Low distortion sine wave generator has a 100mV offset! - Page 1
Forum section: electronics / repair

author: anotherlin
author_rank: Frequent Contributor
timestamp: August 16, 2016, 07:15:54 am
content: Hi everyone,I have HM8037 low distortion sine wave generator. It is designed for audio/hi-fi, with a range of around 5Hz to 50KHz.Everything works fine, I do have a "beautiful" sine wave, except that the output has an around +100mV offset.I always used it "AC coupled" so didn't noticed before looking at it on the oscillosope.Unlike an arbitrary generator, this unit doesn't have an offset control knob. And the manual doesn't mention any trimmer to adjust/calibrate that.Is it normal or should I try fixing it ?Best regards,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2016, 08:37:34 am
content: Funny... depends upon your definition of "THD+N", over what frequency range you consider "N". If it includes DC, your figure is something like 30% I guess...Does the manual have a spec?Tim

author: karoru
author_rank: Regular Contributor
timestamp: August 16, 2016, 08:44:09 am
content: Here's the service manual with schematics.http://www.sm5cbw.se/hameg/hm80/hm8037-serv.pdfOutput stage is just usual push-pull amplifier powered from +15V and -12V, and there isn't AC-coupling anywhere so it looks like this unit assumes that audio devices will be AC-coupled.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2016, 09:16:52 am
content: Holy hell... they used an AD633 ($10) instead of an LM13700 ($0.50?)...And yeah, they only specify noise down to 5Hz or so, so yeah, it's intended for DC coupling. Get a big friggin' capacitor and call it a day.I don't see any obvious reason for a DC offset, so there might be something funny with it (but, something funny would probably cause distortion too, so I don't know). The DC reference for the whole circuit is R9/R26, so I suppose you could put a variable DC current into that node to tweak it.TL082 should be fine, and I don't think there's enough noise gain to screw up the offset by more than 10s of mV, but maybe replacing those would be something to investigate (again, no obvious reason, and other things would probably be broken). If they're socketed, it'd be easy enough to try, anyway..Tim

author: LA7SJA
author_rank: Regular Contributor
timestamp: August 16, 2016, 10:51:00 am
content: Measure pin 1 on U1-A and report back.Johan-Fredrik

author: anotherlin
author_rank: Frequent Contributor
timestamp: August 16, 2016, 11:45:01 am
content: Houston, seems like we have problemThank you for your answers.The service manual is for the HM8037-2, but I have the HM8037 first version, dated 1985!But I think it probably works according to the same principles.Indeed the first place to look would be at the input of the output stage.

author: 45Overload
author_rank: Contributor
timestamp: August 16, 2016, 03:36:49 pm
content: 100mv is a reasonable offset value for a unit like this. Notice that they do not specify d.c. offset in the spec page. Your typical audio amplifier can have this much offset without causing any trouble. Since it is a piece of test equipment, stuff happens on the bench and cables get shorted...I would be tempted to run this a.c. coupled all the time to save yourself the trouble of replacing parts in the power amplifier.The design does not offer any offset trimming, so if you need to experiment just use standard methods for adjusting the offset on (for example) U3B and/or U1B.

author: anotherlin
author_rank: Frequent Contributor
timestamp: August 16, 2016, 09:16:16 pm
content: Ok, I just misunderstood and mis-used my oscilloscope!I'm using a Hameg/R&S HMO722 and it has a quick view option which is very useful: It displays frequency, Vrms, Vpp, Vmax, Vmin, duty cycles, mean voltage, etc.According to the quick view, the mean voltage was around 100mV for an around 10Vpp signal.As a sine wave should have a zero average, and the thing on screen looks like a perfectly good sine wave, I expected a zero or so mean voltage.So I took the assumption that this 100mV was an offset added to the sine wave.However, after re-centering the display to GND, switching back and forth AC and DC coupling of the scope with no change in display, I came to the conclusion that there is no offset.The non-zero mean value must be because the sine wave is not perfect (welcome to the real world!) and the oscilloscope is not the best tool to do that kind of measurement.I'll try with my DMM to be sure of the result.What puzzle me still, is that the scope displays different absolute value for Vmax and Vmin for the sine wave, for instance +4.96V vs. -4.68V.I don't know how that relates to the THD distortion.I'll post some screenshots tomorrow.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2016, 08:45:07 am
content: If you are zoomed in close, you need to use Cycle Mean. Otherwise, the little extra bit of the waveform that doesn't fit on screen / in the waveform buffer won't average out correctly. The average of sin(x) is only zero after every cycle (x from 0 to 2pi, 4pi, etc.), and nonzero inbetween.If you are zoomed out, use plain Mean. Cycle Mean will try to calculate too small of a sample, and give ridiculous results (unless you have deep memory that's keeping a close-up view, in which case the above applies and you're still fine). You may still see errors if the waveform is aliased, so you shouldn't zoom out too far either.Additional precision can be had by applying channel and visual filtering: set the channel bandwidth as low as you can, set acquisition mode for Hi-Res or Averaging (>8 averages), or set a lowpass filter option. The average of a signal is the same as zero Hz (DC), so you can't filter too much: the only drawback to heavy filtering is it may take a while to calculate (i.e., for a cutoff frequency of <1Hz, you must record at least >1s of data).As for differences in peaks, with respect to zero: those should be symmetrical around the average, for a signal that does not contain even harmonic distortion.Mind that your oscilloscope may not measure the peaks to better than 0.4% precision (i.e., 8 bits of resolution). Max/min functions take the single sample that stands out above all others, so you gain absolutely no resolution with deep memory -- to get useful precision, you need to use a more sophisticated measurement, like FFT.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: August 17, 2016, 07:57:50 pm
content: The input amplifier of cheap scopes may not be that linear. This does not really matter for the limited (e.g. 8 Bit) resolution of a scope. So chances are that there is more distortion from the scope input than from the generator. So the scope is not really the right instrument to check for low distortion.The eye is not very sensitive to see moderate distortion in the % range. If not working properly, it is possible that the generator produces more distortion than specified. One possible failure that would lead to higher distortion could be mismatch of the stereo pots and compensation circuit around the AD633 reaching saturation (control voltge).To better see distortion one could build a twin T filter (fixed firequency) and than look at the residuals with the scope.