Problem with measuring function generator

[Tomsik]

Hello all, when I was playing with my Chinese function generator FeelTech FY3200S I noticed a problem with a decreased amplitude of a tested signal. The test signal was 6MHz sine wave, 1V amplitude.

When I put external 50ohm terminator to the oscilloscope, I see amplitude just 460mV, therefore I want to find a problem.
I tried to put terminator directly to the generator and through T-adaptor, I connect oscilloscope, I see amplitude 496mV.

Oscilloscope is set to 1X attenuation, measured with bnc cable not with probes.

Where is the problem? In the coaxial cable?

See attachments
Please help me.



 

[_Wim_]

Your generator has an output impedance of 50 ohm. This means the amplitude measured by the scope will be only 1/2 of the amplitude of the generator. This is the case for most (all) signal generators. Typically you have a setting in the function generator for high impedance or 50 ohm. When you select 50 ohm, the generator will output a signal twice as high to compensate. In summary, this behaviour is normal.

[Tomsik]

Your generator has an output impedance of 50 ohm. This means the amplitude measured by the scope will be only 1/2 of the amplitude of the generator. This is the case for most (all) signal generators. Typically you have a setting in the function generator for high impedance or 50 ohm. When you select 50 ohm, the generator will output a signal twice as high to compensate. In summary, this behaviour is normal.

Please, read my answer again... I understand it But you don't understand my problem. I want to know, why I measure just 460mV instead 500mV!!!
And for comparing I terminated directly generator and I measure 500mV!! Why?
Where is 40mV?
It is the problem...

[TurboTom]

Try another BNC cable. Seems like there's a problem either in the connectors or the cable itself. Could also be a dodgy BNC-Y. Have you got a BNC-feed-through-terminator? That would be a better way to go to terminate the scope.

You may want to check the DC resistance of the complete connection assembly (signal conductor) with a multimeter. Obviously, this should be < 1 Ohm.

[Tomsik]

Try another BNC cable. Seems like there's a problem either in the connectors or the cable itself. Could also be a dodgy BNC-Y. Have you got a BNC-feed-through-terminator? That would be a better way to go to terminate the scope.

You may want to check the DC resistance of the complete connection assembly (signal conductor) with a multimeter. Obviously, this should be < 1 Ohm.

I measured BNC cable with BNC-Y adaptor, it has approx 0.6 ohms.

[egonotto]

Hello,

make the some with 1 kHz and look Dave's Video:



Best regards
egonotto

[_Wim_]

Your generator has an output impedance of 50 ohm. This means the amplitude measured by the scope will be only 1/2 of the amplitude of the generator. This is the case for most (all) signal generators. Typically you have a setting in the function generator for high impedance or 50 ohm. When you select 50 ohm, the generator will output a signal twice as high to compensate. In summary, this behaviour is normal.

Please, read my answer again... I understand it But you don't understand my problem. I want to know, why I measure just 460mV instead 500mV!!!
And for comparing I terminated directly generator and I measure 500mV!! Why?
Where is 40mV?
It is the problem...

That was not very clear from your question. You should also add the 50 ohm resistor from the generator in your drawing to make clear you understood that.

What happens at other frequencies (5MHz, 4MHz, 3MHz...)? Is it always 40mV, does the variation go up and down or only down when the frequency changes? 

[Tomsik]

 It looks that in 1MHz starts decreasing of amplitude.

[Labrat101]

Hi ,  you answered your own question ..
FeelTech FY3200S cheep cheesiness    .
  This unit is the earlier version of the FY6800 & 6600 etc with a few bad updates.
First the 50ohm Must be at the scope end .
Plus they are not accurate..   Your scope  which is properly far better.  Increase the voltage slightly.
to the desired required voltage . Seen on your scope.  with 50ohms @ scope a T will work fine if it fits snug.
example.
 ie require 0.5v  the DDS will be set at 1.024 . scope reads 0.501v.   scope set @ x 1 
 Therefore:: The test signal was 6MHz sine wave, 1V amplitude.
       2.024v on DDS    Scope reads 1.001  (example)
 (The DDS displayed voltages are NOT accurate ) .
   THE TERMINATOR MUST BE AT THE SCOPE END ONLY.

Read  previous post on FY6600 & FY6800 etc . there is thousands of articals on these units.
The output is sort of 50ohms

If you have a good bench meter with a banana BNC connector set the voltage on the DC (wave select )
see what you get . then change to the desired wave and set the voltage required +_ the error.

Also Maybe the power supply is not stable if your using the cheesiness SMP it came with. 

Have Fun

[threephase]

Welcome to the world of transmission line theory.......

The behaviour you are seeing seems reasonable to me.

With the 50 Ohm terminator installed at the function generator end, the end of the coax at the scope is terminated by the input impedance of the oscilloscope, likely to be 1 MOhm. Therefore there will be a reflection of the signal you are applying at the scope termination that will sit on top of the initial signal and give the impression of a higher voltage, 496 mV in your case.

