Products > Test Equipment
FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
Johnny B Good:
--- Quote from: DaveR on November 22, 2018, 04:19:43 am ---Hi Johnny,
Have you seen these SMPSUs?
https://www.ebay.co.uk/itm/SMPSU-Switched-Mode-Chassis-Power-Supply-12v-0-3A-12v-0-2A-5v-2A-SMPS1-432-/141117111577?var=&hash=item667e84f884
https://www.ebay.co.uk/itm/SMPSU-Switched-Mode-Chassis-Power-Supply-5v-2A-15v-300mA-15v-200mA-Toko-783-/361575419603?hash=item542f930ed3
They look the part, if nothing else, but the seller is an anus - so caveat emptor!! See the negative feedback on him (which I can corroborate from personal experience.)
That TCXO module you found must be a new one, and it's cheaper than the oscillator by itself from other suppliers. There's also a Rojon mini TCXO for £22 if you want to be really extravagant:
https://www.ebay.co.uk/itm/Mini-TCXO-50-000MHz-50MHZ-0-1ppm-Ultra-precision-Oscillator-audio-DIY-GQ1-XH/382556866867?hash=item59122a8933:g:nIwAAOSwA3dYL7Cj:rk:3:pf:0
I can't see a problem with leaving pins 1 and 4 floating in the case of the 3095: both pins are unconnected on the PCB, so both will float to Vs+ and the device will be permanently enabled (no differential just means no power down control, but the default state is enabled, as would be expected). In the case of the 3491, although the datasheet doesn't recommend leaving PD floating, the device will still be permanently enabled if it is (subject to a probably unlikely stray signal event :)). I wasn't intending to upgrade the 3095s in my FY6600 to 3491s - I was planning to install them in my FY6800 (when I get round to it!) to get a direct comparison between them and the 3095s, more for curiosity and experimentation than anything else. If there happens to be a problem with them I've got spare 3095s I can drop back to as the upgrade from the single 3022.
Regards,
Dave
--- End quote ---
Thanks for the links to those PSUs,
I've just looked them over. Unfortunately, the first thing that strikes me is the marginal current rating of the 12 and 15 volt rails (0.3 and 0.2 amps) which I suspect won't be enough to support the demand from the opamps.
I noticed the use of a couple of HT smoothing caps (presumably 200 or 250 volt rated ones wired in series) with a flylead jumper on the 12v version for the 120/240 volt mains voltage selector which is a bit of a spoiler (meaning user intervention is required when moving between mains voltage standards - no automatic handling of mains voltage adjustment).
The 15 volt version is using a single 400v rated 68μF smoothing cap suggesting that it's probably a "Universal" smpsu type. That doesn't preclude the possibility that it may be 200-240 volt only of course, but that seems rather improbable given the extra cost over the optimum choice of 22μF since a 68μF would provide ample smoothing even for a 60W 100v 60Hz supply working off the apparent design rule of a microFarad per watt's worth of loading on a 100v 60Hz supply.
TBH, neither of them are, imo, suited for the task. I think the best option in this case (looking for a psu system with the lowest possible level of half mains voltage leakage current with good voltage regulation on all three rails) is to find a 15 or 20 W rated 5v low leakage smpsu and use dc-dc converter modules to derive the two independently regulated 15 volt rails rated for 500mA each. This way, you only need to track down one low leakage psu and not have to put up with the indifferent regulation typical of secondary 12/15 volt rails loosely tied to a precisely regulated 5 volt primary output rail courtesy of secondary turns ratio alone.
I started writing this reply about 12 hours ago but "Real Life"(tm) got in the way and I was still distracted by the, now seemingly impossible task of tracking down a suitable 15 to 25 watt rated Class II 5v smpsu containing a shielded transformer and no frelling Y1 class capacitor (as per that Nokia 5.7v 800mA phone charger that proved to be totally devoid of such vandalism yet was virtually free of the half mains voltage leakage).
I'm still mulling this problem over but for now, I've changed my mind and decided to upgrade the shite rectifier diode in the FY6600's psu. I've just discovered that at 100Khz 20v p2p, I can offset the voltage by +2.9 and -3.6 volts before clipping becomes evident suggesting that I can afford the resulting drop on the 12v rails as a result of such an upgrade to the 5v rail after all!
