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Yes it was like that only much worse. I've got the captured files here somewhere. Spewed forth wide band noise almost like an impulse had occurred.
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Is DAC ultrasonic/RF output important?
- Thread starter pkane
- Start date
OP
- Thread Starter
- #42
I've seen the ultrasonic content in Forte captures before, it is quite a bit more than other quality interfaces and DACs. Should be a good candidate to test for IMD into audible range with a preamp or an amp
EDIT: Found one of my loop-back audio captures using Forte at 192kHz. Looks pretty messy, although low in level. This also passed through the ADC anti-alias filter, so the ultrasonics may be lower in level than straight out of the DAC:
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Hi Paul,
I am glad to see you exploring further into RF.
Even if almost insignificant in the studied case, there is an issue with calibration and measurement at around 1.5 MHz. You will notice some modulated signal.
It may be due to:
- internal Picoscope's noise
- external 'noise' around your measurement bench caught by the probe
OP
- Thread Starter
- #44
Hi Paul,
I am glad to see you exploring further into RF.
Even if almost insignificant in the studied case, there is an issue with calibration and measurement at around 1.5 MHz. You will notice some modulated signal.
It may be due to:
Using a simple BNC cable for the short should help understanding it.
- internal Picoscope's noise
- external 'noise' around your measurement bench caught by the probe
Hi Arpiben,
You can see the Picoscope calibration run with the probe shorted that shows very little of the noise, only some at around 1.5MHz:
Picoscope probes are cheap, but well made, low capacitance and use BNC connectors
I've since then figured out that I can run in differential mode that reduces RF noise even more, unfortunately that's not available using Picoscope Mac software, only on Windows.
Hi Arpiben,
You can see the Picoscope calibration run with the probe shorted that shows very little of the noise, only some at around 1.5MHz:
Picoscope probes are cheap, but well made, low capacitance and use BNC connectors
I've since then figured out that I can run in differential mode that reduces RF noise even more, unfortunately that's not available using Picoscope Mac software, only on Windows.
Eventually, you can select Pico's probe to 10:1 /check the second input channel or change ground point just for curiosity.
Even expensive probes can catch RFI.
(I realize that I may have misguided you with BNC cables since I do have shorts terminations, sorry for that).
I repeat it is not important here taking into account the instrument specifications.
Rather take it as a kind of general modus operandi for better distinction between sources of spuriae.
OP
- Thread Starter
- #46
As discussed, I want to see if there's an effect of ultrasonic noise on the output of an amplifier. To do this, I'm using an old Nelson Pass designed amplifier Forte 1a (class A). Sorry about introducing yet another component named Forte into this thread
Playing 1kHz sine @192kHz into the NOS mode of Holo Spring DAC, RCA cables feeding output of the DAC into Forte amp, terminated with 8ohm resistors. At 1W output with both channels driven, this is what is captured at the speaker terminals with Picoscope peak-hold mode:
1kHz sine bw=24kHz:
1kHz sine, bw=3MHz:
And now playing a 10-22KHz sweep, bw=3MHz
There appears to be more/higher level noise past the signal frequencies, although it's still relatively low in level.
Playing 1kHz sine @192kHz into the NOS mode of Holo Spring DAC, RCA cables feeding output of the DAC into Forte amp, terminated with 8ohm resistors. At 1W output with both channels driven, this is what is captured at the speaker terminals with Picoscope peak-hold mode:
1kHz sine bw=24kHz:
1kHz sine, bw=3MHz:
And now playing a 10-22KHz sweep, bw=3MHz
There appears to be more/higher level noise past the signal frequencies, although it's still relatively low in level.
What's the noise floor of the DSO under the same conditions? Note it will generally change with the gain (V/div) setting and bandwidth, natch.
OP
OP
- Thread Starter
- #50
I'll need to reassemble my test bench to do this. Will do a bit later.
No worries, just curious if it is setting (or at least a major contributor) to the HF noise floor you are seeing. I'd love to take a 'scope and spectrum analyzer from work to piddle with at home. I have permission, just have to sign them out, but they are worth more than my house so I'm (more than) a little nervous about taking them down the highway and playing with them at home. Maybe when I retire, but then again we have Amir, and you, and Wolf, etc. so I can live vicariously through your hard work.
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OP
- Thread Starter
- #52
No worries, just curious if it is setting (or at least a major contributor) to the HF noise floor you are seeing. I'd love to take a 'scope and spectrum analyzer from work to piddle with at home. I have permission, just have to sign them out, but they are worth more than my house so I'm (more than) a little nervous about taking them down the highway and playing with them at home. Maybe when I retire, but then again we have Amir, and you, and Wolf, etc. so I can live vicariously through your hard work.
