BROAD BAND DC BLOCKING CAPACITORS - RF Cafe Forums

 Post subject: BROAD BAND DC BLOCKING CAPACITORS

Posted: Mon Nov 24, 2008 8:28 am 

Hi!

I have an amplifier whose data-sheet says that it can be used from DC to 8 GHz. It also mentions that lower frequency limit is decided by the external DC blocking capacitors. I found many broadband DC blocks of 100pF that can operate from 10 MHz to 20 MHz, however to have things work from DC or say 100 KHz (important for my System), I had to go to some higher value of Capacitance 10nF or more. However I find that the SRF of the best 10 nF Capacitor (That I came across) was few MHz.

Has anyone come across a capacitor that can run from DC or 100 KHz to 8 GHz.

I am using/looking for Surface mount capacitors

_________________

Ashish Bondia,

Design Engineer- RF

 Post subject:

Posted: Mon Nov 24, 2008 1:34 pm 

Hi,

I responded to one of your questions recently. I said to consider the SRF of your blocking cap. However, SRF is not your limiting factor for a blocking cap. Impedance is what is important.

Keep the size of your cap is close to the width of transmission line. I have used 10nF caps up to 6GHz with no problems.

 Post subject:

Posted: Tue Nov 25, 2008 10:07 am 

Hi!

Thanks for your comments.

Upper frequency limit is not what is troubling me. I need capacitors that work from DC (or very low Frequencies) to 500 MHz. My idea was that normally till SRF the impedance decreases and after SRF the impedance increases with frequency. Since I am working with broad band signal, any abrupt impedance change in middle of the band would change the shape of my spectrum (pulse in time domain). I think I have to ask our DSP team if they have something to take care of this.

_________________

Ashish Bondia,

Design Engineer- RF

 Post subject:

Posted: Tue Nov 25, 2008 11:31 am 

It ssems a little excessive to involve DSP guys in a cascaded gain block issue. I avoid the creepy digital guys whenever possible. Asking them to fix an RF problem is like asking a baboon to do your finances.

 Post subject:

Posted: Thu Dec 04, 2008 3:09 am 

If I go by your reasoning the I believe that SRF should also not be the limiting factor for the RF Choke/inductor used in Bias Tees for Biasing amplifier. There also i should consider the impedance.

recently I came across a part whose data sheet says that it works from DC to 1 GHz. When I did simulations using the S-parameter file for the part, I found that the Low frequency performance was not as given in the data-sheet. When contacted the support engineers of the part vendor informed that the measurement was taken using Broad band Bias Tees at the output port. They suggested that to get the desired performance I can use a 300uH Biasing Inductor with SRF >300 MHz. I spent considerable time searching for a surface mountable inductor with that high SRF for a 300uH inductor. The best I got was 7 MHz. Although Simulations using the S-parameter files showed not much degradation in performance beyond 7 MHz, I don't know how the measured result would be

_________________

Ashish Bondia,

Design Engineer- RF

 Post subject:

Posted: Tue Jan 13, 2009 1:00 pm 

For broadband bias chokes (SMT) try with Conical inductors, such as BCL's series, from Coilcraft, wich are SRF free up to 40GHz and could be a bit bulky and expensive (>20u$d)

Concerning to Broadband DC blocks, companies like ATCeramics and DLI make them, parts such as 545L, 530L (ATC) and CO8BLBB1X5UX (DLI) would do fine I guess

 Post subject:

Posted: Wed Jan 14, 2009 4:56 am 

Hi fernetbranca,

I am using CO8BLBB1X5UX (DLI) for Broad-band DC block.

For Inductor I am still searching for a 300 uH inductor with SRF >300 MHz.

The BCL/BCS series fro Coil craft is characterized above 50 MHz and m,aximum inductyor value available is 8 uH.

_________________

Ashish Bondia,

Design Engineer- RF

 Post subject:

Posted: Wed Jan 14, 2009 2:36 pm 

Hi Ashishbondia,

I don't think you need such a big inductor for a choke. The choke should be 3 or 4 times Zo at your lowest frequency. So about 200ohms at 50MHz, which is about 500nH or something.

It may even be much smaller depending on the bias resistor value.

I think a 300uH may cause stability issues.

Let me know if I am missing something.

Rod

 Post subject:

Posted: Thu Jan 15, 2009 4:39 am 

Hi Yendori,

You are correct in your reasoning that 500nH will be sufficient for 50 MHz. however for our application the signal has substantial energy at frequency of few 100 KHz to 250 MHz. Hence the part vendor recommended us to use 300 uH inductor. As for the stability issue I am using a resistor in series with the inductor in the Bias line.

Please let me know if I there is any issue with my choice.

_________________

Ashish Bondia,

Design Engineer- RF

 Post subject:

Posted: Thu Jan 15, 2009 3:21 pm 

Hi,

You are right. I have no idea where I got 50MHz from...getting old I guess.

It sounds like you are on the right track.

Rod

 Post subject: Re: BROAD BAND DC BLOCKING CAPACITORS

Posted: Wed Apr 08, 2009 6:56 pm 

I find it hard to believe that a 10 nF cap resonated at a "couple of MHz". I use 0.1 uF chip caps routinely up to 6 GHz. Many will not give you any trouble, some might--depending on the dimensions and the dielectric constant used.

As far as the inductor goes, if it is a low current bias line, just use a microwave inductor in series with a resistor, say 500 ohms or so. If that is not going to work, you want to use a broadband "cone inductor".

https://www.coilcraft.com/bcl-s.cfm

After this, if you need more isolation, I would put a hundred pf to ground and THEN follow up with a big choke.

_________________

Rich

Maguffin Microwave Consulting

www.MaguffinMicrowave.com

 Post subject: Re: BROAD BAND DC BLOCKING CAPACITORS

Posted: Mon Sep 20, 2010 1:03 am 

You can try using a DC Block, you can search for DC Blocks by specification at https://www.everythingrf.com/Search-RF-Components/DC-Blocks/

 Post subject: Re: BROAD BAND DC BLOCKING CAPACITORS

Posted: Wed Sep 22, 2010 9:14 am 

Try these wideband DC blocks from ATC. They go to 20 GHz

https://www.atceramics.com/pdf/technotes ... eature.pdf

 Post subject: Re: BROAD BAND DC BLOCKING CAPACITORS

Posted: Sat Oct 09, 2010 4:50 pm 

I use Dielectric Lab capacitors in my broadband sma dc blocks, 10khz-18ghz. They're actually 2 caps in parallel; a large cap for low frequencies, & a small cap for the higher frequencies where the large cap self resonates. ATC also makes equivalent caps. You can see the specs of the sma dc blocks at https://www.fairviewmicrowave.com/coaxial_dc_blocks.htm

Bob Hawkins

Fairview Microwave

https://www.fairviewmicrowave.com