Looking for RF New Group

Jan 17, 2007 5 Replies
J

Hi All,



I have a RF questions and I am also seeking suggestions on a RF News Group.



Questions



------------ I have a 900MHz Spread Spectrum Tx 4watts (900~928Mz band) ... and I looking for a low pass filter box to attenuate possible harmonics... (50R IN coax connector, 50R OUT coax connector).



At this stage... I am looking for 10dB attenuation at 2.4GHz and more attenuation at 5GHz, 10GHz and 24GHz, with minimal loss in the pass band (900~928MHz).



Do you have any suggested LPF suppliers?



Is this LPF a common request?



Are there any other NG's I should try.


Thanks in Advance and apologies if my post is OT.


Joe


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I

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do various lpf's, but you will likely have trouble finding one from anyone spec'd at 24GHz.

Regards Ian

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M

Here's a 928MHz lpf .1dB ripple 9-element chebychev you can try building yourself. or order a custom made one from K&L filters and pay the big bucks...

Simulate in LTspice...

*928MHz .1dB CHEBYSHEV LPF v1 1 0 ac 1 sin R1 1 0 50 C1 1 0 4.1pF L1 1 2 1.23E-08H C2 2 0 7.32pF L2 2 3 1.38E-08H C3 3 0 7.56pF L3 3 4 1.38E-08H C4 4 0 7.32pF L4 4 5 1.23E-08H C5 5 0 4.1pF R2 5 0 50 .ac lin 10k 1e6 24e9 .plot ac v(5) .end
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J

A lumped filter like this can make a decent low-pass filter with a 928MHz but -- even with good air coils -- will fall apart *well* before 24GHz... not that this might not still work for him, since he didn't tell us how much attenuation he's looking for out at 24GHz (other than "more than 10dB") -- just that he should be aware that the LTspice 24GHz will be a fiction.

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M

Yep, I was assuming he'd go the K&L Microwave filter route... he'll probably have more problems with low frequency junk than the higher ghz crap, maybe a highpass is needed too... How about a nice bandpass filter like this one...

bandpass filter V1 1 0 AC 1 C1 1 2 0.75PF L1 2 0 15nH C2 2 0 1.16PF C3 2 3 0.14PF L2 3 0 15nH C4 3 0 1.77PF C5 3 4 0.11PF L3 4 0 15nH C6 4 0 1.8PF C7 4 5 0.11PF L4 5 0 15nH C8 5 0 1.77PF C9 5 6 0.14PF L5 6 0 15nH C10 6 0 1.16PF C11 6 7 0.75PF .net V(7) V1 Rin=50 Rout=50 .ac lin 1000 50e6 2.4e9 .print S21(V1) .end

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J

Thanks for your comments guys.

My 900Mz Tx at 4watts causes interference to a microwave PIR (which operates in the GHz range). But I don't have the PIR freq spec's. Distance between the PIR and Tx is 3~4metres.

I am thinking the PIR spec's are between 2.4GHz and 24GHz , more likely to be 10GHz.

Fact - I do know truning down the 900MHz Tx to 3watts - there is no interference detected.

I am thinking one of 2 probable causes.

a. A spurious or harmonic in the GHz range is causing the interference. Action - use a LPF in the 900MHz Tz

So 3 or 4dB loss qat 2.4GHz might be enough.... and assuming the same or additonal loss right through to 24GHz

b. 900MHz is bleading through the electronic circuits in the PIR Action - Tell the PIR guys to decouple and bypass sensitive circuitry in the PIR (PIR guys might not want to talk to me)

Cover the PIR with Aluminium foil and poke a tiny hole for the mirowaves to get out. See if that fixes it.

JG

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