Now i need a high-power T-R switch that can switch state in two microseconds. Drive outputs: +12 volts(current ,0.5 ampers) , +700 volts , -12volts(-0.5 ampers), -12 volts. Tx path may stand 600 watts (peak). I want to use PIN driver IC,but don't find IC that may output
+700 volt. So i have to design the switch by myself. I am going to employ discrete MOSFET as driver . Input to the MOSFET is TTL logic. some specification of my switch is : working frequency band : 3 - 30 MHz T - R isolation : > -80 dB (when transmitting)
Anyone ever do this? Can i get so fast speed(< 2 microseconds)? Would you give me some suggestions? Any help will be appreciated!
What's the carrier lifetime of the PIN switches you'll be using? That probably has more to do with how fast you'll be able to switch between T and R than the FETs you use. What carrier lifetime do you need to do a good job with your 600W at 3MHz? But I'd also like to know why you think you need to switch so quickly for that frequency band, since the transmission modes allowed are not particularly broadband. Also-- why do you need so much isolation? 50dB gets you below 10mW, and I'd consider it a pretty poor receiver front end that couldn't handle that without damage. 80dB attenuation won't be as easy over a decade of frequencies as it would if you were dealing with a single frequency or a narrow band, though you can add however many stages of low power PIN or simple diode switches or other RF switches to get to whatever attenuation you need, after your high power PIN switch. Note that 10 amps in an FET will slew 1000pF 10 volts per nanosecond...
Have you looked for ap notes on the web sites of manufacturers of high power PIN diodes?
(Perhaps your use is something like ultrasound, in which case you might want something faster than 2usec, but the 3-30MHz is suspiciously like a spec for HF over-the-air transmissions...)
The carrier lifetime of PIN diode i use is 10microseconds(Typ.) . Part number is UM4010,manufactured by Microsemi Corp. I have read some ap notes of PIN Diodes, and got to know that switching speed of my antenna switch is not only related to carrier lifetime of PIN diodes ,but also related to driver . A good driver may get faster switching speed:may up to one fifth or one tenth of carrier lifetime(please refer to great book "Semiconductor control',writen by White J F. ,1977) . In order to switch the RF signal(3 - 30 MHz) with less distortion, accroding to calculation , the carrier lifetime must longer than 8 microseconds, So i choose UM4010. The reason of switch speed < 2 microseconds is that transmitting signal will reach receving antenna in 2 microseconds. I agree with you , it is not easy to gain 80 dB isolation over a decade of frequencies. That high isolation may improve the recept signal to noise ratio =A3=ACavoid Saturation of recept preamplifer. I am going to take CPW technology to improve isolation besides care of designing bias circuits. Yes , My switch is used as a key assembly of ionospheric sounding system (HF band) .
So the receiving antenna and transmitting antenna are separate, and about 2000 feet apart?? If that is the case, why do you need a TR switch? I trust that the transmitter is not so high power that the receiving antenna is trying to deliver 600 watts to the receiver...and even if it were, you wouldn't need to reverse-bias the PIN by several hundred volts. Maybe I'm reading too much into what you wrote.
In any event, it seems like it should be reasonably easy to switch ~700V in well under a microsecond. It's not so different from what switching power supplies do using half-bridge configurations, and in your case, you mainly need to supply the current to charge/discharge capacitance, and the DC current to bias the PIN "on" is modest. For example, see the data sheet for the Fairchild FQH8N100C 1kV 8A power mosfet; switching times listed (rise, fall, delays) are in the range of a few tens of nanoseconds up to 250nsec max.
I am sorry ,because i express unclear. in my system ,Transmitter and receiver share one antenna. Accounting for RF choke in DC bias path, A low path filter must be used. thus in order to get faster switch wave rising edge ,we have to decrease time constant of circuit . The filter's cutoff frequency is closer to 3MHz, better rising edge or falling edge of switch wave we achieve. Do you agree with me? As to FQH8N100C you mentioned , i am reading it. Thank you for your advice!
Yes, I would certainly agree that proper filtering to separate the PIN bias from the RF is a good idea. The faster you need to switch, the more critical that becomes. It should be an advantage to consider not only a low-pass filter for the bias but a high pass filter for the RF-- or other methods to reduce the capacitance that the PIN switch must drive. Of course, be careful about the return path for the current that charges the capacitance especially on the receiver side, so that you do not expose the receiver input to excessive current. A power mosfet that switches several hundred volts in a few tens of nanoseconds will be generating quite a bit of available energy to a 50 ohm 3-30MHz input! If there exists between the fast PIN driver circuit and the PIN diodes a low pass filter, and also between the PIN switches and the receiver input (or transmitter output) a high pass filter, and the filter passbands do not overlap, then there should be decent isolation to protect the receiver input. I can tell you that I also use PIN diode clamps on the input to a receiver I recently put into production, and they are able to absorb transient energy quite nicely. They are just diodes normally reverse-biased by several volts. In a switched system, I would consider driving them actively, using differential drive so that there is no (or very low) transient on the RF input that they shunt to ground when they are "on." If the diodes are matched (as they are in a packaged pair, such as Avago HSMP-3822) and you drive them to the "off" state symmetrically, they should inject very low pulse energy into the line they are switching. The data sheet for the HSMP-3822 has several ideas for getting very good isolation on the receiver side, once you've gotten isolation from the high power with a high voltage PIN diode, and there are other PIN diode ap notes on the Avago site.
Maybe, maybe not. Frequencies up to 150 MHz are already common for instruments. 25 years ago that was a bit fancy lab ware, at the same time 320 MHz was experimental ultrasound. I think experimental in now around 3 GHz for ultrasound, AFAIK it is primarily used for microcrack detection in and near metal welds.
We work with 3GHz and up to 30GHz all the time, and we buy and use pre-made pin diode setups. Great for shunting power in rf networks in test and simulation situations.
One of my first projects in grad school, 25 years ago, was an acoustic microscope running at 2 GHz. (I didn't invent it or even build the transducer--I just did the electronics and mechanics.) The guys down the hall were building a 100-GHz acoustic microscope that had to work in superfluid helium. I don't think they ever got it working.
Hmm... perhaps they're working on the 100mpg car X prize by now. :-)
100GHz in the early '80s... definitely pure research, I would think (e.g., you work your best at it, but you don't really expect you'll be able to make it work...).
Hi,Tom Thank you for your reply in my news group"how to design PIN diode driver - high voltage and high speed ?"! I got so much in your answer! In my Antenna switch, in order to avoid input overload of my receiver, a power limiter will be utilized. In your reply , you mentioned that you also use PIN diode clamps on the input to your receiver . My question is , is it able to absorb transient voltage >1 mV (the maximum value of receiver's pre-amp is 1mV )? According your suggestion , i am going to use HSMP-3822 diode pair with 5 volts forward bias(when transmitting ),-5 volts reverse bias(when receiving), on the input of my receiver. Do you think the power limiter plan can absorb the transient energy above 1 mV for the receiver during both the receiving state "ON" and "OFF"? Any suggestions will be appreciated!
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