### Author Topic: PIN Diode Questions  (Read 799 times)

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#### rheb1026

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##### PIN Diode Questions
« on: March 07, 2018, 04:09:21 am »
Not sure if this belongs in the beginner or ham radio/rf area...

Is there a way to roughly estimate/calculate the parallel resistance when the PIN diode is reverse biased? I'm looking at the NXP BAP64-02, and the value isn't given in the datasheet.

Another question: when using a PIN diode in the receive path of a moderate (10W) power amplifier, is there a standard way to reverse bias this diode to keep the reverse voltage higher than the RF voltage? Is a boost converter the only way?

#### rfeecs

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##### Re: PIN Diode Questions
« Reply #1 on: March 07, 2018, 04:46:42 am »
Not sure if this belongs in the beginner or ham radio/rf area...

Is there a way to roughly estimate/calculate the parallel resistance when the PIN diode is reverse biased? I'm looking at the NXP BAP64-02, and the value isn't given in the datasheet.

It's going to be negligible.  Reverse biased it will just look like a capacitor.  You can roughly estimate from the datasheet reverse current is a maximum of 1uA at 20V, so more than 20 MOhm.

Another question: when using a PIN diode in the receive path of a moderate (10W) power amplifier, is there a standard way to reverse bias this diode to keep the reverse voltage higher than the RF voltage? Is a boost converter the only way?

Not sure what you're asking.  Receive path?? 10W at 50 ohms is about 32V peak, so you need to reverse bias more than that.

#### rheb1026

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##### Re: PIN Diode Questions
« Reply #2 on: March 07, 2018, 05:00:45 am »
Sorry, should clarify: I plan to use PIN diodes for a transmit/receive switch at the antenna end. The power amplifier will put out 8-10W.

I've attached a schematic of my intended circuit to help understand better. Note that the PIN diode bias voltage is not sufficient, I just copied and pasted from an identical circuit in a low signal level path

I was wondering if there is any way around using a boost converter of some sort to generate the bias for these diodes? The only other option I can think of is to use a relay instead to do the switching. The voltages available will be +12-14V and +5V.

My concern is with the noise generated by a boost converter and keeping it out of the receive path.

#### rfeecs

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##### Re: PIN Diode Questions
« Reply #3 on: March 07, 2018, 05:56:59 am »
I don't know an easy way around it.  If you could switch off the output of the amplifier and possibly switch off the boost converter while you receive?

There are MMIC TR switches out there that can handle 10W and only require a 5V or even 3V power supply.  A quick Google search yields Mini-Circuits VSW2-33-10W as one possibility:
https://www.minicircuits.com/WebStore/dashboard.html?model=VSW2-33-10W%2B

#### T3sl4co1l

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##### Re: PIN Diode Questions
« Reply #4 on: March 07, 2018, 06:27:52 am »
You can possibly use the RF to self-rectify... but you need to accept that this will introduce distortion.

Tim
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#### rheb1026

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##### Re: PIN Diode Questions
« Reply #5 on: March 07, 2018, 09:31:26 am »
I don't know an easy way around it.  If you could switch off the output of the amplifier and possibly switch off the boost converter while you receive?

There are MMIC TR switches out there that can handle 10W and only require a 5V or even 3V power supply.  A quick Google search yields Mini-Circuits VSW2-33-10W as one possibility:
https://www.minicircuits.com/WebStore/dashboard.html?model=VSW2-33-10W%2B

Thanks, I put in a sample order for a few of those. I'll have to look and see about boost converters with a shutdown pin

I should probably just start a project thread since I'm sure I'll have more questions

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##### Re: PIN Diode Questions
« Reply #6 on: March 07, 2018, 10:23:13 am »
variable or fixed frequency?

#### rheb1026

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##### Re: PIN Diode Questions
« Reply #7 on: March 07, 2018, 10:26:56 am »
Mostly fixed... It should operate from 144-146 MHz, so fairly small bandwidth

#### rfengg

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##### Re: PIN Diode Questions
« Reply #8 on: March 07, 2018, 10:29:46 am »
"Another question: when using a PIN diode in the receive path of a moderate (10W) power amplifier, is there a standard way to reverse bias this diode to keep the reverse voltage higher than the RF voltage? Is a boost converter the only way?"

You do not strictly need a higher reverse voltage from a DC supply to bias it higher than the peak RF voltage due to the self bias developed across the PIN diode.........go thru this seminal paper by Prof Caverly.......it guides you on how to go about this.
Let me know if you need help with the calculations.

https://cdn.macom.com/applicationnotes/AN3022.pdf

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#### G0HZU

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##### Re: PIN Diode Questions
« Reply #9 on: March 07, 2018, 01:08:03 pm »
Forget all the above and revisit your circuit design and all of these problems will go away if you just make a simple narrowband (classic) diode TR switch. If you place a (lumped) quarter wave section between the Tx diode and the Rx diode the Rx diode can be a shunt ON diode to act as a simple crowbar in Tx mode. The Tx signal sees a 90degrees phase shift through the quarter wave section before it reaches the Rx crowbar diode, then a 180degree phase shift occurs in the reflection from the shorting crowbar followed by another 90degrees on the way back (=360 degrees total) so the receive path looks like an open circuit to the Tx path in Tx mode. So you get high isolation from Tx to Rx in Tx mode. In Rx mode just reverse bias the Tx diode and the Rx diode and the antenna signal will then be routed to the receiver input stage. Both diodes need to be ON in Tx mode and both need to be OFF in Rx mode.

