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

[maxwell3e10]

For clarification here is impedance measurement of my oscilloscope 1M input:
- at 10 MHz = 57 - j884 Ohms
- at 100 MHz = 43.9 - j96.5 Ohms
As you can see, my oscilloscope input impedance at 10 MHz is not 1 MOhm, but 57-j884 Ohm. So, you're needs to take this into account.

Hmm, interesting, how did you measure it? It seems a bit surprising. If it has ~50 Ohm real impedance wouldn't it attenuate signals by about a factor of 2? What happens if you go to even lower frequencies.

[radiolistener]

Hmm, interesting, how did you measure it? It seems a bit surprising. If it has ~50 Ohm real impedance wouldn't it attenuate signals by about a factor of 2? What happens if you go to even lower frequencies.

I measured it with vector analyzer. As you can see it has 1 M at DC but with AC things going to be much more complicated, and this is not ~50 Ohm, note it has reactive component (about 18 pF). So, the things are much more complicated, because input impedance depends on frequency and mismatch effect also depends on cable length and cable impedance (because it is also not exact 50 Ohm)...

[BU508A]

Hmm, interesting, how did you measure it? It seems a bit surprising. If it has ~50 Ohm real impedance wouldn't it attenuate signals by about a factor of 2? What happens if you go to even lower frequencies.

I measured it with vector analyzer. As you can see, this is not ~50 Ohm, it has reactive component (about 18 pF). So, the things are much more complicated, because input impedance depends on frequency and mismatch effect also depends on cable length and cable impedance (because it is also not exact 50 Ohm)...

Maybe a Smith-chart would help? 

[maxwell3e10]

But as I said, in this frequency range the "wrong" impedance of the cable doesn't really matter.

Yes, because the wavelength of the signals is so much longer than typical cable length.

If the signal leaves the circuit one have to make sure, that the impedances are adapted, for example by using a Pi-network.

Yes, "signal leaves the circuit" means it has to propagate some distance. But within a circuit where the distances are very small, one does not need to match impedance. That is why as one goes to higher (GHz) frequencies, the circuit building blocks become smaller and there are more strip lines connecting then. Whereas for 1-10 MHz frequencies, one typically only does impedance matching at the input and output.

[radiolistener]

May be...  But when you use proper impedance match, things will be much more easier

[radiolistener]

because the wavelength of the signals is so much longer than typical cable length.

you're also needs to take into account that usual coax cable has velocity factor about 0.6, it means that wavelength is almost twice smaller in the cable in comparison to the vacuum.

In the vacuum 1 meter wavelength is 299.8 MHz. But in the cable 1 meter wavelength is about 197.8 MHz.

Also you're needs to take into account double cable length, because reflected wave needs to move forth and back. By take this into account, 10 MHz = 29.98 m * 0.66 = 19.79 meters wavelength in the cable / 2 = 9.8 meters.

For 10 MHz, 9.8 meters is longer than 1 meter cable, but not so much...

For 50 MHz difference will be even smaller: 50 MHz = 5.99 meters * 0.66 = 3.95 meters in the cable / 2 = 1.98 meters. As you can see 1 meter cable is pretty close to 1.98 meters. So we cannot treat it as "much longer" at all...

[maxwell3e10]

Yes, I agree with this. 10 MHz is kind of borderline above which one has to start thinking about impedance matching and termination.

There was a discussion on some other thread if 10 MHz clock distribution requires proper 50 Ohm termination on each end. In principle, yes, especially since the cables involved are often fairly long. In practice, not always. Some instruments have 50 Ohm impedance for external 10 MHz reference input, others have higher impedance.

[radiolistener]

From my experience with these BNC to alligator clip cables:
https://www.aliexpress.com/item/32822207220.html

the critical frequency is about 2-4 MHz, above that I can notice significant error. I tested it with low bandwidth sine wave. It doesn't means that there is complete fail, no. You can still see the signal, but above 2-4 MHz you can notice difference with properly matched cable.

[radiolistener]

Some instruments have 50 Ohm impedance for external 10 MHz reference input, others have higher impedance.

I think it depends on how device using it. Some device may ignore distortions and even may be tolerant to jitter. But some devices may be very sensitive to external 10 MHz purity.

[rhb]

@ maxwell3e10

BTW What's the "3e10" supposed to mean? 

