Fast, bipolar-tolerant, low-capacitance switch to ground - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: April 02, 2017, 07:10:29 am
content: Hi folks,I have a coil/inductor of about 0.1 to 10uH (wide range I know) which I'd like to feed ("transmit") a 27 MHz sine wave into, and then rapidly switch to capturing the output ("receive") from the same coil. The coil is filled with a sample powder; the application is NQR, for those who are curious why a coil would ever produce an "echo"!I'd like to tie one end of the inductor to a buffer which outputs a +/- 4V (supply is bipolar) 27 MHz sine wave during the transmit phase, and ties to ground (0V) for the receive phase. This is easy to do.But here's my actual question: I'd like to tie the other end of the inductor to ground (0V) during the transmit phase, and then for the receive phase rapidly ( << 50 ns ) switch to open-circuit with a parasitic capacitance preferably at 2pF or less. I'd use a switching MOSFET tied to ground, and it's hard to find a MOSFET with Cd at 2pF or less. I'd use a BJT, but the 0.2V Vce drop and slow turn-off time during to storage time make this difficult. Does anyone have any suggestions on how to implement this? To put it into numbers, I'm looking for some sort of device/circuit which can be switched between two modes, which I will name "Short" and "Open":"Short": Node tied to ground, needs to handle ±100 mA (although I'm open to solutions that are only good to 10 mA), no particularly strong requirement on voltage drop (let's say < ±1V ). So < 10 ohm to ground I suppose."Open": Node not tied to ground, < 2pF, > 10 kiloohm to ground. Voltage will be small; assuming that the signal will not exceed a diode drop either way from ground may be reasonable."Short" -> "Open" switching time: << 50 ns."Open" -> "Short" switching time: < One million ns (i.e., < 1ms, basically unconstrained)I understand that dealing with single digit pF is tricky, I'm already aware that PCB design will need to be adjusted (e.g., no ground plane, etc).(I was originally planning to have one node of the inductor tied to ground, and have the other connected to a strong output buffer that can faithfully reproduce 27 MHz and then rapidly (<< 50ns) switch to high impedance mode with minimal parasitic capacitance, which although far from impossible, is a bit tricky. I'd also have to connect my receiver to the same node, taking care to protect my receiver/amplifier from the full output of the buffer. The approach I'm outlining above seems much simpler.)

author: Bendba
author_rank: Regular Contributor
timestamp: April 02, 2017, 07:46:53 am
content: Hi,This might sound stupid but, couldn't you just have two identical coils, one to drive and one to measure?

author: rs20
author_rank: Super Contributor
timestamp: April 02, 2017, 07:50:21 am
content: Quote from: Bendba on April 02, 2017, 07:46:53 amNot a stupid idea at all, but there's a risk the secondary would be strongly driven during the transmit phase, which could still damage the receiver.

author: Bendba
author_rank: Regular Contributor
timestamp: April 02, 2017, 07:56:37 am
content: Could you then have an open switching device and switch it on at the moment you turn the driver off?Sorry, that got sent too fast.I mean, if your driven coil is connected to the receiver through a fast switching device, you could keep it "disconnected" while the driver sends the signal and quickly connect it to the receiver when the driver has done is job.I'm not sure if what I mean is clear.

author: rs20
author_rank: Super Contributor
timestamp: April 02, 2017, 08:08:47 am
content: Quote from: Bendba on April 02, 2017, 07:56:37 amYeah, a two-node switch that turns on during the switchover works too, as opposed to my original question of a "one-node" shunt to ground than opens/turns off during switchover. But it still needs that < 2pF capacitance to ground, and shouldn't have fancy features like internal 50 ohm terminations that that sort of device often comes with. By all means, I'm open to any specific suggestions that fit that description.

author: SeanB
author_rank: Super Contributor
timestamp: April 02, 2017, 08:15:18 am
content: How about devices that are meant to do switching, like PIN diodes. You can at least get a good low capacitance when off ( depends on the reverse bias for the diode) and reasonable performance when on, and they are quite fast in this application. Impedance on the input side can be whatever is your coil, and output will be 50R to the receiver only. Power depends on the diode you use, but even bog standard 1N4004 diodes can be used in there in many cases, or just cascade a few stages to get better isolation with only little more loss.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2017, 08:28:04 am
content: The main two options for switching with both polarities are JFETs and MOSFETs with a separate substrate. A 2 pF capacitance sounds like more or less impossible - there would normally be much higher capacitance inside the coil already. Often the coil is intentionally made resonant to improve both the driving power and detection SNR.With suitable driving of gate and substrate something like an BSS83 might be an option. It is very low capacitance, but also rather high R_on (e.g. 45 Ohms). There might be similar low R_on versions with more capacitance.If the voltage is limited to < 500 mV even the 2N7000 is not that bad - there might be better more modern and lower voltage MOSFETs that could even reach the < 2 pF and < 10 Ohms range.Only a +-4 V drive sounds rather low power, I would expect a much more powerful driver.I have done somewhat similar experiments (not nuclear resonance, but mechanical in the 1 MHz range). Here diodes where used for switching / clamping of the signal: a transformer with 2 anti parallel diodes between the source and coil. However the power level for driving was rather high ( > 100 V, about +60 dBm).

