> Chris Jones <lugnut808@spam.yahoo.com> wrote:
>
>> Note that the rise and fall time of the 74LVC04 is quoted in the
>> datasheet with a 30pF or 50pF capacitive load. Instead, I measured the
>> risetime with the output connected through a resistive
>> back-terminating network to a matched coaxial cable, which would be
>> expected to give much faster edges.
>
> Where do you see a spec for the rise and fall times? I tried several
> different manufacturer's datasheets and all I can find is the prop delay.
>
> I can see the mfgr's wanting to eliminate every possible spec parameter
> that they can get away with. This reduces cost and removes the loss for
> devices that don't meet the spec.
>
> But I'm old school. I want as many spec parameters I can get:)
>
> JK
>
> Note that the rise and fall time of the 74LVC04 is quoted in the
> datasheet with a 30pF or 50pF capacitive load. Instead, I measured the
> risetime with the output connected through a resistive
> back-terminating network to a matched coaxial cable, which would be
> expected to give much faster edges.
Where do you see a spec for the rise and fall times? I tried several
different manufacturer's datasheets and all I can find is the prop delay.
I can see the mfgr's wanting to eliminate every possible spec parameter
that they can get away with. This reduces cost and removes the loss for
devices that don't meet the spec.
But I'm old school. I want as many spec parameters I can get:)
JK
Reply by Chris Jones●July 4, 20132013-07-04
On 04/07/2013 19:40, John K wrote:
> Chris Jones <lugnut808@spam.yahoo.com> wrote:
>
>> The ~280ps risetime was found by measurement. If you want it
>> guaranteed then you'd need a more expensive chip. It is only a typical
>> value on even ADI's fast comparators ADCMP572 etc. In practice rather
>> than paying a manufacturer extra to guarantee it, you might be better
>> off buying a lot of 74LVC chips from the same batch, measuring a few
>> and then knowing that this is what you have.
>
>> You can see the risetime figure briefly on the screen of the scope in
>> the video I linked. I measured it previously with a similar scope and
>> I got about 280ps, depending on supply voltage etc. That was for an
>> old Philips chip, so now that would be called NXP. I have a few of
>> those old chips left over, and I know they are fast. No idea whether
>> they use the same chip layout and fab now, maybe JT can advise.
>
>> Chris
>
> It was very difficult to follow the video. He kept rotating the time axis
> knob back and forth. Too fast to actually look at the waveform and try to
> understand what it is saying.
>
> I am surprised the risetime can be that low. It takes very fast
> transistors to do that. If that is true, why is the propagation delay so
> long? You'd expect with transistors that fast, the prop delay would be
> down in the picosecond range instead of 4.5ns at 3.3V as shown here
>
> "http://datasheet.octopart.com/SN74LVC04APWR-Texas-Instruments-datasheet-
> 114799.pdf"
>
> I don't have any of these devices on hand and do most of my stuff in
> 10GHZ ecl so I really don't have a need for them. If you have access to a
> fast scope, could you take a picture of the risetime?
>
> Thanks,
>
> JK
>
Note that the rise and fall time of the 74LVC04 is quoted in the
datasheet with a 30pF or 50pF capacitive load. Instead, I measured the
risetime with the output connected through a resistive back-terminating
network to a matched coaxial cable, which would be expected to give much
faster edges.
Each inverter in those 74LVC04 chips would contain several (an odd
number of...) inverter stages in a chain. They would probably also have
ESD resistors at the input which would somewhat slow down the
propagation delay to make it likely to be several times greater than the
best possible rise and fall times. I think they probably use a 0.5um
CMOS process guessing from the absolute max. supply voltage rating. On a
0.18um process I think a single inverter stage has about 100ps
propagation delay.
Jim Thompson might know a lot more about it than that, and might even be
allowed to tell us.
I no longer have access to a fast scope so I can't give you any plots,
but I can confirm that you could get a pretty clean looking square wave
with <300ps rise and fall times out of that very board that was on the
video.
Chris
Reply by John K●July 4, 20132013-07-04
Chris Jones <lugnut808@spam.yahoo.com> wrote:
> The ~280ps risetime was found by measurement. If you want it
> guaranteed then you'd need a more expensive chip. It is only a typical
> value on even ADI's fast comparators ADCMP572 etc. In practice rather
> than paying a manufacturer extra to guarantee it, you might be better
> off buying a lot of 74LVC chips from the same batch, measuring a few
> and then knowing that this is what you have.
> You can see the risetime figure briefly on the screen of the scope in
> the video I linked. I measured it previously with a similar scope and
> I got about 280ps, depending on supply voltage etc. That was for an
> old Philips chip, so now that would be called NXP. I have a few of
> those old chips left over, and I know they are fast. No idea whether
> they use the same chip layout and fab now, maybe JT can advise.
