Author Topic: Fy6800 vs Fy6900  (Read 16493 times)

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Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #75 on: July 17, 2019, 04:48:13 pm »
Before I ran this test, I'd thought the suggestion to use a Schmidt triggered input gate with the sine wave output to generate a square wave at any frequency, sanitised of this 4ns clock jitter, may magically resurrect this clock jitter in a demonstration of "Sod's Law".

It will be surprise for you to find "missing" jitter after trigger, but this is not magic, that's normal behavior...  ;D
This jitter didn't resurrected, it is already present in the sine wave. But you just didn't notice it due to slow slope :)

« Last Edit: July 17, 2019, 05:00:21 pm by radiolistener »
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #76 on: July 17, 2019, 04:54:09 pm »
So now we take the signal and simply invert it.  I did this by reversing the polarity of the signal generator, not the scope. The rising and falling edges swap places and now you have a very jittery signal, totally unacceptable for the purpose I stated.   However, if I change the trigger on the scope to the falling edge, it goes right back to the lower jitter number.  And, as expected, an FFT of the inverted signal is exactly the same.

it doesn't matter what polarity you select, the jitter is the same. This is how oscillscope trigger works. It shifting waveform in such way, so the trigger point will be at the center of display. And this is why you don't see jitter edge at the center. It still jittering, but oscilloscope shifting waveform in such way so you're don't see it. :)
« Last Edit: July 17, 2019, 06:13:04 pm by radiolistener »
 

Offline bdunham7

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Re: Fy6800 vs Fy6900
« Reply #77 on: July 18, 2019, 12:34:07 am »
So now we take the signal and simply invert it.  I did this by reversing the polarity of the signal generator, not the scope. The rising and falling edges swap places and now you have a very jittery signal, totally unacceptable for the purpose I stated.   However, if I change the trigger on the scope to the falling edge, it goes right back to the lower jitter number.  And, as expected, an FFT of the inverted signal is exactly the same.

it doesn't matter what polarity you select, the jitter is the same. This is how oscillscope trigger works. It shifting waveform in such way, so the trigger point will be at the center of display. And this is why you don't see jitter edge at the center. It still jittering, but oscilloscope shifting waveform in such way so you're don't see it. :)

I know how the trigger works.  I'm referring to the first rising edge after the trigger (the last picture). And my scope doesn't agree with you that the jitter is the same.  Why is that?
 

Offline Johnny B Good

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Re: Fy6800 vs Fy6900
« Reply #78 on: July 18, 2019, 01:34:50 am »
Before I ran this test, I'd thought the suggestion to use a Schmidt triggered input gate with the sine wave output to generate a square wave at any frequency, sanitised of this 4ns clock jitter, may magically resurrect this clock jitter in a demonstration of "Sod's Law".

It will be surprise for you to find "missing" jitter after trigger, but this is not magic, that's normal behavior...  ;D
This jitter didn't resurrected, it is already present in the sine wave. But you just didn't notice it due to slow slope :)

 It wasn't any surprise to discover that the theory worked in practice. I can't understand why you keep insisting that this 4ns clock jitter exists in the sinusoidal waveforms when it plainly does not. This 4ns jitter isn't hidden by the "slow slope" as you put it because it was never there in the first place!  ::)

 I've just run some tests using the 5MHz square wave as suggested by bdunham7. This is free of the 4ns jitter everyone seems to be obsessed with in this cheap signal generator. Furthermore, unlike the 10MHz square wave, it has a perfect 50% duty cycle.

 Using this as my trigger source, I see only the 2 or 3 hundred peak to peak picoseconds of noise jitter on sine wave and Sinc-Pulse when zoomed in at 1ns per division and the Y amplifier gain set to 20mV per division (some 26dB in excess of that required to show the full 5Vpp sine wave (2.5Vpp into the 50R dummy load) on the display so as to zoom in on the zero crossing region to increase visibility of this jitter noise). I can't use this much excess gain with the Sinc-Pulse without this making the jitter look worse but the steep flanks don't require as much gain to show the jitter noise anyway.