Here's the capture of 1kHz signal playing at -85dB, the 1k tone is just barely above the noise:
And here's the same at 3MHz bw:
3MHz zoomed in a lot:
Thanks Paul, looks like the noise floor is just a little below what you measured earlier (so significant but not dominant).
Most 8-bit DSOs I have seen only have 6 ENOB or less at HF (GHz in my case) but at audio I'd expect (hope) they are near ideal.
Off-topic: How do you like Pico's interface? Last time I use one was long, long ago but I keep thinking I should get a DSO to spur my hobby activities. Unfortunately that urge is well-countered by all the other expensive hobby equipment I have around (camera, woodworking tools, various electronic gear, cross-country skis, etc.) that I never seem to have time to use...
Most 8-bit DSOs I have seen only have 6 ENOB or less at HF (GHz in my case) but at audio I'd expect (hope) they are near ideal.
Off-topic: How do you like Pico's interface? Last time I use one was long, long ago but I keep thinking I should get a DSO to spur my hobby activities. Unfortunately that urge is well-countered by all the other expensive hobby equipment I have around (camera, woodworking tools, various electronic gear, cross-country skis, etc.) that I never seem to have time to use...
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OP
- Thread Starter
- #54
I know exactly what you mean about expensive hobbies
Pico is fairly intuitive. I started using the Mac version but didn't realize it was a beta that was not updated since 2018. Turns out they've been updating the Windows version only, and that has a lot more features (like math channels, for example). Windows version is behaving much better. The Mac one would occasionally get into a state where it wouldn't capture the correct signal until restarted. Windows seems much more stable. It's nice that all their models, even the most high-end ones, all use the same software.
There are a few more expensive Picoscope models than mine that do have more bits, more memory and larger bandwidth, but these exceed my experimenter budget, some by a lot
The base model 2204A for about $140 turned out to be surprisingly useful. I could use more memory for larger FFTs. 2206B has more memory and larger bandwidth, but is more than 2x the price.
OP
- Thread Starter
- #55
I know exactly what you mean about expensive hobbies
Pico is fairly intuitive. I started using the Mac version but didn't realize it was a beta that was not updated since 2018. Turns out they've been updating the Windows version only, and that has a lot more features (like math channels, for example). Windows version is behaving much better. The Mac one would occasionally get into a state where it wouldn't capture the correct signal until restarted. Windows seems much more stable. It's nice that all their models, even the most high-end ones, all use the same software.
There are a few more expensive Picoscope models than mine that do have more bits, more memory and larger bandwidth, but these exceed my experimenter budget, some by a lot
Oh, and a warning (not for you Don, as you know this already), but in case anyone wants to get started with a DSO -- heed the warning about the connection of DUTs to the DSO completing the path to ground! I went through two sets of fuses on the Forte amp until I realized I needed to float the ground
I know exactly what you mean about expensive hobbies
Pico is fairly intuitive. I started using the Mac version but didn't realize it was a beta that was not updated since 2018. Turns out they've been updating the Windows version only, and that has a lot more features (like math channels, for example). Windows version is behaving much better. The Mac one would occasionally get into a state where it wouldn't capture the correct signal until restarted. Windows seems much more stable. It's nice that all their models, even the most high-end ones, all use the same software.
There are a few more expensive Picoscope models than mine that do have more bits, more memory and larger bandwidth, but these exceed my experimenter budget, some by a lot
Yah, I am really spoiled by all the high-end Tek and Keysight (Agilent, HP, whatever) 'scopes at work. I was looking at a stand-alone Rigol or Siglent, maybe low-end Tek/Keysight but they are pricey compared to the competition, or Pico, maybe the 4000 series though the 500 series would be cool. But so would retiring early... And our bonus this year was about 20% of last year's, bad year for the electronics biz in general, so...
I have pictures someplace of a $30k active differential probe smoked (literally, melted cover when the amp in the tip overheated and blew) by not paying attention to grounding (fortunately I did not do that one). And a collection of standard melted 10:1 passive probes (some mine, some others').
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I've got myself a Digilent Analog Discovery 2 for Christmas. The thing I like most is that their software includes a full API, runs on Windows, Mac, and Linux (even on the new Raspberry Pi 4). The bundle is on sale right now. Don't know how long it will last.
https://store.digilentinc.com/ultimate-analog-discovery-2-bundle/
OP
Well crud, went through the details, noted the tie to NI, ordered the kit... I need to spend less time on ASR and maybe this will help.
Note they ship signature required so I had it shipped to work instead of home. Probably just as well, porch pirates and all... Worst thing is they aren't shipping until next year.
Note they ship signature required so I had it shipped to work instead of home. Probably just as well, porch pirates and all... Worst thing is they aren't shipping until next year.
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