Hello, We need to reshape a rectangular pulse (say TTL, around 100ns wide) into a trapezoidal pulse of the same width, I mean with programmable 2ns to 20ns up to low and low to high rise times. Precision can be medium but the rising should be quite linear (ie not simply low-pass filtered). Any idea of an analog chip that could help to build this function ? Our first idea is to to it digitaly in an FPGA with an external quick DAC and to synthetise this waveform but may be you will have a simpler idea ? U to I converter then capacitor ? Can it be quick enough ? Cheers, Robert.
Programmable slew rate ?
Nov 07, 2006
8 Replies
How big a pulse do you actually want? If you were prepared to settle for a 0V to 3V pulse and had -5V and +10V rail available, I'd definitely go for the current sources and the capacitor (with the capacitor buffered by a fast amplifier capable of driving trerminated
50R transmission lines).3V in 2nsec with a 10pF capacitor is 15mA. Farnell stocks the BFR92 and BFR93 5GHz wideband NPN transistors, and the BRT92 and BFT93 PNP complements. I always needed to put a 27R surface mount transistor in series with the base (and right up against the base) to stop them oscillating, but - given that - they can be made to provide very nice quick steerable current sources at around the 20mA level, not to mention level shifters.
You will need a real resistor - 50R should do - in series with the emitters in any long tailed pair. You can set the current with a DAC driving a Howland current pump or an instrumentation ampliifer, but these outptus will look capacitative at the frequencies of interest, so you need the series resistor to keep the fast edges looking more or less tidy.
Bill Sloman, Nijmegen
a écrit dans le message de news: snipped-for-privacy@b28g2000cwb.googlegroups.com...
Many thanks Bill, that looks like a good path... Of course the initial wish list includes a 0/5V only power supply but we already know that a larger range of power would be needed with an analog solution. We will check your proposal... We also found some RS485 driver chips that have some form of configurable slew rate outputs but too slow for us.
Cheers, and long life to sci.electronics.design.
Robert
Robert Lacoste a écrit :
Ouch...
You may use a diode gate. That's simple but for a say 10pF output node capacitance you'll have a permanent 2.5 to 25mA, depending on your programmed slew rate.
I = 2.5 ... 25mA v | | | Fast 0-5V >---+--|>|--+--|>|---+--->
pulse | | | | | | '--|>|--+--|>|---+ | | | --- | --- 10pF total | | | === | GND v I = 2.5 ... 25mA
For such a high speed and low integration cap you'll need low capacitance schottkies.
Thanks,
Fred.
.---------+-------+---------------+-o+5V | | | | | >| |< | | ??? |- -| | | /| |\\ | | | | | | .---+ | HP5082-2755 | | | .-. .--+--. | | '->| | | | | | | | V V | |\\| '-' - - |\\| | \\ | | | | \\ o---| >--------)----+ +---+------| >-o | / | | | |10p | / |/| | V V --- |/| | | - - --- | | | | | | | | | '--+--' GND | | | | | | \\| |/ | | BFM520 |- -| | | | | | | | '---------+-------+---------------+-o-5V (created by AACircuit v1.28 beta 10/06/04
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Similar to Bill, but very simple. I think there is also a complementary PNP-pair available, but I'm too lazy to search. The thing with the beads applys as well, but since the sources are always on, they are more stable. The first buffer is not needed, if the oscillator can sink/source enough current, it has to be faster than 2ps. TTL in/out. set the pot from 180R to 2k. Eventually you can put small linearizing resistors(100R) in the emitters. The diode ring has to be fast and low capacity, bulk resistance doesn't matter in this case.
ciao Ban
Apricale, Italy
Even fixing the DCT's, it won't work. You need emitter padding resistors.
...Jim Thompson
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I love to cook with wine. Sometimes I even put it in the food.
.---------+-------+---------------+-o+5V | 100 100 | | | | | | >| |< | | ??? |-+-| | | /| | |\\ | | +---' | | | .---+ | HP5082-2755 | | | .-. .--+--. | | '->| | | | | | | | V V | |\\| '-' - - |\\| | \\ | | | | \\ o---| >--------)----+ +---+------| >-o | / | | | |10p | / |/| | V V --- |/| | | - - --- | | | | | | | | | '--+--' GND | | +---. | | | \\| | |/ | | BFM520 |-+-| | | | | | | | 100 100 | '---------+-------+---------------+-o-5V (created by AACircuit v1.28 beta 10/06/04
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I forgot the connections and I wrote about the resistors, here they are. These are dual monolitic 9GHz transistors, and inexpensive.
ciao Ban
Apricale, Italy
"Robert Lacoste" a écrit dans le message de news: 4550a88c$0$25953$ snipped-for-privacy@news.orange.fr...
Many thanks to all of you for your postings, now we are going to switch on the soldering iron... Thanks again, Robert
From about 20 years ago ISTR a programmable pin driver, V(h), V(l), t(r), and t(f) all independently programmable. I wonder if you could find and use some of those?
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.
--Schiller
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