 If I offset the sine wave frequency by +/-10mHz, I see a steady drift against the square wave trace from which I'm triggering the 'scope. If I trigger from the sine wave (or Sinc-Pulse) set exactly on 5MHz and offset the square wave by 10mHz instead, I see it drift in discrete 4ns steps with both edges in unison, unlike the 10MHz case where each edge alternately jumped 4ns at a time giving the frequency drift a shuffling effect (this confirms the 50% duty cycle of the 5MHz square wave, btw).

 If the divisor used to generate the frequency from a 250MHz clock isn't an exact even integer as for the 10MHz case (25) you get this asymmetric effect on the duty cycle. The 5 and 25MHz settings both produce a perfectly symmetric square wave (divisor values of 50 and 10 respectively - both even integers). It works with all even integers from 6 upwards (4 corresponds to 62.5MHz so can't be used in this case). The 6 divisor corresponds to 41.666666666667MHz, an irrational number. The 8 divisor -> 31.25MHz a nice easy number to dial in. 10 we know already, 12 -> 20.833333333333MHz, another irrational number, and so on.

 The "Take away" from all this is that sinusoids are immune to the 4ns 'jitter' that afflicts square waves and can be used to drive a Schmitt triggered high speed logic gate to generate square waves free of this 4ns jitter issue after all. :)

JBG
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #79 on: July 18, 2019, 04:19:12 am »
I'm referring to the first rising edge after the trigger (the last picture). And my scope doesn't agree with you that the jitter is the same.  Why is that?

I think this is because the first picture is taken for 192.00 kHz and the second picture is taken for 192.10 kHz   :)
You can find it in Freq Mean column of oscilloscope statistic measurements  ;)
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #80 on: July 18, 2019, 04:28:29 am »
The "Take away" from all this is that sinusoids are immune to the 4ns 'jitter' that afflicts square waves and can be used to drive a Schmitt triggered high speed logic gate to generate square waves free of this 4ns jitter issue after all. :)

sine waveforms are not immune to jitter. But you're may be right. This 4 ns jitter may appears because square wave is generated with no interpolation. While sine wave cannot work without interpolation, so it definitely uses some interpolation. And this is may be the reason why square wave has much higher jitter than sine wave. But sine wave also has some jitter.

It needs to be tested with high speed digital level buffer which has good enough rise time, for example 1 ns or better.
 

Offline bdunham7

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Re: Fy6800 vs Fy6900
« Reply #81 on: July 18, 2019, 04:35:36 am »
I'm referring to the first rising edge after the trigger (the last picture). And my scope doesn't agree with you that the jitter is the same.  Why is that?

I think this is because the first picture is taken for 192.00 kHz and the second picture is taken for 192.10 kHz   :)
You can find it in Freq Mean column of oscilloscope statistic measurements  ;)

That accounts for the measured jitter being 40 times as high as the same signal measured differently?  :palm:  It's the exact same signal as the second display.  The the way the scope measures frequency keeps it from being very accurate under the conditions displayed.
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #82 on: July 18, 2019, 05:53:19 am »
That accounts for the measured jitter being 40 times as high as the same signal measured differently?  :palm:  It's the exact same signal as the second display. 

That's not the same signal, previously you're wrote:

I did this by reversing the polarity of the signal generator, not the scope.

You're changed parameter of signal generator and now you're talking that this is exact the same signal... We cannot assume that this is the same signal, because we don't know what is going on inside signal generator when you change this parameter.

There are several possible issues for that, for example:
1) signal generator bug, for inverted polarity it may produce a little different frequency
2) your're randomly changed frequency for a little (when applied inverted polarity parameter)

We can see 192.10 kHz average frequency on the second screen, while it was 192.00 kHz on the first one  ;)

So, this is NOT exact same signal :)
And different jitter behavior also confirms that these signals are different.
« Last Edit: July 18, 2019, 06:09:01 am by radiolistener »
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #83 on: July 18, 2019, 06:40:42 am »
If the divisor used to generate the frequency from a 250MHz clock isn't an exact even integer as for the 10MHz case (25) you get this asymmetric effect on the duty cycle.

asymmetric effect appears here due to bad sample rate to signal freqeuncy relation.

10 MHz wave has 100 ns period. In order to reproduce it with no asymmetric issue, you're needs to generate edges with 100 ns / 2 = 50 ns period.

