I built an F5m amp and am now looking to tune the bias.
I tried using my Rigol DS1054Z with 2 leads across a resistor on the amp board ( lead ground wires removed ) at 1X ( and 10X ) with MATH enabled on Chan1 and Chan2 ( A-B ) but get lots of noise on the math line. The math line bounces around within a 200mV range which is not accurate enough.
Any advice on how I can use it to get a reading of 10mV or even 1mV accuracy? ( I think my Rigol is unlocked too )
I get a 10000 meter next week but wanted to try with my oScope in the meantime.
I tried using my Rigol DS1054Z with 2 leads across a resistor on the amp board ( lead ground wires removed ) at 1X ( and 10X ) with MATH enabled on Chan1 and Chan2 ( A-B ) but get lots of noise on the math line. The math line bounces around within a 200mV range which is not accurate enough.
Any advice on how I can use it to get a reading of 10mV or even 1mV accuracy? ( I think my Rigol is unlocked too )
I get a 10000 meter next week but wanted to try with my oScope in the meantime.
Use ONE probe. Ground is the screen lead, place the probe (X1) to the cathode side of the bias resistor.
Set to DC 2mV/Division. Set bias, (no signal condition) to required voltage. If more voltage(bias current) is required, use a 5mV range.
Set to DC 2mV/Division. Set bias, (no signal condition) to required voltage. If more voltage(bias current) is required, use a 5mV range.
@JonSnell Electronic thanks. What is the 'screen lead'? The Ground on the lead is earth ground so I want to be careful 😉
The screen lead is the dangly one coming sideways out of the probe, i.e. not the witch's hat...
This is one of the rare occasions that x1 mode is useful (because its a low impedance part of a circuit, low voltage, and we want to reduce noise (although that can be done by low-pass filtering the scope input too).
This is one of the rare occasions that x1 mode is useful (because its a low impedance part of a circuit, low voltage, and we want to reduce noise (although that can be done by low-pass filtering the scope input too).
Ok thanks. I did have the ground lead attached to the case/earth ground. I'll try again as I suspect I had the probe on the anode side by mistake.
I'm sure I'm overlooking the obvious, but I don't see how I can measure the voltage differential across the resistor with just 1 probe. I connected the ground lead to the case and tried the probe on both cathode and anode sides but was seeing just over 10V. When I connect a volt meter, I get .200V
What are the actual measurements that you get at each end of the resistor? Is the difference of the two equal to 10V? Or is it 0.2V?
Maybe check that the settings and probes are set correctly to 1x or 10X.
Maybe check that the settings and probes are set correctly to 1x or 10X.
Last edited:
The difference seemed to be the 200mV ( math line ) I tried 1x and 10x too. It maybe been 'correct' but the math line was much too noisy. ( 200mV range of noise )
Good idea to use two channels for simulating a differential probe. Using one channel only may lead to magic smoke, when you connect the probe ground accidentally to a point in the amp, which is NOT at ground.
Regarding the noise: Did you connect the ground of the Rigol with the ground of your amp? This should reduce the noise considerably. But dont expect precision measurements because of the insufficient vertical resolution (I guess 8 bit) of the scope. Using a DMM as @Mark Tillotson suggested is much better (and fully differential!).