You seem very busy avoiding identifying and explaining my 485 photos.

Generally this thread has degenerated into a "3 Stooges" skit with an occasional straight man walk on.

[maxwell3e10]

@rhb, see reply #1994

3e10 is something relevant to this discussion.

[rhb]

Sorry, kid, but that's more trouble than you are worth.  I'm not going to play "hunt the wumpus" for a message number.  In the future, post a link and make sure it works.

The real question is the difference in the response of a 485 at 50 and 1 Meg ohm input impedance to a 50 ohm source impedance, 36 ps rise time, 10 MHz square wave.

Which is which and why?

Also sitting next to me are a 1.5 GHz LeCroy DSO with switchable input impedance and a Tek 11801 with four 20 GHz sampling heads.  I have in total 5x 20 GHz sampling heads and 2x 12 GHz sampling heads.  Those are *all* 50 ohms input only.  If you  read the datasheets for serious scopes you will find that above about 500 MHz, the input *must* be 50 ohms.  You think maybe there is a reason?

There are a *lot* of PhDs on this forum.  I've not observed any that put Dr, Wizard or anything similar in their user name.  The normal pattern is initials, name or some combination.  Real PhDs don't need to see  initials before or after a  name to recognize one.  The hallmark of a PhD is the tag line from Clint Eastwood's "Magnum Force". "a man's got to know his limitations".

The real ones do, the failures and frauds don't.  Knowledge is infinite.  Humans are finite.  Getting a PhD is about coming to terms with that. And understanding what to do when you don't know.

At least one friend of mine (Stanford SEP 1982) and I agree that it's an expensive luxury, but if you can afford it, a lot of fun.  I suspect that I could find quite a few more friends who agree.

As for you, I'm not too sure you could do a decent job of sweeping floors at NIST;  so much ego and so little to justify it.

Have Fun (somewhere else)!
Reg

And yeah, I'm not a *nice* guy.  I have this unplesaant habit of telling the truth whether people like it or not.

[tomato]

You seem very busy avoiding identifying and explaining my 485 photos.

@rhb, see reply #1994

Sorry, kid, but that's more trouble than you are worth.  I'm not going to play "hunt the wumpus" for a message number.

His reply to your question is in post #1994. 

[svetlov]

dear DaveR ! detailed photos could not find-probably one of the participants of the forum will want to do photos - so far there's only this video
https://www.youtube.com/watch?time_continue=763&v=FaNvHQW_CsQ

[rhb]

Here is a photo showing, starting at the bottom, the reflection response for an 50 ohm BNC terminator , 50 ohm thru BNC terminator  and the 50 ohm terminator on a tee.  Display is 50 mV/div and 500 ps/div.

The scope BW is 20 GHz with an SMA-M to BNC-F cable.  What you see in the first horizontal division is the reflection from the BNC-F connector.

Points:

The thru shows a capacitive reflection as does the tee & terminator.  I don't have a 1 M ohm BNC termination, so the thru trace is not entirely accurate

The tee & terminator has a lot of ringing in the stub which is not terminated.  The spikes on the trailing edge are multiple bounces.   The tall ones are the open end of the tee.  The short ones are the terminator end of the tee.  The geometry doesn't exactly match the way a tee and terminator are used on a scope input, but it's pretty close.  To match the geometry better I'd have to use a different cable and that would change the BNC-F to a BNC-M  and change the time relationships as well.

From this one may conclude that the BNC terminator has a BW of about 1.5 GHz, the thru about 1 GHz and the tee and terminator about 650 MHz.  These are Chinese, but have been tested and bad ones discarded.

[bitseeker]

I've heard about the differences in terminators due to geometry, but it's neat to see it.

[maxwell3e10]

It would be good to start a thread on passive RF hardware: cables, attenuators, terminators, etc. At work I have a pile of cheap BNC cables and a pile of double-shielded Pasternack cables. For a critical signal its amazing how much difference a good BNC cable can make.

[rhb]

I've been playing with my new toy some more.  It is beginning to appear that there is more variation in the Chinese stuff I'm using than in  the preceding thru vs tee & terminator comparison.  I can radically deform the trace by mashing on the male part of the connectors.

