Use large ferrite bead around R24 and add PU glue or insert wire bead. YOu want >10Ω at switching rate.
Verify impedance of both C49, C82 at switch rate and all harmonics.
If not less than 100mΩ change caps. This will give 10/0.1 or 40 db Min attenuation. >80dB would be better.
Also check impedance of caps at RF frequency and thus SRF of 100pF and change if necessary for same reasons.
let me give you one piece of sage advice. If you see an electrolytic capacitor that gives Tangent of the loss angle specs DONT USE IT FOR ANYTHING EXCEPT AC filter. The reason is that the ESL and ESR are poor, with these low cost parts.
Especially dont use it for RF supply filter.
Do yourself a favour and use a good AC coupling board to a Spectrum Analyzer . verify the insertion loss at RF freq of interest and measure the supply ripple.
YOu want the spurious signals well below the MFg required ripple specs converted to dBm.
Even 100pF can be considered big for mircowave.
For Alum Electr. consider $0.065 /1k
MFG: UCC
PN: EKY-160ELL121MF11D
Catalog Drawings KY Series Bottom_2.5
KY Series_6.3 x 11
Standard Package 2,000
Category Capacitors
Family Aluminum
Series KY
Capacitance 120µF
Voltage Rating 16V
Tolerance ±20%
Lifetime @ Temp. 5000 Hrs @ 105°C
Operating Temperature -40°C ~ 105°C
Features General Purpose
Ripple Current 340mA
ESR (Equivalent Series Resistance) -
Impedance 22 mOhm
Mounting Type Through Hole
Package / Case Radial, Can
Size / Dimension 0.248" Dia (6.30mm)
Height - Seated (Max) 0.433" (11.00mm)
Lead Spacing 0.098" (2.50mm)
The electrolytic ensures low switcher noise.
The ceramic ensures low RF noise.
If this doesn't fix it, then you need to consider isolating the ground plane from switcher currents to ensure they do not induce mV drops under your transmitter. A bigger picture of the layout may show that is not easy and the ferrite used for microwave is not he same as used for SMPS rejection.
µwave ferrite inline filters are conductive ceramic, low permeability material.
SMPS inline or CM filters are insulating ceramic, high permeability material. But in reality, there are hundreds of different blends of ferrite mixes for millions of different parts.
There is insufficient information to completely answer the question.
I assume you are concerned about EMI from external sources affecting your thermometer -- not the other way around.
Questions to ask:
- What kind if EMI is your system going to be exposed to?
- What exactly are you worried about? Damage to the uC, damage to the
sensor, both?
- Corrupted Data?
- Consequences of actual EMI. E.g. danger to equipment or humans, down time, cost.
Taking these in turn:
Likely sources of EMI are from switching transient of high power local loads, especially inductive ones (motors). Nearby high power RF transmitters may also cause problems but cell phone tower are usually not an issue. Static electricity discharge can damage electronic devices too, but that is independent of the cable length.
Damage to the uC and sensor is unlikely if your sensor is connected to the uC by 3 wires in the same cable. Any noise will be mostly "common mode" (all wires are equally affected). You can put a few turn of the entire cable through a ferrite ring core if you are really paranoid.
A suitable RC filter on the uC pin(s) connected to the DQ signal may be appropriate. The details depend on your circuit. E.g. do you use 1 pin with direction control or separate read/write pins, etc?
If your concern is that you might get incorrect readings, then the CRC in the data stream will allow you to detect that and discard occasional bad readings.
Consequences of EMI: EMI protection can be expensive. If there are no dangerous consequences then replacing your thermometer may be the most economical option.
Summary: Your circuit may be fine as is, nothing further required.
The Maxim application note on 1-wire networks may help. See Appendices. The circuit in Appendix B looks appropriate for your application. Guidelines for Reliable Long Line 1-Wire® Networks
Best Answer
If you know the current on the line and resistance, then you will know how much voltage it will generate when the return current goes through the ground. This can be modeled on paper. Either measure the wire or find an AWG chart and estimate it. This type of noise from switching return currents is called common mode noise.
Another thing you will need to account for is inductance, each wire functions like an inductor because when currents flow through a conductor they generate a magnetic field. There can be mutual inductance from one wire to the next when wires are ran together, it may be necessary to design the cable with a transmission line for the return current of the signal. Or use twisted pairs to reduce this effect.
If you can estimate or measure the inductance and resistance, you can use a spice package such as LT spice to model parasitic effects. At speeds over ~50MHz transmission line effects and capacitance will also come into play, however these are harder to model and are probably best measured.
In any case, you need to be able to take a cable and reduce it into a circuit. Remember that a wire has resistance and inductance, if you have two conductors and an electric field between them, you have a capacitor. The real world its too difficult to model all that goes on, you will never have a perfect model. The goal is to try and model all of the relevant dynamics in a circuit so you can get the answers you need to come up with a proper design.
There are also 3d FEM field solvers that you can draw parts up with 3d cad programs and tell the solver what kind of current or voltage is on the wires, the solver then solves all of the EM field equations and can give you the answers. In almost all cases, this type of analysis is just as time consuming as testing the actual physical wires. With the cost of such software in the 10k$ it's usually not worth it.
No, you will need to do this yourself.
A good start would be to read Electromagnetic Compatibility Engineering by Henry W Ott