek Digital" since i havent any serious need for it.

author: MikeK
author_rank: Super Contributor
timestamp: April 20, 2012, 12:25:56 am
content: Quote from: Vredstein on April 11, 2012, 01:58:34 amSo, have you had time to play with it?

author: Vredstein
author_rank: Contributor
timestamp: April 20, 2012, 03:07:49 am
content: "So, have you had time to play with it?"Yes. I'm using it to get familiar with my Tek 2235 scope. Using assorted BNC "t" adaptors, and a BNC to croc clip cable I was able to monitor the voltage of a 1K square wave signal from the fg to the scope. Not sure how much all the adaptors and extra cables might affect the reading versus a built-in voltage meter.I'm a complete noob, so I'm still in the stage where I read about theory, then twiddle knobs and casually notice what knobs make something change.