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RF interference filtering...help!

P

Pico

Jan 1, 1970
0
I have a 40Mhz transmitter and a 433MHz receiver with their antennas about
4" apart.
When the 433 rf input signal is weak the 40MHz is swamping it.
I can't move the devices apart, they are on seperate power supplies and
there is no electrical connection between them.
The pcb's follow convention for immunity (i.e. mostly ground planed) but
it's not enough.
The troublesome RF is entering the receiver circuitry via the antenna, not
the power supply or i/o connections.
I cannot alter anything on the 40 MHz transmitter for legal reasons -
affecting it's certification.
I was wondering whether I could build a highpass filter of some kind on the
receiver antenna input but don't really know the theory.
Anybody please?
Thanks
Wilbur
 
A

Ancient_Hacker

Jan 1, 1970
0
Pico wrote:

I was wondering whether I could build a highpass filter of some kind on the
receiver antenna input but don't really know the theory.
Anybody please?
Thanks
Wilbur

Yep, you need a HPF. Probably at least a 5-node one. See some filter
design applet, such as this really good one:

http://www-users.cs.york.ac.uk/~fisher/lcfilter/


At those frequencies you may get unrealistic component values, that
means you may need to use the impedance tansform options to get
reasonable values.

Another option is to BUY an already designed filter, they're not too
expensive, see www.digikey.com, search for "LC filter".
 
J

Joerg

Jan 1, 1970
0
Ancient_Hacker said:
Pico wrote:





Yep, you need a HPF. Probably at least a 5-node one. See some filter
design applet, such as this really good one:

http://www-users.cs.york.ac.uk/~fisher/lcfilter/

Unfortunately that won't help if it's harmonics or spurious signals in
the 430MHz range generated by the transmitter. The OP mentioned that he
can't change that unit. If it is one of those pre-cert deals that would
include the antenna so you can't (legally) connect a lowpass filter
between transmitter and antenna.
 
A

Ancient_Hacker

Jan 1, 1970
0
Joerg said:
Unfortunately that won't help if it's harmonics or spurious signals in
the 430MHz range generated by the transmitter.

True, you can't do anything there, but it's much more likely its due to
the large transmitter signal getting into the front end and getting
cross-modded there. Since he can't do anything on the transmitter end,
we might as well try to improve the area we can modify.
 
M

Michael A. Terrell

Jan 1, 1970
0
Ancient_Hacker said:
True, you can't do anything there, but it's much more likely its due to
the large transmitter signal getting into the front end and getting
cross-modded there. Since he can't do anything on the transmitter end,
we might as well try to improve the area we can modify.


Then a tuned trap on the transmit frequency would do more good than a
HP filter.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
J

Joerg

Jan 1, 1970
0
Ancient_Hacker said:
True, you can't do anything there, but it's much more likely its due to
the large transmitter signal getting into the front end and getting
cross-modded there. Since he can't do anything on the transmitter end,
we might as well try to improve the area we can modify.

Possibly. Also, it is a good policy to look at all the connections.
Dissimilar metals can caused enough non-linearity to create
intermodulation. And back to back diodes to GND to protect an input
transistor are an absolute no-no here.

Michael's suggestion of providing a trap is a very good one.
 
R

Rich Grise

Jan 1, 1970
0
Unfortunately that won't help if it's harmonics or spurious signals in the
430MHz range generated by the transmitter. The OP mentioned that he can't
change that unit. If it is one of those pre-cert deals that would include
the antenna so you can't (legally) connect a lowpass filter between
transmitter and antenna.

If the device is supposed to be FCC certified, and it's causing spurious
emissions at its tenth and eleventh harmonics and so, then he has a bone
to pick with the manufacturer of the device.

If the 40 MHz is just swamping the receiver, then a HPF, but I have
nothing to offer as to the design of a filter at those kinds of
frequencies. One thing that springs to mind is a parallel resonant LC
at 40 MHz, in series with the antenna lead, but for more than that,
you'll have to see a VHF guy. :)

Good Luck!
Rich
 
J

Joerg

Jan 1, 1970
0
Hello Rich,
If the device is supposed to be FCC certified, and it's causing spurious
emissions at its tenth and eleventh harmonics and so, then he has a bone
to pick with the manufacturer of the device.

Not necessarily. Even the smallest spec of noise will show up if you
have the antennas right in the face of each other. Yet the emissions can
still be below legal limits.

If the 40 MHz is just swamping the receiver, then a HPF, but I have
nothing to offer as to the design of a filter at those kinds of
frequencies. One thing that springs to mind is a parallel resonant LC
at 40 MHz, in series with the antenna lead, but for more than that,
you'll have to see a VHF guy. :)

Better see a UHF guy :)

The best approach is usually a nice sharp bandpass for the frequency you
are receiving at, not trying to muffle other noises one by one. Even
when you manage to get it quiet, next thing you know they put a cell
tower right next to it. This happened to a client. Or the Fedex
freighter comes lumbering in and there is a lengthy chat with the tower.
Happened to me after they re-routed Mather field approach.
 
M

Michael A. Terrell

Jan 1, 1970
0
Joerg said:
Hello Rich,


Not necessarily. Even the smallest spec of noise will show up if you
have the antennas right in the face of each other. Yet the emissions can
still be below legal limits.


Better see a UHF guy :)

The best approach is usually a nice sharp bandpass for the frequency you
are receiving at, not trying to muffle other noises one by one. Even
when you manage to get it quiet, next thing you know they put a cell
tower right next to it. This happened to a client. Or the Fedex
freighter comes lumbering in and there is a lengthy chat with the tower.
Happened to me after they re-routed Mather field approach.


The WACX TV transmitter site in Orange City has several unusable
C-band earth stations. Right after they were built, AT&T built a
microwave c-band relay right over the station, wiping out all 24
channels by being on the frequencies we needed to receive. We ended
having to use the old pair of microwave links from the original studio
site and a dial up DTMF decoder to select the desired receiver and feed
it to the old transmitter site on 11 GHz (CARS band) then another hop at
7 GHz (STL band) to the new site. The only thing we had to purchase was
the DTMF decoder, but every time they had to change the channel or bird,
required a long distance call.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
P

Pico

Jan 1, 1970
0
Thanks for the guidance everyone.
Looks like the problems solved...put a high pass LC on the 433 front end
(10pf and 22nH) which killed the 40 stone dead but left the 433 apparently
unnaffected.
Can now put the antennas almost together before there is any problem.
The simulation also says I have a 1.75dB signal gain at 433 so this really
is a result!
Pico
 
J

Joerg

Jan 1, 1970
0
Hello Pico,

Thanks for the guidance everyone.
Looks like the problems solved...put a high pass LC on the 433 front end
(10pf and 22nH) which killed the 40 stone dead but left the 433 apparently
unnaffected.
Can now put the antennas almost together before there is any problem.
The simulation also says I have a 1.75dB signal gain at 433 so this really
is a result!


Great, but I'd still consider a proper bandpass. It might work now but
what if someone with a walkie-talkie passes by or the cell phone in his
pocket rings?
 
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