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?
Interesting feature of an inexpensive function generator
Started by ●July 1, 2013
Reply by ●July 1, 20132013-07-01
"Charles"> > 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?** Using the rise time of some TTL gate to calibrate a 100MHz scope is not gonna be very accurate. How do you check the rise time of your source ? What about issues with the coax linking it to the scope ? The easiest way to check scope BW is to use an RF generator - any decent one with levelled output will do, long as it goes out to a couple of hundred MHz. Anecdote: On more than one occasion, techs have questioned the practice of using a 10MHz or more BW scope to test a piece of electronic audio equipment for flat response from 20Hz to 20kHz and beyond. They asked : " How do you know your scope has a really flat response ? " I replied: " Square waves ( audio frequency understood) look perfect " . ... Phil
Reply by ●July 2, 20132013-07-02
On a sunny day (Mon, 1 Jul 2013 19:28:37 -0400) it happened "Charles" <charlesschuler@comcast.net> wrote in <kqt30u$8pr$1@dont-email.me>:>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?? Add a TTL chip to a less expensive one? 74FXX series should do.
Reply by ●July 2, 20132013-07-02
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.
Reply by ●July 2, 20132013-07-02
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 Jon
Reply by ●July 2, 20132013-07-02
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.pdfNice - 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.
Reply by ●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.
Reply by ●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 $5 at the flea market. >>> >>>Just search for Tektronix tunnel diode pulser on ebay to see one. >>> >>>As far as I know, they are just diodes with RF connectors attached. You >>>could probably roll one yourself if you knew the secret sauce. > >>Try: >> http://w140.com/tek_067-0681-01.pdf > > Nice - thanks! > > Tunnel diodes are rather scarce these days (like hen's teeth). Seems > to me that one might be able to put together a similar pulser with a > different sort of negative-resistance switching device, such as a > "lambda diode" made from a pair of coupled transistors. The > traditional lambda diode uses a pair of JFETs, but fast P-channel > JFETs are also scarce. An "RF N-channel JFET, plus RF PNP bipolar" > combination might work nicely. Keeping it small and compact (to > minimize parasitic trace inductance) would probably be important in > this application, so as to get the cleanest pulses possible.Search TekScopes Yahoo Group for "DIY TD Pulser". That is my version of the venerable Tek 067-0681-01 using russian TD still available from Ebay. Everything is there including all construction details, photos, and actual test results. YMMV but that one I built and put on TekScopes gives something like 35pS risetime and takes one evening to build. --- ****************************************************************** * KSI@home KOI8 Net < > The impossible we do immediately. * * Las Vegas NV, USA < > Miracles require 24-hour notice. * ******************************************************************
Reply by ●July 2, 20132013-07-02
"miso" wrote in message news:kqtrkh$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 ●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