FY6800 uses 250 MHz sample rate, this is 4 ns step. So it cannot reproduce edges for time period which is not integer multiple of 4 ns.

50 ns is not integer multiply of 4 ns, because 50 ns / 4 ns = 12.5. This is why you cannot generate 10 MHz at 250 MHz sample rate with no distortion.

These distortions is visible on the spectrum as even harmonics (20 MHz, 40 MHz, 60 MHz,...).

This effect can be solved by change DAC sample rate in such way so it will be able to reproduce edges of the signal with no time error. For example, you can use DAC sample rate 200 MHz, in such case it will be able to reproduce edges with 50 ns period, your 10 MHz square wave will be clean with no even harmonics.
« Last Edit: July 18, 2019, 06:50:15 am by radiolistener »
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #84 on: July 18, 2019, 07:14:16 am »
Can you run the same simulation with 9.8 and 10.0 MHz sine waves?

In order to run it for sine wave there is needs to know what kind of NCO they are using in FY6600. It can be implemented with different precision/complexity, and it leads to different spurious performance.

I don't have information how it is implemented in FY6600.
 

Offline bdunham7

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Re: Fy6800 vs Fy6900
« Reply #85 on: July 18, 2019, 04:06:47 pm »
There are three pictures of signals.  The first was not inverted, the second was inverted with the trigger set for falling edge, the third was  inverted with a rising edge trigger.  So the second and third are the same signal, but the measured jitter varies by a factor of 40.  I've stated all that clearly more than once.  The frequency "difference" you've latched onto is just nothing---the signal has not actually changed.
« Last Edit: July 18, 2019, 04:15:46 pm by bdunham7 »
 

Offline bdunham7

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Re: Fy6800 vs Fy6900
« Reply #86 on: July 18, 2019, 04:11:51 pm »
I  think it pays to be a little more hard-hearted with sellers.  You have 30 days from the last delivery date to file INR, and some of these guys will say anything to try and run the clock out.  This is seller honghong20, right?  I haven't received mine nor you yours, so I think it is possible that no packages have been sent.  Talk of refusing packages is pretty strange--you should NEVER refuse a package according to eBay--and the seller wouldn't want to incur the return shipping charges that would likely exceed the value of the transaction.

 Thanks for that worrying advice (I think :-//).

 I've only ever communicated via Ebay's INR Request system which advised me to try and resolve a solution with the trader and telling me I could raise the issue on the 19th if not resolved to my satisfaction by then.

 I've seen mentions of delaying tactics being used by some Ebay traders so I am mindful of the issue. In this case, we're dealing with different traders (although possibly the same under two different names). I'm dealing with a trader with the name of "hgfurniture2018",  "Based in China, hgfurniture2018 has been an eBay member since 09 Jun, 2018", so just over a year in business. His feedback score is 4608 with a 98.6% positive Feedback rating.

 I know that feedback ratings generally need to be larger than 99.8% to instil full confidence and I'm not sure whether a 13 month trading history is long enough to reduce the risk of a fly by night disappearing act being pulled - is it long enough?

 Do Ebay monitor these negotiations or do you have to explicitly describe them when seeking their assistance? The email from Ebay suggests they're aware of the content of these communications so, maybe they do record them for 'posterity' to help resolve any disputes. :-// If Ebay advise against refusing a delivery, that request should raise alarms if not an eyebrow or two.

 I've replied to their suggested remedy with caveats of my own so I'm now awaiting their response before I query with Ebay their request that I refuse a second package turning up after they've sent a replacement to see if this is an acceptable action in this circumstance.

JBG

Well, it looks like I'm not getting my bargain FY6600s after all.  I asked the seller for a tracking number and he replied:

Dear buyer,
sorry for the trouble that carrier informed us that your packages was lost on the way,
we can offer you refund,
please tell us and we can arrange it asap.
Sorry for the inconvenience .


Now he'll offer me a refund thru Paypal (that I'll have to pay fees on but I won't know that until too late) and then maybe he'll claw the money back from Paypal 3 months from now.  Or it is possible he is just wriggling out of a loss from a mistaken listing by refusing to ship.  Either way, a huge waste of my time.  And look at the feedback!

Oh well, I guess I should be glad eBay is in the middle.
 

