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Low Pass Filter and Receiver in Series?
#1
Hi,

to reduce the CPU load and noise I'm planning to build the recommended low pass filter for my 433 MHz receiver.
The components are still on their way but I'm already thinking about the wiring.
My Raspberry Pi 2 has two 5V pins. One of them is blocked by my motion detector (which really needs 5V) and the other one is connected to the 433 MHz receiver.
My first idea was to use a 3.3V pin of my Pi for the filter but according to the specifications for the used frequency of 16 MHz 4.5 - 5.5V are needed for the ATtiny45. I also found this post
(12-20-2015, 10:18 AM)curlymo Wrote: Yes, running it on lower voltages can make the tiny less accurate frequency wise. We are running it on it's top frequency. 5v with resistors is safe.

My next idea was to run the filter and receiver in series. I'm only a beginner in electronics and assume that less accurate frequency results in difficult to find problems. Therefore I want to ask you if it is possible to not only connect the 433 data pin but also the voltage and ground pin of the receiver to the filter instead directly to the Raspberry Pi? So it would look schematically like:

Code:
Receiver        Filter                    Raspberry
###      Data     ###         Data          ###
###-------------- ###-----1KOhmResistor-----###
###       5V      ###          5V           ###
###---------------###-----------------------###
###       Gnd     ###          Gnd          ###
###---------------###-----------------------###
###               ###                       ###

Already many thanks for your comments.
 
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#2
You should do your supply voltage connections in a STAR configuration.

Use blocking capacitors at each subsystem, 4,7µF (tantalum) to stabilize the voltage and at each chip 100nF (ceramic) to suppress high frequency spikes.

Keeping the receiver and any computer chip at least 10 to 20 centimeters away from each other is another good measure to suppress noise.
 
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#3
I found the following drawing by you
(10-12-2014, 08:49 PM)wo_rasp Wrote:
Code:
+5V O----+--------+----O--------------------------------------o +5V
        |        |    O---------------------------o +5V      | Rx
        |        |    O---------------o +5V       | Tx       |
        |        |    O--o +5V        | LPF       |          |
  4,7µF |  100nF |         Pi   100nF |     100nF |    100nF |
      =====    =====                =====       =====      =====
        |        |         Pi         |           |          |
        |        |    O--o GND        | LPF       |          |
        |        |    O---------------o GND       | Tx       |
        |        |    O---------------------------o GND      | Rx
GND O----+--------+----O--------------------------------------o GND

At first it didn't look like a star network for me at all (as I said I'm a beginner) but now I think I understand how it should be read. Thanks for your help.

Just one more question:
If I e.g. search for "4.7 µF Tantal" I find various capacitors but none for 5V. Am I right that I can also use one that is designed for a much higher voltage (e.g. 20 V)?
 
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#4
The voltage specification has an impact on the size of the capacitor.
You want to use the lowest value possible, however 10 to 20V is fine.

Current spikes on the ground wire will lead to a voltage spike on the data input line (due to the resistance of the ground wire and subsequently the change of the voltage of the input signal).

To avoid this you do need to suppress the current spikes on the power supply line. The lower the resistance of the capacitor (called ESR) the better the suppression of spikes.

Compared to standard electrolytic capacitors, tantalum capacitors have a significantly lower ESR value.
The ESR value of ceramic ceramic capacitors is again lower by a factor of 10 to 100, in particular at high frequencies.
The combination of both capacitors plus separating the power supply wires from the data wires this design supports the suppression of spikes and reduction of noise on the data input lines, and that is what you want to achieve.
 
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