dc motoremcinterferencemotorRF
What kind of frequency does a DC brush motor generate? There are a few explanations online as to what causes EMI in DC brush motors (commutator sparking for example) and how to reduce it, but no mention of frequencies at all.
I know this is a poor question, but there I simply cant find an answer to it. I know it varies on the motor and other factors, but I am simply looking for an order of magnitude (kHz, MHz, GHz…).
A Smith Chart is not so much an attenuator design aid
as a means of evaluating and adjusting a design.
So - see attenuator articles below and then Smith chart articles.
Carbon resistors may be carbon film or carbon composition?
Carbon film are not suitable for UHF work as they are formed by cutting a spiral track in a carbon film cylinder, so have very substantial inductance.
Carbon composition have a solid carbon body and may be suitable for UHF work depending on other factors.
UHF Attenuators:
Attenuator design tutorial - looks good.
Interest - Commercial products
What is a Smith Chart?
Wikipedia gives a better than average concise summary:
From here
Somewhat gentle introduction - 27 page powerpoint intro - still gets deep quite quickly BUT a Smith Chart can be very very useful with almost no maths or numerics involved.
Superb Smith Chart resource - essentially an index of indexes - breaks subject up into sections and provides many references for each.
Another good list of references
Smith Chart tutorial from Maxim - reasonable "dense" but looks understandable.
You'll understand this once you've read it :-)
I suggest you use ONLY PLANES for your high-current high-speed-switched-current loops. And have no breaks in the planes where outgoing and return currents need to co-exist to null out the magnetic fields (which by the way reduces the stored energy and thus lets the circuit switch faster).
By the way, you need not allocate entire planes to the high-speed high-current loops. Do plan, with sketches, your high-speed components placement; select components with pinouts and backside solder-regions etc to minimize component heights above the planes (or regions of planes).
Regarding MOSFET oscillation, a "gate stopper" resistor is used to dampen ringing. Assume 10nH to 100nH (1/2" to 4" of wire, 10,000pF Cgate, and transconductance of 1 amp/0.1 volts. Lotta potential to oscillate, in Colpitts config.
Best Answer
The fundamental frequency is determined by the commutator switching speed (which varies with motor rpm), but the rf produced by arcing can go up to GHz frequencies. Here's an example:-
The blue line is before adding ferrite cores to the motor leads, the green line after.
At lower radio frequencies (below 100MHz or so) small capacitors connected from each motor terminal to the case (possibly combined with small inductors in series with the leads) are usually sufficient to reduce EMI to an acceptable level. At GHz frequencies the wiring itself blocks it, but various internal parts of the motor can act as antennas. If the motor has large cooling slots or a plastic end bell these frequencies can easily radiate.
At audio frequencies EMI is mostly conducted through the wires as a result of the pulsing current. This is mitigated by twisting and/or shielding the motor wires and keeping sensitive circuits away from them, putting large capacitors across the supply rails, avoiding ground loops etc. In severe cases the motor might need to be on a separate power supply.