Offline Johnny B Good

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Re: Fy6800 vs Fy6900
« Reply #87 on: July 18, 2019, 06:51:27 pm »
Can you run the same simulation with 9.8 and 10.0 MHz sine waves?

In order to run it for sine wave there is needs to know what kind of NCO they are using in FY6600. It can be implemented with different precision/complexity, and it leads to different spurious performance.

I don't have information how it is implemented in FY6600.

 No one does. It's a black box with closed source encryption protected firmware in its front panel controller driving an fpga that does the donkey work in ways we can only speculate on. We can make educated guesses from published articles on various DDS technologies but there's no easy way to establish the actual details of Feeltech's implementation in their FY6*** series of function generators.

 At just 80 dollars or so,  it doesn't take too much imagination as to how Feeltech implemented their solution to masking this 4ns jitter issue with square and pulse waveforms. I'm guessing they took a pragmatic approach, allowing the 4ns jitter rather than use a more complex solution to compromise rise and fall times as seems to be the way Siglent have done this in their later low jitter models.

 It helps that their choice of unusually high sampling rate would hold this down to a mere 4ns versus the more typical 8ns or worse of the earlier models of expensive DDS signal generators. However, the 7.2ns rise and fall times on square waves (but not on its close cousins possessed of "infinitely fast" peak to peak transitions), suggests Feeltech had deliberately endowed its edges with additional 'slope' in the hope that it would mitigate this issue.

 Unfortunately for Feeltech, if the unusually slow transition times of 7.2ns versus the 3.8ns that can be achieved with a 250Msps sampling rate was the result of such a deliberate design choice, it hasn't done any good. The jitter is still present despite the enforced slower rise and fall times. It seems they may have compromised the square waveform for zero gain. Presumably Feeltech's designer had felt so confident that this "No Brainer" trick would work, he simply neglected testing that it would actually be of benefit.

 Getting back on topic, the square wave output from an FY6900 just might possibly now have faster 3.8ns transition times. ::) Anyone reviewing this latest offering from Feeltech should take particular interest in these two zero cost enhancements, the restoration of 3.8ns edges on the square wave output and the reworking of the 86R 20ishdB attenuator that's switched into circuit for voltage settings below half a volt p-p,  into one with a 50R impedance.

JBG
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #88 on: July 18, 2019, 07:52:28 pm »
At just 80 dollars or so,  it doesn't take too much imagination as to how Feeltech implemented their solution to masking this 4ns jitter issue with square and pulse waveforms. I'm guessing they took a pragmatic approach, allowing the 4ns jitter rather than use a more complex solution to compromise rise and fall times as seems to be the way Siglent have done this in their later low jitter models.

I'm agree, it has pretty good performance. It seems that it prety close to the best quality for fixed sample rate DAC. May be there is possible improvement for sine wave NCO, but I don't think such improvement will be significant. So, it does it's job very well... for fixed sample rate DAC of course... :)

But it can do it better with variable sample rate  ^-^
« Last Edit: July 18, 2019, 07:57:07 pm by radiolistener »
 

Offline Johnny B Good

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Re: Fy6800 vs Fy6900
« Reply #89 on: July 18, 2019, 08:08:21 pm »

Well, it looks like I'm not getting my bargain FY6600s after all.  I asked the seller for a tracking number and he replied:

Dear buyer,
sorry for the trouble that carrier informed us that your packages was lost on the way,
we can offer you refund,
please tell us and we can arrange it asap.
Sorry for the inconvenience .


Now he'll offer me a refund thru Paypal (that I'll have to pay fees on but I won't know that until too late) and then maybe he'll claw the money back from Paypal 3 months from now.  Or it is possible he is just wriggling out of a loss from a mistaken listing by refusing to ship.  Either way, a huge waste of my time.  And look at the feedback!

Oh well, I guess I should be glad eBay is in the middle.

 I'm sorry to hear that. Whilst the exchange of correspondence between hgfurniture2018 and myself during this past week has finally resulted in a second shipment now in the process of being made, the chinglishisms do hint vaguely at the possibility of this being a much better thought out scam than yours, so I may also ultimately suffer the same fate - non delivery and a refund.