When you moved the 50 Ohm terminator to the scope input, this reflection was dampened and you saw a true reflection of the output level of the function generator, that you measured as 460 mV.

The 40 mV drop you saw will be dependent on the amplitude flatness specification of your particular function generator. Over its output frequency range, there will be an expected drop from the output voltage setting, that will increase as the output frequency is increased. The 40 mV drop could be perfectly acceptable for your generator, the reading will also be affected by the accuracy of the 50 Ohm terminator. A terminator with slightly less than 50 Ohms impedance will drop the output voltage more than expected.

You may be able to see the behaviour of the reflection better by switching to a square wave output, but your function generator may not support as high a frequency output as it does for sine wave. With the 50 Ohm terminator at the function generator end, the top of the square wave may slope upwards from left to right, or it may curve upwards at the top right corner of the square wave. As you change frequency, that slope will vary, this is the reflection you are seeing.

When you switch the 50 Ohm terminator to the oscilloscope input, the square wave will now have a flat top as the reflection has been dampened.

Kind regards

[TurboTom]

@Tomsik: How long is your BNC cable? I tried to replicate your findings with a DG800(+) and a DS1000Z with a feed-through terminator and a 1m quality BNC cable. When I switch sine frequencies between 1MHz and 6MHz in this configuration, the effect of an ill-terminated transmission line (with the terminator placed at the AWG output) is barely noticable (maybe 1 or 2% of the amplitude, clearly neglible by the standards of quantitative measurements on an oscilloscope). A visible effect of maybe 5% is present when increasing the frequency to 10MHz.

My setup, if properly terminated (i.e. the terminator is placed at the scope input), doesn't show any deviation of the amplitude when changing frequency.

There may be another issue depending on how you measure the amplitude: The automatic readout (measurement) function may be affected by the frequency. I found this to be the case on the DS1000Z to some extent, even though the amplitude of the waveform on the screen stayed perfectly the same. You've always got to keep in mind that an oscilloscope isn't an instrument for precision measurements into the sub-percent tolerance range. Moreover, automatic measurements may impose considerable tolerance. For the DS1000Z, I didn't find tolerance specs for the measurements, but Rigol specifies a gain (amplitude) error of +-3% full scale for input ranges >10mV/div at the BNC.

Since I'm not familiar with your AWG, I cannot tell for sure if this may also be a contributing factor to the problem. At least, at the frequencies that you selected, and provided you're using a BNC cable of moderate length, transmission line effects are still neglible.

[Labrat101]

Hi ,
We have one of these in our Lab ..
That's why I made the previous Post with how to set this thing ..
Sorry to Say the FY3200 was one of feeltechs cheepo's
with Just a few out of spec 's ..
You will not create his problem on your signal Gen  . as it does not exist .
What you guys mentioned is 100% correct .  Fy3200 has a noisy SMPS. voltage not stable etc.
so the reading out are about right and he will have to correct them by adjusting the voltage out put to correct
the error.  from unit to unit the error can vary anywhere 200mv to 400mv so no two are the same.if lucky ..
Calibration is required . 
Sorry but these are what they are .. So just terminate @ the scope
The 6600 & 6800 are a bit better and the 6900 is a mish of both . also not too bad.

[CDaniel]

If a dedicated oscilloscope probe has a bandwidth of just ~6MHz when used 1x , then I don't think your direct cable is better .
Use a probe with devider 1/10 that has a good bandwidth and see the result .
With such a probe and no 50ohm termination my Feeltech 6600 is pretty linear up to 60MHz .

[Labrat101]

If a dedicated oscilloscope probe has a bandwidth of just ~6MHz when used 1x , then I don't think your direct cable is better .
Use a probe with devider 1/10 that has a good bandwidth and see the result .
With such a probe and no 50ohm termination my Feeltech 6600 is pretty linear up to 60MHz .

Yes but you have the 6600 the 3200 is sort of there first proto type . And non of the bugs were smoothed out.
The 6600 / 6800 give quite a nice sine all the way upto 60Mhz I can get a 25Mhz square as well with the 4ns jitter
which is an inheritance of the these units.
 6mhz on his unit is not one of the Magic frequencies so there will be errors .
Yes a good probe set @ x10 has a higher bandwidth .
He didn't Mention What Scope he was using !!
He may be using the simple $20 Kit version with a 10mhz bandwidth .
or something with a low BW ..

[newbrain]

If a dedicated oscilloscope probe has a bandwidth of just ~6MHz when used 1x , then I don't think your direct cable is better .

I hope it is, unless it's a complete dud.
After all BNC+RG58 are used with much higher frequency than 6 MHz and much longer than the usual 1m lab cable.

Here is just one of the first hits on google, see the declared attenuation.

See, of course, our kind host's video:

[Labrat101]

What scope is being used ?
It's like saying  I bought 
  A car and it doesn't work .
What's wrong..   
You can't solve a problem without all the Details..

[Tomsik]

I use Tektronix TBS1052B with 50MHz bandwidth. It must be able to measure that...