> Chris
It was very difficult to follow the video. He kept rotating the time axis
knob back and forth. Too fast to actually look at the waveform and try to
understand what it is saying.
I am surprised the risetime can be that low. It takes very fast
transistors to do that. If that is true, why is the propagation delay so
long? You'd expect with transistors that fast, the prop delay would be
down in the picosecond range instead of 4.5ns at 3.3V as shown here
"http://datasheet.octopart.com/SN74LVC04APWR-Texas-Instruments-datasheet-
114799.pdf"
I don't have any of these devices on hand and do most of my stuff in
10GHZ ecl so I really don't have a need for them. If you have access to a
fast scope, could you take a picture of the risetime?
Thanks,
JK
Reply by Chris Jones●July 3, 20132013-07-03
On 03/07/2013 20:16, John K wrote:
> Chris Jones <lugnut808@spam.yahoo.com> wrote:
>
>> On 02/07/2013 09:28, Charles wrote:
>
>> If you are satisfied with rise and fall times of about 250 - 300ps,
>> then you can use a cheap 74LVC04AD or similar.
>>
>> http://www.youtube.com/watch?v=oUiEeOp8ynY#t=698s
>
>> Unfortunately that particular board didn't have sufficient
>> low-frequency decoupling (it was intended for use at much higher
>> frequencies where the decoupling was adequate), so the power supply
>> cable inductance rings badly with the small decoupling capacitors on
>> the board. If he had looked at the fall-time instead of rise time, and
>> left my ac-coupling and back termination in, it might have been a bit
>> cleaner. The best option would be to improve the local low frequency
>> decoupling.
>
>> Chris
>
> Chris, how do you get 250 ps risetime? I tried to find the datasheet on
> Octopart and they have started using captchas for rate limiting. I could
> never get past that screen.
>
> I did get into Mouser and found the part is made by TI and NXP. Neither
> datasheet said clearly what the output rise and fall times were. The best
> I could find was a table that had numbers like <2.5ns, but it wasn't
> clear if that was for the input or the output.
>
> I'd like to find out for sure if that family really does 250ps risetime,
> but I'd be surprised if it does. Do you have any more information?
>
> Thanks,
>
> JK
>
The ~280ps risetime was found by measurement. If you want it guaranteed
then you'd need a more expensive chip. It is only a typical value on
even ADI's fast comparators ADCMP572 etc. In practice rather than paying
a manufacturer extra to guarantee it, you might be better off buying a
lot of 74LVC chips from the same batch, measuring a few and then knowing
that this is what you have.
You can see the risetime figure briefly on the screen of the scope in
the video I linked. I measured it previously with a similar scope and I
got about 280ps, depending on supply voltage etc. That was for an old
Philips chip, so now that would be called NXP. I have a few of those old
chips left over, and I know they are fast. No idea whether they use the
same chip layout and fab now, maybe JT can advise.
Chris
> Unfortunately that particular board didn't have sufficient
> low-frequency decoupling (it was intended for use at much higher
> frequencies where the decoupling was adequate), so the power supply
> cable inductance rings badly with the small decoupling capacitors on
> the board. If he had looked at the fall-time instead of rise time, and
> left my ac-coupling and back termination in, it might have been a bit
> cleaner. The best option would be to improve the local low frequency
> decoupling.
> Chris
Chris, how do you get 250 ps risetime? I tried to find the datasheet on
Octopart and they have started using captchas for rate limiting. I could
never get past that screen.
I did get into Mouser and found the part is made by TI and NXP. Neither
datasheet said clearly what the output rise and fall times were. The best
I could find was a table that had numbers like <2.5ns, but it wasn't
clear if that was for the input or the output.
I'd like to find out for sure if that family really does 250ps risetime,
but I'd be surprised if it does. Do you have any more information?
Thanks,
JK
Reply by Chris Jones●July 3, 20132013-07-03
On 02/07/2013 09:28, Charles wrote:
> Hi,
>
> I have been using an GW Instek SFG-2110 and find that the TTL output has
> a fast rise time. It is about 2 ns and this could be useful for
> oscilloscope bandwidth measurements for instruments of less than 100 MHz.
>
> The TTL option seems to be missing on more expensive function
> generators. Any thoughts?