 After being assured the FY6600-60M they were going to send me was of new stock and my request that they at least provide estimated dates for its delivery in my penultimate message, they responded with this:-

"'dear customer, sorry for the problem . It may need 10-15 days to wait another one .Is it fine ? best wishes"

to which I replied:-

"Dear Sir, Ten to fifteen days delivery time is acceptable. You may proceed with the shipment. Regards"

 This elicited the following reply:-

"dear customer, We have arranged to send you a new one. Everything would be fine.please wait patiently. Please don’t worry. Any question later, please kindly come back to me firstly, i will try my best to help you solve it well. Best wishes and have a nice day! Best wishes"

 In their original reply, they had offered me the options of sending another generator or provide a refund. Ever since then, the conversation was all about meeting my caveats and verifying the address details they had before sending the replacement out. If it is a ruse to avoid selling an item they could otherwise charge double for, the address verification request was rather a nice touch. ::)

 I'll not be closing the request until it's either delivered or I've given up and requested a full refund. The incident won't be closed until I have that sig genny in my grubby mitts or else a full refund into my account. It's possible I might effectively land up in your situation, only another 3 or 4 weeks down the line.

 Now all I can do is sit tight and wait in the hope that Ebay's involvement will force them to keep their promises. Luckily, it's not a "Life or Death" matter, just a waste of time with only a refund for consolation should the worst come to the worst. It's just that it would be rather nice to have an unmodified 'spare' to play around with to compare with the original unit I've upgraded "beyond all recognition".

 Ah well, only another 10 to 15 days to go, according to the seller. :-//

JBG

 

Offline Johnny B Good

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Re: Fy6800 vs Fy6900
« Reply #90 on: July 18, 2019, 10:46:11 pm »
At just 80 dollars or so,  it doesn't take too much imagination as to how Feeltech implemented their solution to masking this 4ns jitter issue with square and pulse waveforms. I'm guessing they took a pragmatic approach, allowing the 4ns jitter rather than use a more complex solution to compromise rise and fall times as seems to be the way Siglent have done this in their later low jitter models.

I'm agree, it has pretty good performance. It seems that it prety close to the best quality for fixed sample rate DAC. May be there is possible improvement for sine wave NCO, but I don't think such improvement will be significant. So, it does it's job very well... for fixed sample rate DAC of course... :)

But it can do it better with variable sample rate  ^-^

 I've just been looking at the specifications for the Siglent SDG1062X Signal Generator as the most comparable to the FY6600 of the relatively inexpensive upmarket class of signal generator - a snip at only £440 compared the £75.66 I paid for mine (admittedly, I've spent some 40 to 50 quid on parts to upgrade it ::) ).

http://www.labtronix.co.uk/drupal/sites/default/files/sdg1000x/SDG1000X_DataSheet_DS0201X_E01A.pdf

 Prior to my purchasing the FY6600, I had been considering the cheaper 30MHz version, the SDG1032X for a mere £360 (just a fiver less than I'd paid for my SDS 1202X-E dual channel 200MHz BW DSO a week or two before). I got the impression that there was a large element of "Swings and Roundabouts" between the Siglent and the Feeltech generators so thought, in view of all the upgrading fun to be had, the best "Starter" option would be to go for the FY6600. A mistake at £75.66 is far easier to swallow than one costing £360 or £440. :)

 You have to start spending ten times the price of that FY6600 before you get a signal generator that shows no weaknesses compared to any of the Feeltech's best performance figures. For anyone with a fresh or renewed interest in electronics as a hobby, these Feeltech generators are an excellent starter choice - even its limitations are an education alone. ::)

 Anyway, I thought I'd try and record 30 second movie snippets of what happens when you configure a square wave on one channel with a sine wave on the other, both set to 20.833333333333MHz with first one offset by 30mHz with the other triggering the 'scope, then a reversal of which triggers the 'scope and which has its frequency offset.

 I recorded 320 by 240 30fps MOV files which I was able to compress below the 1MB limit using handbrake. The low resolution leaves a lot to be desired (like more resolution - but, what the hey! :) ) but is just sufficient to show how each waveform performs its 'drift' against a jitter free reference (20.833333333333MHz being one of those awkward 'golden' jitter free frequencies for our square wave output).