Regarding TI's advice in the THS3491 data sheet about not letting the Vref pin float, perhaps the absence of such explicit advice in the 3095 data sheet is because it doesn't matter after all... perhaps. I just felt it was a point worth raising considering the distortion at a mere 20MHz that was evident in your(?) 'scope traces.
Bear in mind that the similar distortion shown in the data sheet was for 32MHz 20v p2p with 100 ohm loading. The adjacent 32MHz trace was with two of them paralleled up to demonstrate the effect of halving the load on each amp to the equivalent of a 200 ohm load per amp, the benefit of which was even more starkly demonstrated in the 64Mhz plots. I'm just sayin' is all. :)
DaveR:
--- Quote from: Johnny B Good on November 21, 2018, 08:45:04 pm ---
Thirdly, if you feel the need to match the existing 50MHz 100ppm frequency reference to the counter's display resolution, then install a 50MHz 0.1ppm TCXO (if you're going to bother at all, why only take half measures with a mere 1ppm rated TCXO when you can get hold of a 0.1ppm ROJON part from a Chinese supplier on Ebay for as little as 15 or 16 quid? :)).
--- End quote ---
I bought one of these not long ago:
https://www.ebay.co.uk/itm/External-TCXO-clock-PPM-0-1-for-HackRF-one-GPS-Applications-GSM-WCDMA-LTE/173432677234?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
and have just looked up the spec in the seller's listing: I thought it was 1ppm, but it's shown as 0.1ppm in the headline, and referred to as 0.1 - 0.5ppm in the description. It's Rojon, but only half the price of those referred to earlier. I wonder how much confidence you can have that 0.1ppm means anything like that accuracy, or that the £15 and £22 TCXO cans contain anything different to this £8 one? The answer, of course, is "none". Caveat emptor, again!
Johnny B Good:
--- Quote from: DaveR on November 23, 2018, 03:13:02 am ---
--- Quote from: Johnny B Good on November 21, 2018, 08:45:04 pm ---
Thirdly, if you feel the need to match the existing 50MHz 100ppm frequency reference to the counter's display resolution, then install a 50MHz 0.1ppm TCXO (if you're going to bother at all, why only take half measures with a mere 1ppm rated TCXO when you can get hold of a 0.1ppm ROJON part from a Chinese supplier on Ebay for as little as 15 or 16 quid? :)).
--- End quote ---
I bought one of these not long ago:
https://www.ebay.co.uk/itm/External-TCXO-clock-PPM-0-1-for-HackRF-one-GPS-Applications-GSM-WCDMA-LTE/173432677234?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
and have just looked up the spec in the seller's listing: I thought it was 1ppm, but it's shown as 0.1ppm in the headline, and referred to as 0.1 - 0.5ppm in the description. It's Rojon, but only half the price of those referred to earlier. I wonder how much confidence you can have that 0.1ppm means anything like that accuracy, or that the £15 and £22 TCXO cans contain anything different to this £8 one? The answer, of course, is "none". Caveat emptor, again!
--- End quote ---
Hi Dave,
I had I a reply nicely typed up and waiting to go until I decided to hit the refresh whilst previewing my missive. Apparently, hitting the refresh button is a shortcut to sending replies to the bit bucket. Whoops! I did it again (but not without copying my text beforehand). There's no doubt about it, hitting refresh is a sure fire way to lose a nicely typed out reply... :'(
Apologies but you're gonna have to wait a little longer for a proper reply. I'm done with this klunky bug ridden system for today. I'll leave it till tomorrow (er, later today - it's already "tomorrow").
Regards, Johnny
soundtec:
Hi Johnny ,
Its does happen on many Blogs ,Ive found ,long and carefully typed responses can simply end up gone .
A very simple way to help prevent loss is to type into a text file on the desktop ,then once your done ,copy and paste to the Blog page . Alternatively before hitting send , copy and paste what you've written to a text file and save .
Johnny B Good:
--- Quote from: soundtec on November 25, 2018, 06:16:11 am ---Hi Johnny ,
Its does happen on many Blogs ,Ive found ,long and carefully typed responses can simply end up gone .
A very simple way to help prevent loss is to type into a text file on the desktop ,then once your done ,copy and paste to the Blog page . Alternatively before hitting send , copy and paste what you've written to a text file and save .