author: Marco
author_rank: Super Contributor
timestamp: April 02, 2017, 08:38:43 am
content: How about using two antiparallel schottky diodes, with a fast closed loop amplifier to provide a virtual ground at the bottom of the coil while it's being driven. Then when you turn off the driving signal at zero current, you drive the output of the amp to ground.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2017, 02:48:58 pm
content: Realize what you're asking for...Even if the switch is ideal, and the transmitter stops instantly, you have the L/R time constant of 200ns, during which whatever energy was left in the coil will decay. If it's a very small coil, it won't have higher resonant / transmission line modes, but a large one will (which may be desirable as far as increasing the NQR return signal?).It's probably easier to leave the "transmitter" connected, though. An open-collector output is as good as open circuit when its current goes to zero. You do need a final with much less than 117pF (Ccb or Coss).You should probably also consider generating a ringdown waveform. A sharply terminated tone burst requires much more bandwidth to emit faithfully: a 50ns rise time is 50ns, no matter what it's windowing, and that means you need at least 7MHz (BW ~= 0.35 / t_r) bandwidth beyond 27MHz to get that wave shape. For sure, the exact phase shift between the tone and the falling edge should be adjustable, so you can null the inductor energy that way.But why something so arbitrary as 50ns? The NQR signals you're looking for, are they really so brief (low Q) as to demand this sudden of an acquisition period? NMR signals have Q's in the hundreds of thousands. And, really, if you're looking for such low Q signals, the fact that it's hard to acquire in this method (time domain) really does imply you shouldn't be looking for it, that way,at all-- do it in the frequency domain instead, with a sweep!Protecting the receiver, blanking its input away from transmitted power, will be the bigger priority. Mind that a ~5 ohm 30V MOSFET will have on the order of 15pF Coss, which is ~400 ohms at 27MHz, so you'll need several (perhaps in an L or T attenuator arrangement) to do a good enough job blanking it that way.Diodes create distortion, but if you aren't concerned about that, it's a good way to reduce the dynamic range at the receiver, without needing a blanking circuit.Tim

author: langwadt
author_rank: Super Contributor
timestamp: April 02, 2017, 03:03:49 pm
content: common way to do it for NMR is anti parallel diodes in series with the transmitter and anti parallel diodes in parallel with the receiver after a 1/4 wave stub

author: David Hess
author_rank: Super Contributor
timestamp: April 02, 2017, 05:04:51 pm
content: Whenever I hear "fast low capacitance switch", I think "schottky diode bridge". And they can also be used to protect low impedance inputs from overload by going open instead of shorting.

author: Marco
author_rank: Super Contributor
timestamp: April 02, 2017, 08:19:58 pm
content: It seems unlikely that the little bit of distortion from a pair of anti-parallel Schottky's is really relevant at all in retrospect, so wouldn't it be enough to just put them below the coil to ground? Slam the generator to ground at zero current to turn it off and have the receiver measures the signal across the Schottky's. They inherently limit voltage during transmission and their video resistance creates the reception.

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2017, 08:41:57 pm
content: In my MRI days (similar to NQR in the hardware), we used PIN diodes to switch a coil from transmit to receive. The PIN diode would connect to the coil through a quarter-wavelength line (actually, an L-C network), so that the high-current forward bias gave an open circuit at the other end and the low-current high-reverse-bias gave a short circuit at the other end.

author: rs20
author_rank: Super Contributor
timestamp: April 06, 2017, 01:17:36 am
content: Thanks for the replies everyone. I was thinking in terms of any capacitance acting as a load/shunt, weakening the incoming signal. But as some have pointed out, the C will just pair with the L to form a tank, so I shouldn't be worried about that at all (beyond ensuring the the resonant frequency of the tank is >= the frequency of interest).I indeed have the ability to precisely adjust phase shift between tone and falling edge; I look forward to experimenting with nulling the L's energy using this.Regarding the comments that +-4V sounds low power, would you be more impressed with +-12V, or are you feeling that I'm even further off than that? I appreciate that classic NQR implementations use LC tank resonance to achieve much higher powers than could be achieved with a non-resonant approach like I'm using (for silly+fun reasons).For those curious, I'm still tossing up between two ideas. Either way I'm looking at inputting the signal into one end of the coil, but the other end of the coil is either:A. Tied to ground via antiparallel Schottkies -- robs a little bit of voltage drop/power from the driver.B. Tied to ground via a 2N7000 FET or similar -- have to actually drive the switch.With the receiver/amplifier looking at the voltage across the Schottkies/FET.