 Having discovered eeblog's limited choice of file extensions, I've stored them as zip files. Being already compressed, there's no point in asking any lossless compression algorithm to compress the uncompressible. ::)

 They're named to reflect the test. Whatever is reference always has its frequency set to the 'golden' one with the other being offset by 30mHz. The timebase is set to 5ns per division with both channels set to 500mV per division. The generator is set for a p-p of 5v both channels terminated with 50 ohms. TBH, I'm impressed that the square wave looks as square as it does at this frequency.  ;D

JBG

PS, the damned kakamaimee interface eevblog is using has screwed my attachment selection yet one more time so I've had to do a follow up post since there's no obvious way to retrieve the situation.
« Last Edit: July 19, 2019, 03:36:32 am by Johnny B Good »
 

Offline Johnny B Good

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Re: Fy6800 vs Fy6900
« Reply #91 on: July 18, 2019, 10:54:58 pm »
 Hopefully, this is the missing zip file. >:( >:( >:( :palm:

JBG

[Edit]  I see it worked this time.
 

Offline radiolistener

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Re: Fy6800 vs Fy6900
« Reply #92 on: July 19, 2019, 01:11:39 pm »
Johnny B Good, is it possible to calibrate FY6600 for real amplitude voltage?

I mean is it possible to calibrate it in such way, so I can set 1 Vpp, connect 50 Ohm load on the output and get real 1 Vpp. This is expected behavior and it is too complicated to convert voltage into some strange "Chinese volts" which is twice higher than standard volts.
 

Offline pantelei4

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Re: Fy6800 vs Fy6900
« Reply #93 on: July 19, 2019, 02:20:44 pm »
I mean is it possible to calibrate it in such way, so I can set 1 Vpp, connect 50 Ohm load on the output and get real 1 Vpp.
It has an output resistance of 50 ohms.
And the amplitude is set without load.
 

Offline Johnny B Good

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Re: Fy6800 vs Fy6900
« Reply #94 on: July 19, 2019, 04:28:56 pm »
Johnny B Good, is it possible to calibrate FY6600 for real amplitude voltage?

I mean is it possible to calibrate it in such way, so I can set 1 Vpp, connect 50 Ohm load on the output and get real 1 Vpp. This is expected behavior and it is too complicated to convert voltage into some strange "Chinese volts" which is twice higher than standard volts.

 I'm glad you asked me that question, radiolistener.  >:D  It gives me an excuse (and the opportunity to attach higher resolution movie files of the previous 'scope traces) to explain why this lack of accounting for high impedance versus matched impedance voltage levels when setting the output level is not the heinous crime many posters to this thread seem to think it is.

 I guess the Chinese didn't want to add that particular 'Bell' to the "Bells and Whistles" list. Not only that, they may have wanted to avoid any possible insult to our intelligence by suggesting that us 'westerners' aren't even in possession of the most rudimentary of mathematical skills (i.e. being unable to handle simple multiplication and division by two and optionally 1.4141414) ::)

 This halving of the output voltage when driving a correctly terminated cable (or just a terminating dummy load directly) from a generator with an output impedance matching said cable and dummy load, is expected behaviour. If you observe unexpected behaviour such as no to very little volt drop or much more than the expected 6.02dB drop, you know something is wrong when driving a circuit that supposedly has an impedance of 50R.

 Indeed, when it comes to having the generator output impedance match the load, it doesn't have to - the key thing being that the terminating impedance matches that of the connecting cable (transmission line). For instance you can use a generator with an impedance of 75 ohm to drive a 50 ohm line, allowing for the additional 1.94dB drop (a 60% drop versus the matched case of a 50% drop).

 In any case, the operator usually has to press a button to tell the signal generator that it should scale the p-p voltage setting to match the operating mode (matched or high impedance loading). The fancy and more expensive signal generators may well offer such a feature. Considering your question, they probably do and possibly by monitoring the current versus voltage output to calculate the actual impedance it is connected to so as to leave the operator a hand free to scratch their balls.

 Personally speaking, I'm rather glad that the Chinese designers of cheap affordable T&M kit have avoided this insult to the intelligence of their hobbyist target market demographic by not adding such unnecessary "Bells and Whistles" which would detract from the budget allowed for offering the  best possible performance at such a low price point. There's only so much money to go round and I'd rather it was all spent on the core features of my signal generator rather than diluted by such "inessentials".

