Electronic – How serial resistors actually reduce EMI

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.

Experimentally wrap tinfoil round the cable, and earth it at the driving end. Or the chassis, but not both. If that works, you can find a better way of screening the cable.

Or a series termination at the driving end of the clock signal; this will somewhat slow up the clock edges; in other words reduce the harmonic content.

I have my doubts about this : if the clock is 30MHz and the peak is at 60MHz that's only the second harmonic. Which means either : the clock is a LONG way from being a square wave (duty cycle is far from 50%) or: the EMI is from other sources (perhaps the RGB data : does the emission go away with the screen black?) Some further information from these experiments and questions may help.

EDIT: If you can scrape up a spare wire in the connector, add a differential driver for the clock at the driving end (the LCD controller) and transmit the clock and its complement down adjacent wires. At the receiving end (the LCD) use a differential receiver to regenerate the clock. This has two advantages : first, the clock waveform will be more accurately preserved despite the cables and EMC filters, and second, the radiation from the two clock lines will cancel each other out, reducing your EMC emissions.