--- End quote ---
Sensible advice. In this case, it's best to save the text before even taking a chance with the preview button (in my case, just in case I forget the lesson about refreshing the view being so terminal). I'm typing this into a text file rather than into the rather cramped reply box. I suspect this is probably what many of the longer established members do.
Anyway, it was probably for the best. I was going to suggest converting the seemingly bi-phase half wave rectified 12v circuit on the PSU to full wave, forgetting that with the simpler (and cheaper if you discount the need for a doubling up of the transformer rating compared to that needed in the more efficient but complex push pull circuit) single ended drive smpsu topology, only half wave rectification can be used as I discovered to my chagrin, luckily without damage when I tested my hypothesis that this would give a small boost in voltage without the need to add a couple of turns to the 12v bi-phase secondary winding, along with a halving of the ripple voltage.
I tested my adaption with the PSU fully connected up, voltmeter poised, eye on the wattmeter scale ready to hit the off switch should it go north of the 10W mark. Switching on showed a mere 2W reading as if the PSU had gone into current foldback due to an overload. The 5v was showing just 2.5 volts and the 12v lines, afaicr, less than 6 volts (possibly just 2 or 3 volts).
Thinking I'd miswired the extra diodes, I removed the PSU to my 'bench' for a closer look but as far as I could tell from visual and electrical inspection, I had managed to create a full wave centre tapped bridge circuit exactly as planned so back it went into the sig generator for another test where it did the same thing.
More curious now, I left the output disconnected to try again and observed some 21 volts on the 12v rails (the 5v was still its usual 5.48 and the wattage was still the 2 watts of earlier. Needless to say, I didn't stop and stare at this marvel of excess voltage and hit the off switch quick smart.
What I'd seen as a rather curious abuse of a bi-phase secondary winding where the opportunity to use either a single winding to generate a centre tapped voltage doubled supply or else a missed opportunity to use the bi-phase winding to create a centre tapped full wave bridge circuit, turned out to be simply a means to allow all three rectifiers (the +5 and 12 and -12 volt rectifiers) to conduct on the same phase as the active single ended drive pulses. Lucky for me that my "Bull in a China Shop" approach hadn't resulted in a fatal mistake. :-)
I realised my 'skoolboy howler' during my musings of the problem whilst waiting for the iron to warm up for the diode-ectomy. This led me to wonder whether replacing the extremely efficient 30A per diode element 45Vrrm rated diode on the 5v winding with a slightly less efficient SB340 (3A average 40Vrrm DO-201AD) I'd recovered from an old external HDD power supply would provide the slight lift on the 12v rails to give a little more than just a tenth of a volt's worth of postive offset range at the 20Vp2p into 50 ohm loads I'd been getting with the +12.35v/-12.7v rail voltages after the transformer mod. Since it looked like the component symbol printed on the PCB, I swapped out the giant 30&30A diode and mounted my prize diode in its stead.
Retesting revealed a rather surprising boost of the 12v rails to +13.55 and -13.70 volts which proved sufficient to let me go +2.0v and -2.3 volts on the offset adjust without clipping. A rather ample margin for such a modest boost on the 12v rails. The maximum power consumption after the diodes have warmed to a more efficient operating temperature (takes about a minute or two from the 10.25 watt initial cold switch on state) is now just a whisker over the 9W mark.
I reckon this final modification to the PSU is just about the optimum between its improved efficiency and the overall energy consumption, the vast majority of which results in raising the internal temperature of the generator. It can be reduced by another watt but this leaves you with no offset margin.
For anyone else interested in modding the PSU for better efficiency and voltage stability on the 12v rails, it's sufficient to replace all three rectifier diodes on the outputs with SB340s all round. For those of a less nervous disposition, a nice finishing touch is to lift the transformer out to add a couple of extra turns to each end of the bi-phase secondary winding which serves the 12v rectifiers.
This, of course assumes the obvious addition of the 47K resistor across the lower 10K resistor of the 5v sense network to raise the 5v (and the 12 volt rails tied to it via the transformer winding ratio) to 5.49v (still just within the +/- 10% Vcc spec for 5 volt logic chips).
[EDIT 20190330]
I've since undone that transformer mod (removed the two turns from the 12v windings) and added a single turn over-wind on the bobbin, made up with 4 separate single turn windings in parallel for better coupling and minimum I sq R loss and leakage inductance to wire in series with the 5 volt diode to buck the transformer output voltage (effectively removing one turn from the 5v secondary winding).
