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

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Johnny B Good:

--- Quote from: Zenith on February 12, 2019, 12:39:39 pm ---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.

--- End quote ---

 I bought my Siglent SDS 1202X-E from their UK agent (Labtronix, if anyone's interested) a lttle over 3 months ago as a result of my attention being drawn to this thread by a usenet posting into SED. It was the startling revelation of the amazingly low prices for brand new modern T&M kit that prompted me to replace my very neglected "Boat Anchor" 'scope and sig generator with high performance replacements I could carry around one handed.

 A week after getting hold of the 'scope, I realised I needed a signal generator to complement it so seriously contemplated Siglent's SDG1032X (just a fiver shy of the 365 quid I'd already shelled out on the 'scope). When I compared the specs, I realised I could get a poor man's version (the FY6600) which offered similar features (some better, others not quite so good) for just 21% of the SDG1032X's asking price, so re-read the whole of this thread... (twice!) before placing an order with a Chinese supplier who held stock in the UK and less than a week later, I had myself an AWG of my own. From then on it was downhill into the depths of DIY electronics improvement.  :-\

 As you've noted with the supplied 'scope probes, they leave a little to be desired (certainly with regard to their BNC adapter tips) but aside from that niggling shortcoming, they (and the 'scope itself) appear to meet their specifications just fine. Of course, as I've pointed out several times here and elsewhere, all test kit has limitations, even the expensive kit costing ten to a hundred times more.

 With regard to the limited resolution of the voltmeter function, that's inherent to the 8 bit high speed ADCs used in these DSOs to capture the signal waveforms. With only 8 bits resolution, you're only going to see a maximum of 256 steps (twos complement signed integer - -128 to +127) in the Y axis of the 'scope's display. The ENOB setting using oversampling can extend the resolution by another 1 to 3 bits but that introduces a compromise which may or may not be acceptable depending on which aspects of the waveform you're trying to examine.

 Luckily, for the purpose of displaying a waveform on the screen, this is generally ample resolution. Even luckier in the case of DSOs, if you need to see finer detail, you can alter the gain and dc offset so as to concentrate the view over a limited Y axis range of interest in the waveform. However, as I've already mentioned, this technique has its limits and is yet another compromise (one, however, which does extend the capability beyond that of the traditional CRT based scope).

 Although my FY6600 has some DC offset, it's mercifully only a matter of a millivolt or so in the sub 500mV range (the range where that silly 85ohm pad is switched into the output circuit) and still low enough to be insignificant at the higher output ranges so I've never been tempted to tweak the trimmers (let sleeping dogs lie undisturbed lest they bite you in the bum). I may have a go at trimming out such residual offsets later on when I'm better tooled up to make sense of any such adjustments but, for now, they're not a problem and I don't see any benefit in taking the risk of making things worse.  ;)

 Being mindful of what I'd said in my previous post about the impedance of the generator being immaterial when driving a load matched to the impedance of the connecting transmission line (in this case, a 50 ohm dummy load hanging off the BNC T adapter plugged into the 'scope's Y channel input socket using the very short 50 ohm BNC patch lead supplied with the FY6600), I've just checked the frequency response of my FY6600 and see 74 and 75 percent of the amplitude (or, in this case, the Peak to Peak) at the 60MHz limit compared to the amplitude produced at 1MHz (the 100KHz amplitude was just a tiny fraction higher by way of a sanity check).

 This is less than the 29% drop associated with the -3dB bandwidth point of 70.70707% voltage amplitude. Since this frequency is well within the 'scope's claimed 200MHz bandwidth, it would seem that my FY6600 example comfortably meets its own bandwidth specification. However, I'm just waiting for the half hour screen saver setting to time out before running the self calibration routine to repeat those tests (which event has occurred just now). I'm now waiting for the self calibration to complete and I can then repeat the bandwidth test...