It's amazingly difficult to figure out how to get valid comparisons.  To really do it right I'd need a pair of BNC-F  1 M ohm input active probes with  50 ohm SMA-M outputs which went to 20 GHz.  That would let me look at what the scope would see.  My LeCroy DDS-125 goes to 1.5 GHz, but it's got 20% overshoot on a step which makes tests like this impossible.


https://www.eevblog.com/forum/rf-microwave/testing-rf-connectors-and-cables/msg2640531/#msg2640531

[Dbldutch]

A while ago I decided to upgrade and tune my rather old (30MHz Version 2.9) because I wanted to have a higher precision counter function.
After reading this enormous number of postings, with some very good information (thank you guys), I decided on three items.
The power supply, the oscillator and the driver opamps for the waveforms.
The latter two are easy and have been described various times.

The power supply has been talked about many times, with several options, but there are only a few examples for a replacement.
I decided to design a completely new supply, that would let me adjust the +/- 12V outputs to +/-15V as well.
Going through my parts collection, I picked a 24VA block transformer that I could mount on a protoboard. The transformer is a little heavy with 2 seperate windings of 12VA each, and an output of 15VAC. Both secondary windings are fused with 0.8AT PTC fuses. The primary has two seperate 115V windings. 

I decided to use simple LM317 and LM337 voltage regulators for both analog supplies. The only specialty is the circuit around the trimmers, because they are China quality, and therefore cannot really be relied upon. The worst case is when the runner looses contact, creating a much higher output voltage then you intended, and this could potentially blow up the FY6600.
For the digital voltages, I selected a simple DC-DC Buck convertor because doing that with normal regulators would create too much heat. I specifically did not want a fan, with all it's generated noise inside the box. I also used some parts from the old supply, specifically the line filter and the 5V choke.

My unit is intentionally still floating. This can be fixed easily with a BNC/USB connection to another instrument.

To reduce some heat hot-spots from the three on-board regulators, I added some sticky heat-sinks to them.

More information can be found on my blog: http://www.paulvdiyblogs.net/2019/09/upgrading-tuning-my-fy6600-waveform.html

Enjoy!

[StillTrying]

The transformer is a little heavy with 2 seperate windings of 12VA each, and an output of 15VAC.

Why did you use 2 bridge rectifiers, with 2 matched secondaries I'd have made a center tap using just one BR.

[Johnny B Good]

A while ago I decided to upgrade and tune my rather old (30MHz Version 2.9) because I wanted to have a higher precision counter function.
After reading this enormous number of postings, with some very good information (thank you guys), I decided on three items.
The power supply, the oscillator and the driver opamps for the waveforms.
The latter two are easy and have been described various times.

The power supply has been talked about many times, with several options, but there are only a few examples for a replacement.
I decided to design a completely new supply, that would let me adjust the +/- 12V outputs to +/-15V as well.
Going through my parts collection, I picked a 24VA block transformer that I could mount on a protoboard. The transformer is a little heavy with 2 seperate windings of 12VA each, and an output of 15VAC. Both secondary windings are fused with 0.8AT PTC fuses. The primary has two seperate 115V windings.

 The secondary winding voltages are specified for full load at the minimum mains voltage input limit so tend to be a little bit higher than you might expect when running on a light load at normal mains voltage. That "15v rms" may be more like 17v rms no load in practice which, after allowing for a generous 2 volt drop in the bridge rectifier and the 1.4 crest factor of an rms voltage for sine waves, would see the smoothing capacitor(s) storing a peak voltage close to the 22 volt mark which will likely drop to an average voltage around the 20 volt mark with an average current loading of 150mA when driving 50 ohm loads with a square wave peak to peak voltage swing of 24 with a 30mA 'vampire load' allowance.

 When the regulators are set for 15 volts under this condition, they'll have to dissipate 750mW each. At a 12v setting this increases to 1200mW each which is a significant amount of heat energy to dissipate via the unsinked tab of a TO220 device (free air dissipation for most TO220 packaged devices is typically 1.5 to 2 watts maximum @25 deg C ambient).

 That heatsinking you're going to use will be vital in (literally!) this case on account the ambient temperature within the case will be considerably higher than a mere 25 deg C (more like 50 deg but I still don't have a suitable temperature sensor to actually check this - that's just an 'educated' guess right now. It could be even higher in the absence of any active cooling - I have a small 50mm sq by 10mm deep 5 volted 12v fan fitted into mine which has made the two original hot spots on the top of the case disappear without trace.

