Electronics > Projects, Designs, and Technical Stuff
DIY oscilloscope (yet again)
Yansi:
Thank you for some clarifications.
I am not sure of hand how the GND coupling switch is accomplished. Do you have any sources I can refer to?
Not sure how difficult it'd be to inject any kind of test signal, but I guess even the low end scopes do that for self-calibration, don't they?
gf:
--- Quote from: Yansi on November 17, 2019, 07:07:41 pm ---Thank you for some clarifications. I am not sure of hand how the GND coupling switch is accomplished. Do you have any sources I can refer to?
--- End quote ---
Well, on my Hantek scope the "GND" position is a fake. Obviously it just sets the signal to zero in the digital domain (after ADC). Seems they did not spend money for any associated component (e.g. relay) in the analog frontend.
--- Quote from: Yansi on November 17, 2019, 07:07:41 pm ---Not sure how difficult it'd be to inject any kind of test signal, but I guess even the low end scopes do that for self-calibration, don't they?
--- End quote ---
One signal is typically injected anyway, and that's the vertical position offset. One potential injection point is at the DC servo. My understanding ist that at least some of the low-end scopes simply sweep the vertical position (and measure the resulting ADC output) in order to self-calibrate offset/gain (requiring by documentation that no signal must be connected to the inputs during self-calibration).
[ This self-calibration eventually compensates the overall offset errors of all frontend stages and the ADC, so it does not matter if any of the stages has some residual offset error - what matters is rather offset drift which invalidates the self-calibration again after some time. ]
gf
PMA:
I haven't found many schematics with ability to inject test signal, but there is at least one example (see attachment (don't remember original author, sorry)).
If there is arb waveform gen on the board, it isn't big step to route signal to inputs.
David Hess:
--- Quote from: Yansi on November 17, 2019, 07:07:41 pm ---I am not sure of hand how the GND coupling switch is accomplished. Do you have any sources I can refer to?
--- End quote ---
In old designs ground coupling is part of the AC and DC coupling circuit and requires another mechanical switch or relay. Functionally it connects the input through a resistor to the AC coupling capacitor while grounding the high impedance buffer. This provides two functions; it grounds signal to the high impedance buffer but it also precharges the AC coupling capacitor to the average DC input voltage which can be important in some applications to prevent damage to the input circuits. The Tektronix service manuals show various circuit configurations for achieving this but the most relevant ones are probably their early automated oscilloscopes where it was just a pair of relays.
Interestingly enough, on the Tektronix 2247A analog oscilloscope which provides input calibration like gf mentioned, ground coupling does not actually connect the input to ground. Instead it connects the input to a variable calibration voltage which may be serve as "ground" by being set to zero volts when ground coupling is selected.
Selecting between AC and DC coupling is possible without a relay and lots of oscilloscopes do it now (the paper by Steve Roach that I mentioned shows an example) but I am not sure how to select ground coupling without a relay which may explain why so many oscilloscopes cheat for this.
gf:
--- Quote from: David Hess on November 17, 2019, 06:53:49 pm ---Many low cost commercial designs use the extra resolution provided by a higher resolution converter as a free programmable gain stage.
--- End quote ---
You mean the polular HMCAD1511? Admittedly, it makes relatively simple/cheap frontend designs possible if the demands are not too high. When I look at my Hantek 6xx4BD, there are two cascaded 10:1 attenuators at each input (-> switchable between 100:1, 10:1, 1:1), then the "classical" JFET/BJT hybrid darlington voltage follower with DC servo, and finally a fully differential amp with ~2x gain, driving diferentially into the HMCAD1511. At 100mV/div, the HMCAD1511 operates w/o digital gain, and for all smaller V/div steps, simply the digital gain is increased (up to 50x at 2mV/div). There is no dedicated VGA in the signal chain. Due to the extra internal resolution the HMCAD1511 has only a moderate ENOB loss up to a gain of 32. The noise level I see (about 350-450µV RMS) is IMO more significant than the ENOB loss due to digital gain. One drawback of this design is that enabling the the 20MHz BW limit in the frontend does not reduce the noise level at all - which makes me conclude that most of the noise is obviously introduced after the BW filter (maybe even inside the ADC chip?).
gf
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version