 I've gotten pretty well the same figures on both channels of the generator and ditto for the 'scope channels, doing only one channel at a time. If anything, there seems to be a slight discrepancy with the 'scope's frequency response where it rises slightly below the 1MHz mark on the Y2 channel compared to the Y1 channel. Just a small fraction of a dB, nothing to get worked up about.

 All in all, the FY6600 seems to be performing to its specification on sine wave bandwidth at least. Since the PSU, opamp and TCXO mods, it now exceeds its original DC offset/signal clipping limits and frequency stability capabilities (and the impedance mismatch below the 500mV setting isn't quite so shabby as it originally was - 45 ohm versus the original 85 ohm, whilst still not ideal, is a noticeable improvement just the same).

 Since the FY6800 is essentially a revamped FY6600, I'd expect that to likewise exceed its sine wave output bandwidth claims, barring any out of the ordinary manufacturing defects (they have enough ordinary cost cutting induced deficiencies to begin with).

 I've only addressed the matter of the frequency bandwidth aspect, ignoring the question of output voltage level calibration which seems to be within +/- 10% (according to my 'scope which, for the moment I'm assuming to be within its own +/-3% tolerance range - ICBA to knock up a simple peak voltage detector using a point contact diode to sanity check the voltage levels right now).

 It's all too easy to obtain misleading results when checking out such kit when you overlook the transmission line effects of the connecting cables at radio frequencies and neglect the need to terminate the cables with a resistive impedance matching that of the cable (50 ohm dummy loads with 50 ohm BNC cables). It's also worth keeping in mind the limitations of your test and measuring equipment (DSOs don't, for example, make a very good substitute as a digital voltmeter).

 If you're seeing unexpected results when checking out new kit, it's more than likely you have overlooked something vital like properly terminating a test cable. Take a pause from what you've been doing and consider the possibility that you've overlooked something in your test setup before concluding you have been 'sold a pup'.  If that doesn't resolve the issue, you can always seek advice from any technical discussion group, whether it's EEVBlog or a usenet group just as Andreax1985 has done in this case. There's nothing wrong in seeking expert advice or a second opinion from your peers.

JBG

 

rhb:

--- Quote from: Johnny B Good on February 12, 2019, 07:14:25 pm ---
 If you're seeing unexpected results when checking out new kit, it's more than likely you have overlooked something vital like properly terminating a test cable. Take a pause from what you've been doing and consider the possibility that you've overlooked something in your test setup before concluding you have been 'sold a pup'.  If that doesn't resolve the issue, you can always seek advice from any technical discussion group, whether it's EEVBlog or a usenet group just as Andreax1985 has done in this case. There's nothing wrong in seeking expert advice or a second opinion from your peers.

JBG

--- End quote ---

I'd like to add that if you are using cables and connectors which are not *known* to be good you give that careful scrutiny.  I bought 10x 50 ohm BNC jumpers on eBay, 5x from two sellers.

When I plugged them into my 8560A and swept them with the TG there was a sharp notch around 50 MHz on all the cables.  So I cut one open.  Despite being screw on shield clamp connectors, the OEM had twisted the braid and soldered it at one point.  So except for use as a power feed for my RF Design RFD 2305 they are useless because the notch is not even stable.

So regardless of price, buy at least 1 or 2 name brand devices of anything so you have a reference to compare to.  But watch out.  Chinese SMA-M connectors are known to often have out of spec pins which are oversize. So if you connect one to a good quality SMA-F it's destroyed.

The only way to address that that I can think of at the moment is to buy a Pasternak or similar SMA-F and
 mount it with a little spring scale so you can push it into a Chinese SMA-M and if the SMA-M is oversize the spring scale will warn you to throw it away.  If anyone knows of a source of steel gauges for checking connectors please post a link.

All cheap Chinese connectors, adaptors and cables have to be swept over the intended operating range on receipt.  I didn't believe it was as bad as it is, but I now have personal experience and an unknown amount of scrap metal bought at inflated prices.

radiolistener:

--- Quote from: Andreax1985 on February 12, 2019, 02:30:26 pm ---And why DC offset is higher with X10 probe than with the unterminated BNC-BNC cable?

