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FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator

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Andreax1985:
I did some testing. Here's what I got:

i) If I use my multimeter and set the FY6800 to DC I get almost perfect agreement at all voltage levels (+/- 2%, but my multimeter is a 15euro unbranded device, so nothing to heavily rely upon).

ii) If I use my DSO (Siglent 1104X-E, 1M input) with 1X probe onto the FY6800 BNC input, I get attenuation starting from 3MHz onward, but this is to be expected. Every 1X probe is strongly bandwidth limited.

iii) If I use my DSO (1M input) with 10X probe onto the FY6800 BNC input, I get no attenuation until 60Mhz. I observe a 10mV DC offset.

iv)  If I use my DSO (1M input) with the BNC-BNC cable supplied with FY6800, I get slight attenuation at higher frequencies (as expected, I think, from this kind of cable), and only slight 1-2mV DC offset (as experienced by Zenith).

So: are my probes (or my DSO) not so good? Should I get better quality 10X probes?

Zenith:
Andreax1985,

I redid the measurements using my Siglent SDS 1102CML+.

I had the scope do its auto calibration. I set both channels to 2mV/div and  set acquisition to averaging over 16 readings (which gets rid of rapid fluctuations in readings) then called up all voltage measurements. Channe1 1 gives Vpp of between 0.08mV and a mean of -0.08mV. Channel 2 gives Vpp between 0.16mV and 0.24mV with mean between 0.00mV and -0.08mV. The scope looks as if it only deals in voltage steps of 0.08mV on that range.

A 10mV, 0.000mV offset, 100KHz  sine from channel 2 of the FY6800 gives a Vpp of 9.4mV and a mean of 1.04mV. Changing the coupling on the scope to AC gives a Vpp of 9.4mV and a mean of zero. Changing back to DC and setting the offset on the FY6800 to -0.001V changes the mean to -0.016mV. Changing the amplitude on the FY6800 to 100mV and offset zero, gives Vpp 0f 102mV and a mean of 1.6mV. A square wave produces near identical results. Note that the mean doesn't change much with amplitude.

I'm inclined to believe that the output of the FY6800 has an offset of about 1mV and that maybe the waveform isn't quite symmetrical, but these are measurements on the limits of what both the scope and DVMs can do. The offset is nothing to worry about, in fact it's pleasantly small, and it's certainly not worth trying to adjust it down further. It seems as if your FY6800 has a similar small offset. If you have some particular need to get the offset down further, you can reduce it by setting the offset on the FY6800.

I had doubts about the 100MHz probes which came with the Siglent, so I compared them against an HP10040A probe using an HP8640A sig gen. The conclusion I came to is that the supplied probes are up to the job. Sometimes they need wiggling so they make a good contact. Top flight probes are nice, but expensive. From what you've said, I think you should spend the money on a better DVM instead.

Generally, I've found my Siglent scope to be surprisingly good and well worth the money, although it has the odd quirk. There's no doubt if you pay more, you get a more capable scope, but I see no need to sell mine and get a better one.

radiolistener:

--- Quote from: Andreax1985 on February 12, 2019, 09:57:31 am ---iii) If I use my DSO (1M input) with 10X probe onto the FY6800 BNC input, I get no attenuation until 60Mhz. I observe a 10mV DC offset.
--- End quote ---

This is because your oscilloscope probe has circuit to correct impedance mismatch on both sides of probe cable. And standing waves in the cable doesn't affect measurement. Because probe designed to work in such way.


--- Quote from: Andreax1985 on February 12, 2019, 09:57:31 am ---iv)  If I use my DSO (1M input) with the BNC-BNC cable supplied with FY6800, I get slight attenuation at higher frequencies (as expected, I think, from this kind of cable), and only slight 1-2mV DC offset (as experienced by Zenith).

--- End quote ---

This slight attenuation appears because your cable isn't terminated properly and you're see effect of increasing/decreasing amplitude due to standing waves in the cable. These standing waves appears due to reflections from non-matched cable end. When there are standing waves in the cable, your amplitude measurements will depends on the cable length and wavelength of signal. So, if you will increase frequency, or reduce cable length the amplitude will be changed.

Andreax1985:
Thanks! And why DC offset is higher with X10 probe than with the unterminated BNC-BNC cable?

CDaniel:
The DSO is switching ranges between 1:1 and 1:10 ( and it is not the simple divider found in multimeters ) , so it is not the cable ... thats why I said don't trust the oscilloscope for small offsets.
But you can investigate this by using a small DC variable voltage , a battery and a potentiometer for example .

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