08-10-2017, 05:37 AM
Thanks Mark. Think we may have got our wires crossed? Anyhoo, didn't think about the IP Z of any test gear. I was more concerned about the attenuator which sits in parallel with the dummy load, changing the value of the load.
In the end I built a 100:1/ 10:1 stereo switchable attenuator using a 68k and 22k both 2% to give 90k and a 22k preset to get 99k and a 1k 10 turn pot. I used what I had, I had no 100 ohm pots. A SPDT SW switches between 10 and 100:1.
"I'd build a simple 100:1 attenuator based on the lowest value resistors I can get away with. The lower you use, the less "loading effect" you'll get." I'm confused about this, surely a higher R or Z "load" is better, hence why scope and DMM have 1M or 10M Z. So if our test gear has a 10k Z and we use a 100 ohm R in parallel then any signal will reduce by 100 times? If we have 10k the signal will be halved, no? I'll admitt I'm struggling to visualise this.
As regards HF losses though a lower Z is better. With 10p of capacitance in a cable say inc stray C, there'd be 3dB loss at 159khz with a 100 ohm Z in parallel, I hope I have that right. But as were measuring audio frequencies, who cares.
BTW, I measured the DC res of my soundcard and it's 10k, so the Z will be a bit less than this. As it is the attenuator works well so far. I been mucking about with various PC based SA's as I mentioned. Rtspect though very simple shows harmonics. Anaprof though more sophisticated is only a demo version and I think has certain features locked. You can't change the IP range, I couldn't get the bugger to work properly and it turns itself off after 6 mins. The SA on audacity is good, though you can't see a signal "live". You have to record it. REW is quite nifty, you can do frequency response graph's and it "nulls" out the frequency glitches in your soundcard/DI. Not sure if it has a THD measurement doodad in it, but heard the sig gen is good. Need to play more.
Andy.
In the end I built a 100:1/ 10:1 stereo switchable attenuator using a 68k and 22k both 2% to give 90k and a 22k preset to get 99k and a 1k 10 turn pot. I used what I had, I had no 100 ohm pots. A SPDT SW switches between 10 and 100:1.
"I'd build a simple 100:1 attenuator based on the lowest value resistors I can get away with. The lower you use, the less "loading effect" you'll get." I'm confused about this, surely a higher R or Z "load" is better, hence why scope and DMM have 1M or 10M Z. So if our test gear has a 10k Z and we use a 100 ohm R in parallel then any signal will reduce by 100 times? If we have 10k the signal will be halved, no? I'll admitt I'm struggling to visualise this.
As regards HF losses though a lower Z is better. With 10p of capacitance in a cable say inc stray C, there'd be 3dB loss at 159khz with a 100 ohm Z in parallel, I hope I have that right. But as were measuring audio frequencies, who cares.
BTW, I measured the DC res of my soundcard and it's 10k, so the Z will be a bit less than this. As it is the attenuator works well so far. I been mucking about with various PC based SA's as I mentioned. Rtspect though very simple shows harmonics. Anaprof though more sophisticated is only a demo version and I think has certain features locked. You can't change the IP range, I couldn't get the bugger to work properly and it turns itself off after 6 mins. The SA on audacity is good, though you can't see a signal "live". You have to record it. REW is quite nifty, you can do frequency response graph's and it "nulls" out the frequency glitches in your soundcard/DI. Not sure if it has a THD measurement doodad in it, but heard the sig gen is good. Need to play more.
Andy.