 These dissipation estimates are rather conservative based on the information you gave and the actual, measured, values are unlikely to be less without the effects of drop out from supply ripple making its presence known (at least as far as the 15 volt option goes).

 It makes sense to use a switching type drop in substitute for the 7805 regulator (better than 90% efficient units with <30mV ripple at 1MHz or higher being cheaply available). As far as I can see, none of the logic ICs are powered from this 5 volt rail directly which is only used to feed the three on board LDO ICs (3.3, 2.5 and 1.8 volt rails iirc) so any switching ripple here is of no consequence. The other 5 volt rails (both plus and minus) are derived from LDO regulators fed from the +/-12 rails (via 8 volt LDO regulators istr) so are also immune to the ripple on this 5 volt psu rail.

 Given a modest airflow around the regulator heatsink (best to use a single large heatsink for both 12/15 volt regulators), this analogue PSU design should be reliable enough. Without the addition of a cooling fan, reliability becomes somewhat questionable (it was already questionable to start with without any modifications being applied).

I decided to use simple LM318 and LM338 voltage regulators for both analog supplies. The only specialty is the circuit around the trimmers, because they are China quality, and therefore cannot really be relied upon. The worst case is when the runner looses contact, creating a much higher output voltage then you intended, and this could potentially blow up the FY6600.
For the digital voltages, I selected a simple DC-DC Buck convertor because doing that with normal regulators would create too much heat. I specifically did not want a fan, with all it's generated noise inside the box. I also used some parts from the old supply, specifically the line filter and the 5V choke.

 As I've already hinted (and also in previous postings), I think it's a mistake to discount the cooling fan option. Although removal of the original PSU along with its own 2 watts contribution to the heat loading will compensate for the additional heat produced by a replacement analogue supply, you'll still see a net increase of waste heat raising the temperature within the box which not only isn't good news for the original components it's also counterproductive to keeping those analogue regulators at a safe working temperature even when well heatsinked.

 Unless everything else on your bench is passively cooled and totally silent to protect unusually sensitive hearing, I doubt you would find any objection to the barely perceptible whisper quiet noise of a small 5 volted 12v cooling fan which has proved in my case to be more than sufficient to drop the internal temperatures by a good 20 deg C. For all the cooling effect of those vent slots, particularly when propped up on its bail stand, you might as well seal them all up to prevent dust ingress and make it proof against liquid spillage accidents.
 

My unit is intentionally still floating. This can be fixed easily with a BNC/USB connection to another instrument.

 Which begs the question, "Why not fit a low impedance (1K to 10K resistor rather than the conventional 1 or 2M) "Static Drain resistor" between signal ground and the PE tag of a C14 socket?". You can buy filtered, switched and fused C14 connectors which will fit the back panel (along with some strengthening in that area of the flimsy fibreglass board used for the back panel) to replace the original C8 two pole connector (this is an FY6600, right?) which will save you recycling parts from the original psu (keeping it intact for use as a spare or to allow reversion to the generator's original state).

 This, for example:-

https://www.ebay.co.uk/itm/Male-C14-IEC-RFI-Filter-Flange-Mount-5-x-20mm-faston-Rated-At-2-4A-250V-ac/223637128377?epid=1905379730&hash=item3411cfc4b9:g:aeAAAOSwE5Vc6tiR
 

To reduce some heat hot-spots from the three on-board regulators, I added some sticky heat-sinks to them.

Enjoy!

 I'm not quite sure what "sticky heat-sinks" are. Hopefully, if they're relying solely on a 'sticky glue' to secure them, the glue used won't be failing at temperatures less than 150 deg C. Also, three regulator heatsinks? The 5v dc-dc converter surely won't be in need of a heatsink (not at half an amp or less) and, in view of the possibility of outputting a 24Vpp square wave at mHz frequencies into a 50 ohm load, it would be better, as I've already mentioned, to mount both 12 or 15 volt regulators on a common larger heatsink to keep each individual regulator cool during the resulting protracted periods (seconds to tens or hundreds of seconds) that they could end up handling each half of such waveforms in turn.

 Incidentally, the circuit you've attached seems overly complicated to me, at least compared to a more pragmatic dc-dc converter alternative to the analogue regulator solution you're proposing. I rather think you're "making a rod for your back" with this upgrade project. Assuming it provides the hoped for benefit of eliminated switching noise (with the lid off, of course!), you may yet be forced to change your mind over the wisdom of keeping it "fanless".

