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BSEE - KB3UON
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500 MHz RF choke, DC block and bypass capacitor selection - RF Cafe
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Below are the old forum threads, including responses to the original posts.
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Post subject: 500 MHz RF choke, DC block and bypass capacitor
selection Posted: Sun May 13, 2007 6:14 pm
Joined: Tue Apr 03, 2007 12:52 pm
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.
Post subject: Posted: Mon May 14, 2007
Joined: Thu May 03, 2007 2:55 am
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
Post subject: Posted: Mon May 14,
2007 1:32 pm
Joined: Mon Jun 27, 2005
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.
Post subject: Posted:
Tue May 15, 2007 12:43 am
Joined: Tue Apr
03, 2007 12:52 pm
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...
Post subject: Posted: Tue May 15, 2007 3:45 pm
Joined: Mon Jun 27, 2005 2:02 pm
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.
following series should fit your needs from SFR point of view:
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