This narrowband TR switch topology has been around for decades, it gets used in CB radios and ham radios and it means you don't need to have a high reverse bias for the Rx diode. Some CB radios just use a pair of limiter/clamp diodes for the Rx crowbar and that saves having any bias in the receive diode(s) but that is to save cost and circuit complexity.

See below for a barebones version of this type of switch. This is a simplified version to help show the basic building blocks and it only has a simple common bias control. It's up to you how you want to add complexity to improve the biasing as you might want to have extra reverse bias voltage  in the diodes in Rx mode. You don't have to use a transmission line for the quarter wave section, you could just use a lumped LPF equivalent made from an LC Pi section(s).
« Last Edit: March 07, 2018, 01:29:11 pm by G0HZU »

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##### Re: PIN Diode Questions
« Reply #10 on: March 07, 2018, 02:04:10 pm »
Mostly fixed... It should operate from 144-146 MHz, so fairly small bandwidth

might be able to used something really simple like this often used for NMR

https://my.vanderbilt.edu/mrengineering2014/files/2014/03/031914_blog-4-fig-3.jpg

#### rfengg

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##### Re: PIN Diode Questions
« Reply #11 on: March 07, 2018, 02:38:16 pm »
A word of caution on advice regarding unbiased limiter diodes.......there is a finite delay before the incident RF turns the limiters ON and during this delay the spike leakage can very well destroy sensitive front ends / preamps.
The advantage of using biased PIN diodes is that you can allways switch in the dc bias before the Tx RF is applied.

#### rheb1026

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##### Re: PIN Diode Questions
« Reply #12 on: March 08, 2018, 01:52:43 am »
Thanks very much for all the replies! I had forgotten about the 1/4 wave TL, I think that's definitely the way to go for this.

I'm going to start a project thread when I get the time to write everything out, but this will be used in a transverter in the end

"Another question: when using a PIN diode in the receive path of a moderate (10W) power amplifier, is there a standard way to reverse bias this diode to keep the reverse voltage higher than the RF voltage? Is a boost converter the only way?"

You do not strictly need a higher reverse voltage from a DC supply to bias it higher than the peak RF voltage due to the self bias developed across the PIN diode.........go thru this seminal paper by Prof Caverly.......it guides you on how to go about this.
Let me know if you need help with the calculations.

https://cdn.macom.com/applicationnotes/AN3022.pdf

Thanks, I hadn't seen this app note before. I gave it a quick read through, but will have to sit down and take more time to go over it. I appreciate the offer

#### w2aew

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##### Re: PIN Diode Questions
« Reply #13 on: March 08, 2018, 08:15:44 am »
Forget all the above and revisit your circuit design and all of these problems will go away if you just make a simple narrowband (classic) diode TR switch. If you place a (lumped) quarter wave section between the Tx diode and the Rx diode the Rx diode can be a shunt ON diode to act as a simple crowbar in Tx mode. The Tx signal sees a 90degrees phase shift through the quarter wave section before it reaches the Rx crowbar diode, then a 180degree phase shift occurs in the reflection from the shorting crowbar followed by another 90degrees on the way back (=360 degrees total) so the receive path looks like an open circuit to the Tx path in Tx mode. So you get high isolation from Tx to Rx in Tx mode. In Rx mode just reverse bias the Tx diode and the Rx diode and the antenna signal will then be routed to the receiver input stage. Both diodes need to be ON in Tx mode and both need to be OFF in Rx mode.

This narrowband TR switch topology has been around for decades, it gets used in CB radios and ham radios and it means you don't need to have a high reverse bias for the Rx diode. Some CB radios just use a pair of limiter/clamp diodes for the Rx crowbar and that saves having any bias in the receive diode(s) but that is to save cost and circuit complexity.

See below for a barebones version of this type of switch. This is a simplified version to help show the basic building blocks and it only has a simple common bias control. It's up to you how you want to add complexity to improve the biasing as you might want to have extra reverse bias voltage  in the diodes in Rx mode. You don't have to use a transmission line for the quarter wave section, you could just use a lumped LPF equivalent made from an LC Pi section(s).

I did a video on the lumped quarter wave T/R switch, if you're interested:

======================================
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#### rheb1026

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##### Re: PIN Diode Questions
« Reply #14 on: March 08, 2018, 08:35:04 am »
Thanks, Alan! I'll have to watch tonight

Love your videos, by the way. Keep up the good work

#### rfengg

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##### Re: PIN Diode Questions
« Reply #15 on: March 08, 2018, 09:58:20 am »
You ARE the best Alan.......all your videos are concise and very well explained.
The contribution you have made thru your videos is beyond measure......keep them coming

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#### LA7SJA

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##### Re: PIN Diode Questions
« Reply #16 on: March 08, 2018, 07:22:11 pm »
Steve Jobs " There’s An App For That™"

w2aew "I did a video on that™"