 In view of the fact that only 5% or less of the input energy appears as useful signal power on the BNC outputs, it's worth taking a note of the mains input energy consumption (wattage) beforehand so as to compare this wattage figure to the new figure after completing your PSU upgrade exercise. The difference will represent the additional heat being dissipated within the box, giving you some indication of the magnitude of the thermal issues this upgrade will present you with.

 Most other contributors to this thread who've upgraded to an analogue PSU of their own design have recognised the need for a cooling fan but some have underestimated the cooling requirements, usually recognising just in time the need to fit a fan with one or two seeing the consequences where they've had to replace damaged parts when a regulator IC has failed, input to output short circuit, causing a massive overvolting event. You need to be very mindful of the potential thermal problems with such analogue supply upgrades so do take care.

JBG

[Johnny B Good]

The transformer is a little heavy with 2 seperate windings of 12VA each, and an output of 15VAC.

Why did you use 2 bridge rectifiers, with 2 matched secondaries I'd have made a center tap using just one BR.

 Well spotted! I missed that particular faux pas.  Not only would it have reduced the bridge rectifier count, it would also have avoided  a diode's worth of volt drop to boot.

 The only time you'd see the use of two separate diode bridges like this is when you don't have a 79xx to complement a 78xx where the isolation of the two separately rectified and smoothed supplies allows you to use a 78xx for the negative rail (output pin to ground, common pin as the negative rail connection). I think the same trick can be used with the dc-dc regulators as well which can prove handy in view of affordable negative output and inverting output dc-dc converters seemingly being as rare as rocking horse shit on Ebay and Banggood.

 The most pragmatic solution to upgrading the Feeltech psu board to something less noisy would appear to be an analogue/switching regulator chimera. At a maximum power demand of just ten watts one could choose a 24VA 240v 50Hz mains transformer with a pair of 32v secondaries to drive buck converters capable of withstanding an input voltage of at least 48 volts to ensure you don't have to forego the 90 to 265vac universal mains voltage feature of the original.

 If you can get hold of a suitable "R" type, you can keep the losses down to that of the original switching supply or better meaning you don't have to squander any of the cooling fan benefit on preventing a conventional analogue PSU from burning itself out or cooking the main board and front panel components into premature failure.

JBG

[Dbldutch]

Good question, I probably should have elaborated on that.

I used the two seperate windings of the transformer to my advantage and make it easier to create a single “star” groundpoint by creating two separate supplies. With center-tapped transformers it’s easier to create grounding issues when using perf or protoboard and not a real pcb. Initially, I was 't sure how I was going to create the negative supply. Using a positive (dc-dc) regulator would need a separate bridge. Another reason was that I could use full bridges and therefor smaller buffer electrolytes. Diodes are cheaper than good buffer caps.

The only connection of the two supplies is at the connector to the main board.

Thirdly, I wanted to isolate the high frequency noise that inevitabilly comes from the 5V DC-DC convertor away from at least the negative supply as much as possible.

[Dbldutch]

Dear Johnnie,

In my humble opinion, you have been polluting this thread since about page 70 with your endless and repeating ramblings and rants.  I suggest you stop that. It’s not your personal thread and you are not nearly as good or skilled as you think you are to critique the work or ideas of other contributors and give us your lectures. As a minimum you should assume other people know what they are talking about. Be a bit more humble, and please, please be more brief!

[CDaniel]

Good question, I probably should have elaborated on that.

I used the two seperate windings of the transformer to my advantage and make it easier to create a single “star” groundpoint by creating two separate supplies. With center-tapped transformers it’s easier to create grounding issues when using perf or protoboard and not a real pcb. Another reason was that I could use full bridges and therefor smaller buffer electrolytes. Diodes are cheaper than good buffer caps.

The only connection of the two supplies is at the connector to the main board.

Thirdly, I wanted to isolate the high frequency noise that inevitabilly comes from the 5V DC-DC convertor away from at least the negative supply as much as possible.

Please take note that central tapp rectification with 1 bridge is full wave so you don't need bigger caps . You just used 1 more diode bridge unnecessary . The star ground and "isolation" from 5V buck converter could be made just as easily ...