[Labrat101]

I use Tektronix TBS1052B with 50MHz bandwidth. It must be able to measure that...

OK  now .
Yes your scope is ok 6mhz sine . Does it have a self calibration setting in the utility menu .
which all Tekronixs have. turn on your scope for at least 20mins and run the self Cal.
 MAKE sure there is nothing connected to the Scope while self Cal is in progress . (very important)
After .. that has completed ..
Then do this . don't ask why just do it.
First set your DDS to 6MHZ @ 1  P-P volt .
Connect a 50ohm Patch lead direct to the scope  (Yes NO. 50 ohm terminator )
Press the AUTO button ..  Measure  P-P voltage  as this will match DDS Gen output .
What voltage appears on the screen ?  example 0.95v P-P   (Peek to Peek) 
Raze the voltage on your DDS so your scope reads 2.000v. P-P  OK.
Now put a 50ohm Terminator on your scope .
OK..
Your scope should now read 1.000 volt  P-P  OK.   If its more than a few mv out .
Check that the connections on your T or Terminator are sitting snug and not loose .
 The voltage reading on a scope voltage vary to the setting . set measure to P-P  is what your DDS gives .

BTW your scope is OK for Sine upto 50MHz apx
Square wave about 10mhz apx.
Other waves about 5Mhz  .
If you have the original Tektronix Probes . X 1 is about 5Mhz  X 10 is Hi or as stated on the probes 70 Mhz. (lable)
ALso remember the probes also have to be calibrated .

The FY3200 is Not a Precision instrument Some are better than others .

See what your results are .  This will get you very close . If you require Plus or minus 10uv this possible via another
procedure .
 
  I guess you are a beginner of the electronic world of Why . How .. And Magic Smoke . 
 
   But Not now . first steps @ a time .

[Tomsik]

I've solved the problem. Problem is due to the cable.
I bought another BNC cable from Testec for comparing. When I measured amplitude of tested wave I measured just 464mV, but when I change cable beyond new cable I measured amplitude 496mV!

See to attachments.

  I guess you are a beginner of the electronic world of Why . How .. And Magic Smoke . 
 
   But Not now . first steps @ a time .

Yes, you are right. I'm beginner, I have experience just 4 years of PCB boards testing and 3 years experience with PCB designing (nowadays).
I just needed your advice, idea or solution because I'm at home, I work from home due to coronavirus restrictions and I don't have more equipment at home.
I had just one BNC cable, BNC-Y adapter, oscilloscope and just one generator (Chinese) I wasn't able to compare.

Many thanks to everyone for the advice!!!

[nctnico]

Unexpected measurement results are cable problems in 99% of the cases so the first thing should be to try a different cable.

[Labrat101]

Hi I am Glade that ur happy
And you can tweet the volt on the DDS to 1.004 to give that little more required .
I know all the other say cables etc but these Units are not Calibrated so it would not make a lot of difference .

 If you have a Banana plug to BNC change the Wave to DC and 1.0000v and connect it to a digital volt meter
 make sure you are sitting and you will see how in accurate it is and slowly raise it to 10v.
If your DDS does not have DC function set to square wave with Duty set to 99.99% as this will give a DC out.
and see what it reads .. Just for fun ..   
Remember you always need twice the voltage setting when using a 50ohm terminator .

But the cable/s you are using is just fine  for this DDS FY3200 and a 50Mhz BW scope .
 A better cable will not give you any thing that you will see on this set up.
 BTW  can check the square wave on you fy3200 to its Max 10Mhz  .. How stable is it . ??  just curious .

 Good luck on your project . 
 
  "If its Getting HOT its working"  If its Getting COLD Its your Coffee..

[tubularnut]

If it's any consolation, I've just hooked up an FY3200 to my Siglent SDS1104X-E. 6MHz sine, 1V, 50ohm term at scope via coax.

The scope shows 488mV p-p, so you are in the ball park with your setup.

Meanwhile my SDG1062X shows 501mV p-p.

So overall, I shouldn't worry too much about your cables or setup, the FY is what it is, just tweak it to match what you desire 

Edit: Though I see you've got better results with another cable  and my FY is worse than yours 

[Labrat101]

BTW No  2  FY units will Ever be identical  .. Maybe 1:10,000 may be close.

Hi Over the last 50yrs I have calibrated a lot of stuff.
and after calibrating  getting two identical scopes and after a few hours in a non climate controlled room.
They will show a slight +_ difference .  Due to so many factors you don't want to know,
 If your doing a project you should always do a Self calibration with no inputs connection after 20 / 30 minutes warm up.
 Every one uses there scope differently  ( unless you are working in High tech Then you go by THE book )

 Most common scope in 100- 200 mhz range are not accurate to uv .
  Voltage showing will be within the scopes rated tolerance Spec  .
     Not as good a  8 digit Bench Fluke  'example'.

  Most people here don't manage to find a $50k scope in the dumpster . "O how I wish ."
   These Babies are good . Wear cotton clove when touching them .

   Have Fun & stay safe .