Short Pulse Generation - Page 1
Forum section: electronics / rf-microwave

author: SergeantFreeman
author_rank: Newbie
timestamp: April 22, 2022, 10:42:21 am
content: Hello there,I'm quite new at RF (not sure if I should post in "Beginners").I want to characterize an RCL circuit using the time domain. It's kind of the same idea ashere.My question is on the Tx side of things: I have a single turn of wire (Tx) and want to generate a very short pulse to stimulate an RCL circuit (let´s say 5ns). My "Tx-coil" has a diameter of 30mm and the wire has a radius of 0.1mm. I will have an inductance in the ballpark of 200-300 nano Henrys.How do I calculate/choose the needed equipment to generate that short pulse? In my understanding that short pulse will need a huge amount of power to generate the needed stimulation in the coil.I have ordered this PCB I´ve made. I know there are many things that I need to change. It's just a test to see if I get anything like that. It's my first time with antennas.Thanks!

author: RoV
author_rank: Regular Contributor
timestamp: April 22, 2022, 05:52:59 pm
content: You can try something like this.I have assumed an AC-series logic at 5 V (typ rise and fall time 3 ns). The resistor matches the logic to the coaxial cable, which is shorted in the end and does the trick. You should obtain something like the waveform shown. Clearly, the antenna and the relevant connection will load the circuit, so you'll have to elaborate a bit on it. The lenght of coax cable shall keep velocity factor into account.You can find logic faster than the AC, but mostly at lower voltage.

author: Marco
author_rank: Super Contributor
timestamp: April 22, 2022, 06:45:42 pm
content: Capacitor charged to 10V and EPC GAN MOSFET next to the TX coil, slam it on and off, freewheeling diode with resistor to let current decay slowly or a TVS if you want it to decay fast.

author: antenna
author_rank: Frequent Contributor
timestamp: April 22, 2022, 10:34:51 pm
content: Are you just trying to ring it to measure the resonant frequency and q-factor? A simple push button coupled through a capacitor would do that if connected to the RLC under test rather than a TX coil.As for fast rise times, my first TDR was a transistor configured as an avalanche oscillator. The rise time is faster than my 200MHz scope can measure (it says 1.35ns 10%-90%, but I know its sub-nanosecond). You can do a search for avalanche oscillators. Or....If you are just ringing it, you can use a square wave oscillator if it has a fast rise time and the oscillation will be superimposed on the square wave. Something like the 74AC14 schmidt trigger would do nicely.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2022, 01:27:45 am
content: Doesn't have to be pulsed, response is the same in either [frequency or time] domain. For the bandwidth expected of a structure that size, a NanoVNA would do fine, I guess.If you insist: another option is a BJT avalanche pulse generator, needs >100V supply (usually from a small flyback converter, not a big deal). Signal is quite large obviously(?), usually stopped down through attenuators but you can do whatever of course.Quote from: antenna on April 22, 2022, 10:34:51 pmIndeed, mechanical contacts are not to be underestimated: the risetime is typically fractional ns. The timing is just very unreliable. But all you need is one (if your scope is fast enough, that is).Tim

author: SergeantFreeman
author_rank: Newbie
timestamp: April 23, 2022, 07:02:59 am
content: Thanks for you help!Actually, I have forgotten to mention that the connection to the DUT needs to be wireless. So I cant use the pushbutton idea. I just need the resonant freq. and Q-factor.My problem with using freq-domain is the changing mutual inductance M when misplaced. That will constantly alter M and thus f_r and Q. In time domain this displacement is just damping and to an extent still usable.I will try the GaN MOSFET. But I'm still wondering If I wouldn't need a huge amount of power. Since this kind of pulse will produce a huge impedance in Tx.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2022, 08:11:52 am
content: What are you actually after? This is sounding like an X-Y problem...Tim

author: SergeantFreeman
author_rank: Newbie
timestamp: April 23, 2022, 11:07:54 am
content: I want to characterize an RCL circuit (with printed coil) (i need f_r and Q) through a wall (or other substrate). The problem with a VNA is that I need to know the exact position of the DUT to get a reliable measurement. Let's say the DUT is moving. Sendung a short impulse to get and analyze the impulse response should get the faster and more reliable measurement than a VNA.Normally done like that:I would like to do it like that:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 23, 2022, 12:24:57 pm
content: How fast is it moving? Even 10ms between positions would seem unbelievably fast (train speed indicator as it were?). But a frequency sweep can be done in that time quite easily, let alone a wideband technique (e.g. white noise or PRBS or other sequence, followed by FFT).The two-coil method has very little difference to a single coil in a bridge. You can use backscatter methods just fine here.Tim

author: Marco
author_rank: Super Contributor
timestamp: April 23, 2022, 03:23:44 pm
content: Quote from: T3sl4co1l on April 23, 2022, 01:27:45 amIf you know how the system of Tx/Rx/DUT interact so you can de-embed the whole thing.By striking the DUT as a bell with the Tx and making sure the Rx is damped far more than the DUT, there's less to need to know.

author: Marco
author_rank: Super Contributor
timestamp: April 23, 2022, 03:33:13 pm
content: Quote from: SergeantFreeman on April 23, 2022, 07:02:59 amI was just going on the signal from the paper, 10V in 200nH for 5ns is 250 mA, not a negligible current.It kind of suggests they let the current in the Tx decay due to loop resistance BTW. Applying zero volt means shorting it.