 In this case, the voltage setting is based on the unloaded condition, leaving the operator to expect half this voltage if and when driving a matched load. This characteristic of your 'basic' signal generator is a very useful diagnostic aid in the hands of anyone familiar with basic transmission line theory, in particular the bit that says maximum power transfer between the generator and its load occurs when their impedances match each other as well as any transmission line linking the two.

 Calibrating the voltage setting for the unloaded condition is the most logical choice (it's very easy to check this well defined unloaded voltage output). It's also useful that it's calibrated this way since anyone working with audio frequency gear outside of communications equipment maintenance or design, will generally be driving high impedance loads, making the voltage setting immediately useful without the complication of having to divide by two or subtract 6dB. >:D

 In answer to your question... what was it again? Oh yes, the business of the unloaded voltage being a "Chinese Thing"(tm) and the possibility of having the voltage setting expressed as that which a matched load would produce (i.e. display 1Vpp instead of 2Vpp to account for the loading effect of a termination matching the output impedance of the generator).

 The answer in the case of these Feeltech generators is no (at least for the moment - if development of an OSS front panel firmware package ever resumes (seems rather unlikely though) you may well get the opportunity to add this feature to your "Wish List").

 However, in the case of other more expensive T&M kit there's nothing to stop designers from implementing the additional complexity to achieve this, and to varying degrees of sophistication, ranging from a simple press button to toggle between the two basic calibrations which match the two reference impedance conditions of either a matched or open circuit (Hi Z) loading (a feature that could trivially be added to these cheap Feeltech generators), to a more sophisticated monitoring of both output voltage and current to not only compensate for the loading but also to calculate the impedance of the attached load  for display to the operator (only stopping short of wiping said operator's arse, of course! ::) ).

 Anyway, that's my view of the matter. You're free to accept or reject this viewpoint; I don't really care, I've said my piece on the subject.

 I've (hopefully this time!) attached higher definition movie files of the traces I posted in my previous two postings mainly to reduce the pain of having to watch the effect at such an execrable resolution of just 320x240. Please enjoy!  ;)

JBG
« Last Edit: July 19, 2019, 07:28:55 pm by Johnny B Good »
 

Offline Johnny B Good

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Re: Fy6800 vs Fy6900
« Reply #95 on: July 19, 2019, 04:41:16 pm »
I mean is it possible to calibrate it in such way, so I can set 1 Vpp, connect 50 Ohm load on the output and get real 1 Vpp.
It has an output resistance of 50 ohms.
And the amplitude is set without load.

 That is so succinctly put!  :)

JBG
 

Offline CDaniel

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Re: Fy6800 vs Fy6900
« Reply #96 on: July 19, 2019, 04:53:14 pm »
So the output is "known" by the microcontroller , nothing new ,  but a Siglent or a similar generator is actually measuring the output in some way if you load it down ?
About this you don't find anything in manuals .
 

Offline bdunham7

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Re: Fy6800 vs Fy6900
« Reply #97 on: July 19, 2019, 05:00:58 pm »
So the output is "known" by the microcontroller , nothing new ,  but a Siglent or a similar generator is actually measuring the output in some way if you load it down ?
About this you don't find anything in manuals .

On my Siglent SDG2042X/2112X you "select" the load impedance and it displays the voltage, halved when 50R is selected, but nothing actually changes in the output and no measuring is done.  The only difference between it and the lowly Feeltech is that it calculates--estimates--the amplitude with a hypothetical 50R load. 
 

Offline maxwell3e10

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Re: Fy6800 vs Fy6900
« Reply #98 on: July 19, 2019, 05:49:37 pm »
That is how all function generators work. They either assume a certain load or let you set the load resistance and adjust the displayed voltage.
 

Offline Jacon

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Re: Fy6800 vs Fy6900
« Reply #99 on: July 19, 2019, 06:18:20 pm »
I've (hopefully this time!) attached higher definition movie files of the traces I posted in my previous two postings mainly to reduce the pain of having to watch the effect at such an execrable resolution of just 320x240. Please enjoy!  ;)
Previous files was in common .mkv format, easily playable.
What should I do with these new files in unknown format ?!
 


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