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500 MHz RF choke, DC block and bypass capacitor selection - RF Cafe Forums

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jayleung
Post subject: 500 MHz RF choke, DC block and bypass capacitor selection Posted: Sun May 13, 2007 6:14 pm

Lieutenant

Joined: Tue Apr 03, 2007 12:52 pm
Posts: 3
Hi, everyone. I have a question about a project working on around 500 MHz.

I am wondering what kind of RF choke will be suitable for this frequency range. It seems that most of the inductor-type choke Self Resonant Frequency is lower than 500MHz, and I am not sure whether I should use inductor as choke any more in this frequency range.

And the other question is that for the DC-block capacitor and RF by-pass (shunt to ground) capacitor, is 220 pF large enough?

Thanks in advance.


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melkhouly
Post subject: Posted: Mon May 14, 2007 8:35 am

Captain

Joined: Thu May 03, 2007 2:55 am
Posts: 8
Location: Egypt
u can use the SMT inductors in this , but be sure form SRF , and what will be the reactive part in this frequency
about the blocking capacitor in 900 MHz the 27 to 100 pF works well , u can use the 100 to 200 Pf in the 500 MHz range

_________________
Khouly


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IR
Post subject: Posted: Mon May 14, 2007 1:32 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi jayleung,

As a general rule of thumb, the reactance of the RF choke should be 10 times of the characteristic impedance i.e. 500 ohm and this is for the lowest frequency of your operating BW. The RF choke should also have an SRF which is abvoe your operating frequency. A reasonable value for 500MHz would be 150nH.

DC block capacitors should have a reactacne of 3-5 ohms at the lowest operating frequency (Since the reactance is getting smaller as frequncy increases). So a reasonable value for 500MHz would be 100pF.


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jayleung
Post subject: Posted: Tue May 15, 2007 12:43 am

Lieutenant

Joined: Tue Apr 03, 2007 12:52 pm
Posts: 3
Hi, IR, thanks for your reply.

But since I am going to use the RF choke to feed the DC control voltage to a varactor with only a few pF capacitance, I found that with a couple hundred nH inductance will degraded the varactor capacitance to about 20% on more.

That's why I am searching for a large inductance with a few uH. But it seems the trade-off is the SRF will drop below 500MHz...

I am not quite sure what kind of RF choke is used for commercial varactor tunning in UHF range. If you have any information about that, please tell me...


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IR
Post subject: Posted: Tue May 15, 2007 3:45 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi,

From the very few designs that I have seen by others - the type of inductors used for the tuning voltage of the varactor is chip inductors like the 0603 of coilcraft or similar.

The following series should fit your needs from SFR point of view:

http://www.coilcraft.com/0603ls.cfm

The function of the inductor is to prevent the RF frequency to reach the tuning voltage. In my previous post, the rule of thumb I mentioned is commonly used for amplifiers, but it should apply (I think) also to the particular use of the inductor for the tuning voltage of the varactor.

Hope this helps! Please update us.







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