checking my bandwidth and rise time help - Page 1
Forum section: electronics / beginners

author: Gazmon
author_rank: Regular Contributor
timestamp: March 12, 2019, 10:23:18 am
content: hi,I want to try and get a fairly accurate or near to measurement of my scope, I only have a 24mhz feeltech sig gen, could anyone give me some tips as to how I could set this up on my scope to check my bandwidth and rise time pleaseany help appreciatedgazmon

author: particleman
author_rank: Regular Contributor
timestamp: March 12, 2019, 02:09:54 pm
content: What is the bandwidth of the scope you are using and the rise time of the signal generator? You are most likely going to need something that provides a fast rising edge. I use a PG506 and add a tunnel diode pulser for higher bandwidth. Im doubting the rising edge on your signal generator will be fast enough to check scope rise time. The rise time on the TD pulser is about 130 picoseconds.

author: rstofer
author_rank: Super Contributor
timestamp: March 12, 2019, 02:19:49 pm
content: Search around (here or Google) for 'fast edge pulse generator' - there are many and most are simple.Here is a thread on the topic:https://www.eevblog.com/forum/projects/yet-another-fast-edge-pulse-generator/That 24 MHz signal generator probably isn't capable of delivering fast rise time square waves. You would like to have the source some multiples faster than the scope.https://interferencetechnology.com/rise-time-vs-bandwidth-and-applications/

author: magic
author_rank: Super Contributor
timestamp: March 12, 2019, 06:07:20 pm
content: Much simpler fast edge generators have been posted, seehttps://www.eevblog.com/forum/testgear/fast-rise-time-2ns-or-1ns-signal-generator/I particularly recommend the w2aew video if you don't understand the principle of operation and the 74LVC series gates shown by Lukas because they allow shorter connection to the 100nF capacitor, which reduces overshoot and ringing. You want to eliminate overshoot to get really meaningful rise time results, and for that end, short connection to the capacitor is essential.I got good results using two paralleled gates of SN74LVC2G14 with a 91Ω resistor on each output. The "remaining" 9Ω comes from internal resistance of the chip's output. A third gate works as an oscillator and drives the inputs to the signal output gates.

author: tggzzz
author_rank: Super Contributor
timestamp: March 12, 2019, 06:15:44 pm
content: Quote from: magic on March 12, 2019, 06:07:20 pmI use 3 and 143 ohms, and get ~250ps risetime into 50ohms at Vcc=5V.Given thedi/dtand the 50mA into 50ohms, good decoupling is very important.

author: magic
author_rank: Super Contributor
timestamp: March 12, 2019, 07:35:27 pm
content: That's quite fast, I didn't even realize. Do you happen to know what's the propagation delay / how fast it would oscillate with input shorted to output?

author: tggzzz
author_rank: Super Contributor
timestamp: March 12, 2019, 08:01:21 pm
content: Quote from: magic on March 12, 2019, 07:35:27 pmRing oscillators are horribly sensitive to temperature, voltage, noise, phase of moon etc. The frequency is determined by the propagation delay, such can be found in the datasheet.

author: tggzzz
author_rank: Super Contributor
timestamp: March 12, 2019, 10:26:18 pm
content: Quote from: magic on March 12, 2019, 07:35:27 pmJust how fast can be seen here:https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

author: David Hess
author_rank: Super Contributor
timestamp: March 12, 2019, 11:10:26 pm
content: The simplest thing to do is use a fast logic gate and ground plane construction to make a reasonably clean fast edge as shown in the w2aew video. You can drive the logic gate with your existing signal generator. AC or LVC logic running on 5 volts is suitable.