T/R switch for ultrasound application - Page 1
Forum section: electronics / projects

author: Paptrack
author_rank: Contributor
timestamp: October 25, 2024, 08:26:55 am
content: Hi,I am looking for some componentes to design an ultrasound pulser, as HV7360 as signal pulser and MD0100 as T/R switch. I´m not confidence about the election of MD0100. I´m no sure the MD0100 is the right component for ±100V and 20MHz signals. I mean, in a ±100V 20MHz square signal, the positive pulse takes 25ns and other 25ns the negative pulse. Datasheet of MD0100 says that TON and TOFF are less than 20 ns, which provides a quick transition between the Transmit and Receive modes.Question #1: So if the pulse applied has 25ns and MD0100 needs 20ns to switch, could we say that MD0100 meets the requirements to protect the receiver path (LNA+VGA+ADC)? Or should I choose a faster T/R switch?Question #2: In the other hand, the same datasheet says as well that Small Signal Bandwidth is 100MHz. What does it really mean? If the small signal refer to the ultrasound echo signal that is coming back to the receiver path, shouldn´t be as well the bandwidth of 100MHz for HV signal? But again, this 100MHz high voltage square signal would be much faster than TON and TOFF.As you can see, I´m a bit confused. Any help will be very welcome.Thank you in advance.

author: moffy
author_rank: Super Contributor
timestamp: October 25, 2024, 10:02:39 am
content: How long after the end of the Tx signal does the Rx have before it needs to detect a signal? That should determine if the On/Off transition timing is suitable. The 100MHz BW just means that when it is On it will pass signals with a BW(-3dB pt) of 100MHz.

author: Paptrack
author_rank: Contributor
timestamp: October 25, 2024, 11:00:17 am
content: Thank you moffy for your reply.I think it is not about the time between the end of TX signal and the echo signal that will come back to RX. The point is that MD0100 must protect the RX in each of the HV pulses (positivo or negative) transmitted by TX.In the file attached, I show the time response when 10V@1MHz and ±100V@5MHz is applied by TX.So, now I would like to apply a ±100V@20MHz signal, i.e. a half-cycle of 25ns. If TOFF and TON are 20ns, we still satisfy that 20ns < 25ns, but isn't that a bit tight? Or can I use it without fear?BR.

author: moffy
author_rank: Super Contributor
timestamp: October 25, 2024, 11:12:36 am
content: Page 3 of the datasheet:https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/MD0100-Single-and-Dual-ChannelHigh-Voltage-Protection-TR-Switch-Data-Sheet-20005738A.pdfShows that the MD0100 is in series with the Rx circuit. It should only be enabled when an Rx signal is expected, at all other times it should be off, so the critical timing is that between the ending of the transmit signal and the time an Rx signal is expected or when the Rx should be enabled.

author: voltsandjolts
author_rank: Unknown
timestamp: October 25, 2024, 11:24:50 am
content: Just to make sure the OP is clear on this: the transmitter is not continuous, you need to transmit 'bursts' of the carrier, each perhaps a dozen cycles or so. Perhaps a few dozen bursts per second. The receiver is only enabled after a transmit burst is completed, then disabled before the next transmit burst.

author: Paptrack
author_rank: Contributor
timestamp: October 25, 2024, 11:32:00 am
content: Please take into account that MD0100 is a high-voltage, two-terminal protection device. It hasn't got an enable-disable pin. The MD0100 can be considered as a normally closed switch and when the voltage drop across the two terminals exceeds a nominal value of ±2V, i.e. when ±100V applied, the device turns off.Thanks.

author: voltsandjolts
author_rank: Unknown
timestamp: October 25, 2024, 11:38:38 am
content: At 20MHz it's probably simpler to control a switching element 'manually' rather than try to find an 'automatic' one like this which is fast enough.

author: moffy
author_rank: Super Contributor
timestamp: October 25, 2024, 11:50:34 am
content: OP you are quite right, no enable or On/Off signal, it is automatic. Page 9 of the datasheet shows a +/-90V signal at 5MHz, I think the fast transitions of your signal should prevent it switching On during a transmit but you would have to test it and see, just to be sure. Sorry about the misunderstanding.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 25, 2024, 02:02:42 pm
content: It's unclear from the datasheet what the usable frequency range is. Figure 3-4 in the datasheet shows that it definitely reacts within a few nanoseconds (i.e., the output voltage starts to drop from its peak), but in that case it takes 11ns to reduce to 10% of the peak. That is with a drive voltage of 10V, and it's unclear how this waveform changes as this voltage increases.As frequency increases, more TX power will be dissipated by both the MD0100 and the receiver. Whether it's enough depends on details of your application.

author: Paptrack
author_rank: Contributor
timestamp: October 28, 2024, 08:39:51 am
content: Even in figure 3.10 and 3.11 in the datasheet shows that ±90V@5MHz is feasible. Making zoom in figure 3.11 I would say that it takes a bit less than 20ns. However, working at 20MHz will be unknown.