If you are satisfied with rise and fall times of about 250 - 300ps, then
you can use a cheap 74LVC04AD or similar.
http://www.youtube.com/watch?v=oUiEeOp8ynY#t=698s
Unfortunately that particular board didn't have sufficient low-frequency
decoupling (it was intended for use at much higher frequencies where the
decoupling was adequate), so the power supply cable inductance rings
badly with the small decoupling capacitors on the board. If he had
looked at the fall-time instead of rise time, and left my ac-coupling
and back termination in, it might have been a bit cleaner. The best
option would be to improve the local low frequency decoupling.
Chris
Reply by Charles●July 2, 20132013-07-02
"miso" wrote in message news:kqtrkh$c19$1@speranza.aioe.org...
They use a special pulse generating diode to calibrate scopes. These
items are peddled all the time on ebay at exhorbintant prices. Like $250
for what gets about $5 at the flea market.
Just search for Tektronix tunnel diode pulser on ebay to see one.
As far as I know, they are just diodes with RF connectors attached. You
could probably roll one yourself if you knew the secret sauce.
http://www.jensign.com/avalanchepulsegenerator/index.html
A way to do it using the avalanche mode of a readily available transistor.
That's the good news. The bad news is that the avalanche mode is device
dependent.
Reply by Sergey Kubushyn●July 2, 20132013-07-02
David Platt <dplatt@coop.radagast.org> wrote:
> In article <3hu4t8l76osq0vcv3iqqrvj66brd81r8tt@4ax.com>,
> Jon Kirwan <jonk@infinitefactors.org> wrote:
>
>>On Mon, 01 Jul 2013 23:21:46 -0700, miso <miso@sushi.com>
>>wrote:
>>
>>>They use a special pulse generating diode to calibrate scopes. These
>>>items are peddled all the time on ebay at exhorbintant prices. Like $250
>>>for what gets about $5 at the flea market.
>>>
>>>Just search for Tektronix tunnel diode pulser on ebay to see one.
>>>
>>>As far as I know, they are just diodes with RF connectors attached. You
>>>could probably roll one yourself if you knew the secret sauce.
>
>>Try:
>> http://w140.com/tek_067-0681-01.pdf
>
> Nice - thanks!
>
> Tunnel diodes are rather scarce these days (like hen's teeth). Seems
> to me that one might be able to put together a similar pulser with a
> different sort of negative-resistance switching device, such as a
> "lambda diode" made from a pair of coupled transistors. The
> traditional lambda diode uses a pair of JFETs, but fast P-channel
> JFETs are also scarce. An "RF N-channel JFET, plus RF PNP bipolar"
> combination might work nicely. Keeping it small and compact (to
> minimize parasitic trace inductance) would probably be important in
> this application, so as to get the cleanest pulses possible.
Search TekScopes Yahoo Group for "DIY TD Pulser". That is my version of the
venerable Tek 067-0681-01 using russian TD still available from Ebay.
Everything is there including all construction details, photos, and actual
test results. YMMV but that one I built and put on TekScopes gives something
like 35pS risetime and takes one evening to build.
---
******************************************************************
* KSI@home KOI8 Net < > The impossible we do immediately. *
* Las Vegas NV, USA < > Miracles require 24-hour notice. *
******************************************************************
Reply by tm●July 2, 20132013-07-02
"David Platt" <dplatt@coop.radagast.org> wrote in message
news:2ghbaa-mn.ln1@coop.radagast.org...
> In article <3hu4t8l76osq0vcv3iqqrvj66brd81r8tt@4ax.com>,
> Jon Kirwan <jonk@infinitefactors.org> wrote:
>
>>On Mon, 01 Jul 2013 23:21:46 -0700, miso <miso@sushi.com>
>>wrote:
>>
>>>They use a special pulse generating diode to calibrate scopes. These
>>>items are peddled all the time on ebay at exhorbintant prices. Like $250
>>>for what gets about $5 at the flea market.
>>>
>>>Just search for Tektronix tunnel diode pulser on ebay to see one.
>>>
>>>As far as I know, they are just diodes with RF connectors attached. You
>>>could probably roll one yourself if you knew the secret sauce.
>
>>Try:
>> http://w140.com/tek_067-0681-01.pdf
>
> Nice - thanks!
>
> Tunnel diodes are rather scarce these days (like hen's teeth). Seems
> to me that one might be able to put together a similar pulser with a
> different sort of negative-resistance switching device, such as a
> "lambda diode" made from a pair of coupled transistors. The
> traditional lambda diode uses a pair of JFETs, but fast P-channel
> JFETs are also scarce. An "RF N-channel JFET, plus RF PNP bipolar"
> combination might work nicely. Keeping it small and compact (to
> minimize parasitic trace inductance) would probably be important in
> this application, so as to get the cleanest pulses possible.
>
>
>
Try Ebay for the Russian tunnel diodes. Cheap and most work. Get the Ge
ones, not the GaAs types.