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Zeeweii toyscopes - How do they do it?
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tatel:

--- Quote from: Fungus on April 23, 2023, 12:05:17 pm ---
--- Quote from: wasedadoc on April 23, 2023, 11:17:35 am ---It would appear to be sampling at 40MHz as stated in specification.  There is no or minimal anti-alias filtering. The displayed frequency is measured from the samples, not the input signal.

--- End quote ---

Yep. If you see an 18MHz sine wave on one of these 'scopes  there's NO WAY to know if it was a 22MHz input signal or not.

(and it's not a specific problem with these 'scopes, that's just how sampling works)

You need a hardware frequency counter to know the truth, and these don't have one.

--- End quote ---

Amplitude under -3dB isn't a clue?
Fungus:

--- Quote from: tatel on April 23, 2023, 12:11:47 pm ---Amplitude under -3dB isn't a clue?

--- End quote ---

That assumes you know what the amplitude is supposed to be.  :-//

(and the one I measured wasn't -3dB)
TimFox:

--- Quote from: Fungus on April 23, 2023, 12:05:17 pm ---
--- Quote from: wasedadoc on April 23, 2023, 11:17:35 am ---It would appear to be sampling at 40MHz as stated in specification.  There is no or minimal anti-alias filtering. The displayed frequency is measured from the samples, not the input signal.

--- End quote ---

Yep. If you see an 18MHz sine wave on one of these 'scopes  there's NO WAY to know if it was a 22MHz input signal or not.

(and it's not a specific problem with these 'scopes, that's just how sampling works)

You need a hardware frequency counter to know the truth, and these don't have one.

--- End quote ---

There is an easy way to see if your display is an alias:  change the time base by one notch in both directions.
An alias will change its displayed frequency.
(I remember chasing an apparent almost 60 Hz waveform, not line-synchronous, with an early DSO, where the signal was actually an external 10 MHz clock that was undersampled when the DSO time base was appropriate for 60 Hz.  The apparent frequency resulted from a small frequency mismatch, 6 ppm, between the external clock and the internal clock of the DSO.) 
Fungus:

--- Quote from: TimFox on April 23, 2023, 02:02:05 pm ---change the time base by one notch in both directions.
An alias will change its displayed frequency.

--- End quote ---

Only if the sample rate changes.
radiolistener:

--- Quote from: tatel on April 21, 2023, 07:51:40 pm ---Also, if we get an aliased waveform that is, say 48 MHz, how could the toyscope show an accurate frequency measurement of 52 MHz?
--- End quote ---

it depends on exact device implementation. Usually frequency counter is implemented in hardware on FPGA GPIO pin which is not sampled with ADC, but connected directly to the signal with analog comparator, so it can measure frequency which is higher than half sample rate of ADC. But some toy may use software trigger implemented after ADC, in such case it can't measure frequency above half sample rate.

Software trigger is used in a toys which doesn't have FPGA, just CPU. But Zeeweii has Lattice FPGA, so it has hardware trigger and can measure frequency which is higher than half sample rate. Usually hardware frequency counter on FPGA has synchronous design, so it's higher frequency is limited with FPGA clock frequency, which is equals to external oscillator, but also may be multiplied inside on a PLL.
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