This forces the switching module to raise the 12v rails as it compensates for the effect of the lower transformer output to the 5v rectifier and smoothing circuit which drives the voltage feedback for all three rails. It's a far more elegant method to rebalance the voltage distribution between the regulated 5v rail and the "we'll take pot luck" turns ratio mediated voltage output level on the "Twelve Volt" plus and minus rails.
Adding this single turn to buck the voltage on the existing 5v winding which lies beneath the HT windings, neatly avoids interference to any of the existing windings and their connections (very unlike the case with my first transformer mod) since it simply terminates directly onto a vacated PCB connection and the lifted out anode of the 5v rectifier diode.
The only gotcha with this is that you may have to reverse the connections to achieve the buck winding state. When I tried this earlier today, that was exactly how it went -Sod's holds its sway as per usual. What made it a little more awkward for me was my choice of needlessly heavy gauge wire which obliged me to unsolder the transformer again in order to preform the ends after reversing the polarity in order to refit it back to the board.
Just because there IS ample room on the bobbin, that's not a valid reason to try and fill the gap with heavy gauge wire when thinner and more flexible wire will be more than adequate to keep the I sq R losses vanishingly small anyway. It would have been trivial, given a more sensible choice of wire gauge, to reverse the buck winding connections without having to lift the transformer from off the PCB. Indeed, the whole mod can be done without removing the transformer at all if sufficiently flexible wire is used. If it hadn't been for the need to undo my earlier mod, I'd most likely have made a better choice of wire to begin with.
Anyway, this alternative solution DOES work, and rather well at that! :) Before starting this modification work, I measured the voltages provided by the previous modifications (diode upgrades, the two turns add ons to the + and - "12v windings" and the 47k shunt of the voltage feedback potentiometer lower arm to raise the original 4.95v to 5.49v just to get a an extra boost on the 12v rails). The voltages were +5.46, -13.78 and +13.60 volts. After reverting the transformer to its original state and adding a single turn to buck the 5v secondary and removal of the 47K resistor, I then got the following voltages, +4.94, -12.98 and +12.83. I then fitted a 200K resistor to bump the +4.94v rail to a less marginal (for the 12v cooling fan) 5.07 volts which boosted the "twelve volt" rails to -13.29 and +13.13 volts which still provides a useful DC offset margin at the 20v p-p level.
One final thing I can add before ending this edit, is that you can forget what I said about upgrading to 'medical grade' class II smpsu modules using transformers blessed with a proper foil shield layer as the ONLY effective solution to the problem of the half live mains 'touch voltage' and the risk of ESD damage to any fragile devices being tested.
In the end, I realised that such ESD hazards still existed even with such medical grade' psus and after trying several schemes to cancel out this touch voltage whilst avoiding the use of an earth connection which would, as it does in the case of the FY6800, introduce troublesome mains earth loop interference, I finally admitted defeat in achieving such an impossible ideal and rather pragmatically, replace the IEC C8 connector with an IEC C6 (trefoil) connector just so I could link the zero volt rail via an 11K drain resistor to flatten down the 90vac half live mains touch voltage to a mere 500mV ac which nicely solves the ESD risk issue yet avoids the earth loop issue that stupid, stupid Feeltech have introduced into their FY6800 - Fools that they are!
[END_EDIT]
Anyway, back to the point that Dave made regarding the TCXO upgrading options, I had been about to mention (as a result of my original transformer mod prior to my doomed bridge diode mod) that I'd changed my mind about hanging fire on ordering that £15.53 "50MHZ hifi TCXO 0.1ppm Ultra precision Golden Oscillator CLOCK power supply" with its JYEC TCXO (the object of my desires).
It's a simple enough job to transplant the TXCO from the board onto the FY6600's main board to directly replace the original cheap 100ppm XO module and the little circuit board it came with won't be going to wast since I have several XO modules to hand that I can test on this board (I'll fit a set of gold turned pin sockets to the board to turn it into a plug in XO test rig).
The order went in on Friday and I have an estimated 3 to 8 weeks delivery (12 Dec to 25 Jan). Just as well I'm not in a blazing hurry to upgrade this aspect of the FY6600-60M. :-)
Regards, Johnny B Good
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