The classic problem that happens when you use a CMOS or TTL buffer is that you can power up the circuit through the protection diodes in the buffer chip, if your circuit is unpowered but receiving signals.
What are the ways to prevent this?
- Diode protection of the buffer's power supply. But this reduces the device's VCC and thus the performance. This still allows the buffer itself to be powered up and driving its outputs...
- AC coupling of input signals, not really feasible on signals like video with variable average levels, or is my gut feeling off on this?
- Series resistance on each line?
- optocoupling, but these are big devices and do they work with 16MHz TTL signals?
- Special diode-less-input chip but since it's a buffer it's the chip that most needs them. Maybe if it had 7V transorbs instead of diodes? That way no 5V signal can power it up but it still protects the chip more than nothing. Is there such a chip equivalent to the 74HCT541/540?
I'm sure there are ways to deal with this, any thoughts?
It's for a personal, non-production (or very small, like 20) project. It's not critical but it annoys me.
Or simply source from a 74HC05, open-drain-output inverter, with pull-up on the receiving end.
Or:
(2N2369) Input o----E C-----+-------o TO INPUT of UN-POWERED GATE B | | o---4.7K----o +VDD of UN-POWERED BOARD | o-----4.7K----------o +VDD of UN-POWERED BOARD
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
In fact, Mike, that's _exactly_ why I have a stash of them here. Harold Johnson tells me that they get around +48dBm TOI with T-mixers built with them, and can do better with SD5000 series parts, but the SD5000's are difficult to get and much more expensive, I believe.
I've recently come across some other parts that may be exceptionally good in that application, too...
I'm far too drunk to quite work out if this would be useful or not, but have you looked at LCX logic? Low-voltage with 5V-tolerant (up to 7V rated) inputs. There is such a thing as a 74LCX541 if you believe Fairchild. I think it was Fairchild who knocked my elbow while I was playing pool, so I don't trust 'em myself. It it'll handle 7V input while running off 2.3V without thermonuclear meltdown, so I guess it doesn't use the usual protection diodes. Or perhaps it uses 8 in series, eh? If the damn datasheet would stop dancing about I might be able to tell you more. Is it fast enough though? You can make them fast by locking them in a box without food you know.
Thanks, I'll have a Grolsch. And rack up before you go to the bar so I can carefully position the balls to my advantage and claim you missed my spectacularly fortuitous break.
+48dBm! That is very good. Now I'm really interested. At 1ns, they should work well into the VHF region. And +48dBm makes them an excellent candidate for test equipment.
The SD5000 has the same typical 1ns, but 10 times the on resistance:
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I'm surprised Harold says they perform better. Wouldn't the higher Ron increase the losses and degrade the TOI? And you'd think the extra 75 ohms in series with the input would degrade the noise figure. Very curious.
I thought LCX540/541 sounded familiar. I redid those for ON-Semi in
2001.
I can't comment on how it's done, but the inputs are "signal-tolerant" when there is no power.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I might do that but I much prefer an IC solution that I just plop on the PCB. Also, if other people want to build one it's easier to build that way. I'd need 6 transistor buffers and associated biasing stuff vs one IC. Anyways thanks for the input.
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Even more obvious, 74LCX541, just like the part on my design! This is the best kind of design, no change required, just some reading and testing. 0.83$ for a SOIC at Digikey, anyone can source this.
Don't count on +48dBm TOI for mixers running with inputs above HF...that number is for use at HF. I'm not sure just what folk have done at the higher frequencies. You should hook up with Harold directly...you can find him on rec.radio.amateur.homebrew.
That 75 ohms doesn't match my recollection of the SD5000 at all. I'll have to look into it further, but that may take some time--too many other things above it in priority.
No hints on the other parts right now...sorry. I want a chance to look into them myself further first.
I redesigned the 'LCX540/41, as well as many other digital parts, for ON-Semi in 2001. I didn't realize you wanted that functionality.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I think I found the thread you were thinking of on the SD5000. Here is Harold's comment:
"The SD5000 series, which started all this I believe, has a high conversion loss due to high Rdss. I have used those and measured variously 10-11 dB CL. It's appreciably more expensive but is a wonderful mixer, I use several in test equipment in the shop where I occasionally NEED high intercepts and G3SBI has measured third order intercepts in the mid +50's using it over volted which immediately made the following filters the culprit in obtaining anything higher.. It may be getting difficult to find, but is also available in DIP. I have had no difficulty with SSOP and have even used a few TSSOP packages in a homebrew workshop. It takes a little care, but you'd better start getting a handle on it, more and more desireable devices are only available in the smaller packages."
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There is some more good info in another thread:
"Certainly a better choice when it comes to conversion loss. I've built a half dozen SD5000 mixers and although at least one of them managed a measured +50 dBm Ip3, (G3SBI measured it not me!) I never managed to get the conversion loss below 9 dB and more often 10. Conversely, the 3125/ family readily does 4.8 dB conversion loss thanks to the very respectable Rds on."
"REALLY simplifies things in the front end department, to at least 30 MHz, with attention paid to the front end filters, you can get away without an RF amplifier."
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I have some FST3125's ready when I get time to breadboard. Gian I7SWX F5VGU talks about it in the above link:
"If you are looking for a low consumption and simple mixer you may go OK with the classic NE/SA602 and similar ones. BUT ... if you are looking for high performance you need the H-Mode Mixer in the 3 transformer G3SBI or the 2 transformer I7SWX configurations. The H-Mode Mixer has been tested on downconversion (CDG2000, STAR projects) and also on upconversion projects (I7SWX). For upconversion I have devised a new squarer using LVDS and test it at IF of 35 to 70 MHz (and 100MHz) with conversion loss around -5dbm and IP3 between +35 to +40dBm (3.3V FST3125)."
"The H-Mode Mixer has been tested for RF input up to 50MHz in RX and converter to 27-28MHz IF. At 144MHz it performs like a standard diode db mixer having a conv loss around -8dB. The input limit is due to the internal gates timing and unbalancing as the FST3125 may have a bandwidth between 300 to 400MHz. For those interested I may suggest a visit to JA9TTT web page where he has reported measurement on both G3SBI and I7SWX versions using the 74AC86 squarer with balance adjustment. The comments are in japanese but a translator is making the reading understandable: ja9ttt.homedns.org/. I have posted a copy in my web page
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in the subdirectory homebrewing. JA9TTT page permits to enlarge the spectrum analyzer screen pictures. I have also developed a 1 transformer double balance mixer/demodulator/modulator using the FST3125 and 74AC/HC86 squarer with an IP3 of around +25dBm. Notes on the H-Mode Mixers and 1T DBM have been reported in RadCom, G3VA's Technical Topics column. For those interested on more detailed information I can make available notes in English and Italian. Please write to my e-mail address."
I also found the mother lode on Tayloe mixers used in software defined radio:
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This really gets your blood going. I need to finish off a bunch of projects so I can clear a space on the workbench and fire up the soldering iron.
Thanks, Tom, for mentioning the 74CBT3125. That's what started all this, and you have given me renewed energy to tackle the hard stuff after I have been procrastinating far too long.
I hate to sound picky, but i really question a +48dBm TOI on any mixer. The spec sheet on the SD5000 show it as completely out of reach for that device used directly. Perhaps after a 20dB pad.
--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.
--Schiller
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