--- End quote ---

Let's assume that generator output impedance is 50 Ohm, and we set it's output to 1 Vpk on 50 Ohm load. It means that we have 0.5 divider, because input and output impedance is the same = 50 Ohm. So Vpk on Hi-Z load (no load) will be 2 Vpk.

For 1 MOhm oscilloscope with BNC-BNC cable, there is 1000000/(1000000+50) = 0.99995 divider and we will have Vpk1MOhm = 2 * 0.99995 = 1.9999 V.

For 10 MOhm with x10 probe, there is 10000000/(10000000+50) = 0.999995 divider and we will have Vpk10MOhm = 2 * 0.999995 = 1.99999 V.

The difference is 1.99999 - 1.9999 = 90 uV. You cannot measure so low difference with oscilloscope.

So, I think this is definitely related with RF interference and oscilloscope accuracy, because 10 MOhm input is more sensitive for RF interference, than 1 MOhm input. And there are other issues that may affect on the result, because 10 mV is too small value on the background of oscilloscope and probe accuracy.

radiolistener:

--- Quote from: rhb on February 12, 2019, 08:55:53 pm ---All cheap Chinese connectors, adaptors and cables have to be swept over the intended operating range on receipt.  I didn't believe it was as bad as it is, but I now have personal experience and an unknown amount of scrap metal bought at inflated prices.

--- End quote ---

yes, I completely agree. Also it's better to avoid these "golden plated" yellow SMA, because they tinned with thin foil of some unknown metal (it's probably titanium nitride or something like that) and it peels off and contaminates the connectors. It's better to buy Chinese "nickel-plated" SMA, because it's much better - it has better contact and don't contaminates the connectors with small pieces of titanium nitride.

Also I noticed that all RF things sold on aliexpress has much worse specs than it's claimed on product description page. You can divide all specified dB at least by half. :)

I also bought 1 meter LMR195 cable with PL239 connectors and get SWR=1.2 on 27 MHz with this cable!
For comparison I have SWR=1.01 with the same conditions on 1 meter of RG58 cable.
I even don't have idea why it has so high SWR, there is just 1 meter of cable...
I tried to heat connectors of this cable with soldering iron (I think there is a bad contact somewhere), but it doesn't helps.
The seller sent me replacement, but it has the same bad SWR.

rhb:

The thing that *really* makes me mad is the US owned the electronics industry after WW II. And we *gave* it to the Chinese so that a small number of people could accumulate more money than they can ever spend.

We taught the Chinese how to do it *and* paid for the factories!

The USA has done more for the benefit of the Chinese than it has for its own people.  All so a few people could add digits to 9 figure bank accounts. I am proud that we have raised the standard of living in China, but I am disgusted that this was done to the detriment of many for the benefit of a few.

Given the disasters of the "Great Leap Forward" and the "Cultural Revolution"  and the long history of abuse and exploitation of the Chinese by Western countries the Chinese population deserved help.  But not at the expense of bankrupting the American public.

We need an analogue to Shenzen in the US. Silicon Valley was at one time, but now almost nothing is made in the US.  And if you're not working for a big corporation, you can't obtain exotic parts.  In Shenzen, you just wander to the right part of town and anything you want is available at good prices in quantities from 1 to 1 million.  They give you a good deal on 1 in hopes you'll succeed and buy 1 million.

But we all get what we deserve whether we want it or not, either as individuals or members of a group.  Sometimes this is as punishment and sometimes it's a blessing.  Which is always ambiguous and depends entirely upon what we do next.

I hope Dave and Simon will forgive my transgression.  But I really feel this needed to be said.  The Western world transferred opportunity to China so the rich could become even richer.  Just because they only care about their position in the Forbes rankings.  Personal reputation is just another  commodity they buy from a PR firm.

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