# Questions tagged [controlled-impedance]

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### Formula for coupled stripline of 100 ohm

I am designing a PCB which involves MIPI and LVDS routing. What is the formula to calculate the impedance of asymmetric coupled stripline for MIPI and LVDS routing (100 ohm)? The required calculator ...
137 views

### Routing ethernet differential pairs over ground plane

I am routing a flexible PCB which connects a panel-mount ethernet connector (without integrated magnetics) to a rigid PCB which contains the ethernet magnetics. I plan to route the differential pairs ...
• 21
1 vote
268 views

### Trace impedance for two signal layers between ground planes

When watching a few videos about stackup, EMI and impedance (especially this video from Fedevel Academy: https://youtu.be/52fxuRGifLU), I got confused how to compute trace impedance in the case of two ...
• 317
458 views

### Unattached tails (stubs) on SPI lines

I have a circuit design coming together that uses either of 2 sockets to plug in a controller. The idea is to allow a user to enable either of the controllers to control a radio using SPI. In the ...
900 views

### When to Remove the Ground Plane under SMD components for High-Speed Signals?

I'm reading TI's recommendations on routing high-speed signals. Most of the guidelines are common sense, but there's one guideline that I haven't read before: It recommends completely removing the ...
• 3,877
131 views

### Which Signals require impedance matching?

There are two types of connections in PCB. Which are in between the PCB.(e.g- A SD card is connected near to Controller) One is going out through some connectors.(Some UART or SPI going out through ...
1 vote
33 views

### Controlled impedance/coplanar wave design

We're using Diptrace at work which has no support for controlled impedance tracks. I'm making a small 2 layer pcb for a TDR pulse generator, and would like to make one track about 50ohms impedance. ...
1 vote
2k views

### How much RF power can a microstrip line handle?

Transmission lines on printed circuit boards must be impedance controlled lines since we want to transmit the full power and avoid the reflections. Microstrip lines have three parameters that we can ...
34 views

### Trace impedance on 4layers (Top and internal_plane_1)

I'm currently design a 4layers pcb with 50r trace for an Ceramic chip antenna (on top layer). Due of a lot of trace, my trace antenna is a coplanar. Trace on top layer with polygon pour around AND ...
81 views

### Pads reflecting in controlled impedance strip?

Background I'm designing a controlled impedance line for a 1900MHz signal travelling between SIM7000A cellular chip and an antenna. Question Tuning circuits are recommended for antennas with series ...
• 63
163 views

### PCB Thickness Variability

How much variability is there in the thickness of a standard PCB? Let's say I order a 1.6mm FR-4 board. Will the actual thickness range 1.5mm-1.7mm? 1.4mm-1.8mm? 1.59mm-1.61mm? What is the (...
• 105
245 views

### Altium - design rule check does not match with signal integrity for impedance

I have designed a 2-layer PCB with Altium Designer. I placed an impedance rule of 50 Ohm in trace width rule section. When I run the design rule check, there is no impedance error. However, when I ...
• 39
152 views

### When is it important to handle reflections in pcb design?

I am watching some videos explaining how reflection can affect a signal and how you should treat these signals as transmission lines to remove reflection. I wanted to understand a more practical ...
• 15
1 vote
197 views

### PCB controlled impedance

Hi) I want to calculate the width of my 100Mb Fast Ethernet MDI diff pairs. I've got some software (TXLINE 2003 Microstrip). I have 4 layers. The second layer is GND, the third is power, top and ...
• 197
4k views

### Controlled Traces Length in SD card

I my new design I need to use SD card communication. I would like to match all the impedance traces and control the length of the traces. Regarding the controlled length, is there a rule of thumb to ...
• 47
2k views

### Implementing 1000BASE-T (Twisted pair) over PCB backplane [duplicate]

Problem: We are designing a small payload which will contain several ethernet capable devices connected via a backplane we must design. All guidelines concern the scenario where the PHY is ...
• 101
116 views

### Transformer getting hot

I have build an LED array consisting of 75leds, 5 connected in series with a 39ohm-1/4watt resistor and 15 such series connected in parallel to the rectifier circuit having 2-IN5408 diodes and 1- ...
3k views

### What should the single-ended impedance be for LVDS?

For LVDS, it seems like the traces are usually specified as 100 ohm differential impedance. What should the single end impedance be from each trace to ground? I'm looking at PCB impedance ...
• 3,299
1 vote
3k views

### Designing a 50Ω coplanar waveguide on 1.6mm FR4

I'm trying to design a 50Ω coplanar waveguide to connect the output of a LoRa module to a SMD metal antenna. The two images below show the top layer (red) and bottom layer (blue). Vias are green and ...
• 4,082
1 vote
46 views

### While performing the controlled impedance routing what about the current flowing through it?

While calculating the conductor impedance the parameters that we consider are the conductor width, conductor thickness, frequency & substrate parameters. For achieving the particular impedance ...
• 63
4k views

### Why doesn't controlled impedance depend on track length? [duplicate]

High speed design noob here. Resistance increases as wire gets longer, but I found in saturn pcb calculator the impedance depends only on track geometry and distance from plane. Say for DDR3 single ...
• 936
1 vote
277 views

### BGA break out and impedance control

I'm connecting two BGAs (0.8mm pitch) which are DDR3 signals. Vendor recommends 40 and 80 ohm for SE and diff traces. My PCB manufacturer suggests the following specifications for the diff traces: <...
• 91
773 views

### Digital pot (MCP41100) as LED strip dimmer

I want to be able to dim a led strip with a MCP41100 but I have no idea how to connect the digipot to the led strip... It is connected to an ESP8266. I will have to figure out how to address the ...
• 133
1 vote
693 views

### Reflections on a differential transmission line

I understand why reflections occur when there is an impedance mismatch on a single-wire transmission line. However, when we introduce a differential line, I am having trouble picturing what would ...
• 1,484
810 views

### PCB Trace Impedance - Multiple Layers

I'm in the process of designing an 8 layer PCB with a stackup of: Signal, GND plane, Signal, Power plane, Power plane, Signal, GND plane, Signal. For calculating the trace widths necessary for ...
1 vote
556 views

### New led lights flickering in shop

I have replaced several light fixtures with LED in my company. The 400 watt hi-bays I replaced with 95 watt LED. They work great. I replaced a set of office lights that were track halogen with 7 watt ...
• 11
2k views

### Is this layout good? (RF - 2.45 GHz)

I finished laying out HACK, my first RF (2.45 GHz) layout. The sensitive part is the connection between the IC (Atmel SAM R21G18A), the balun (Johanson 2450BM15A0015), and the PCB antenna (copied from ...
• 131
962 views

### Testing of impedance test coupons

I have a board with a 50 Ω impedance controlled track. I have got the boards manufactured. The manufacturer has sent me 2 additional boards viz - test coupons. I understand that test coupons have the ...
• 1,879
2k views

### What are "slash sheets", and how do they relate to PCB substrates?

I am trying to specify the stackup for a PCB. Previously I have used whatever the board house considered to be their "standard" stackup. This time I am looking at using FR-408 (pdf) because of its (...
• 16.4k
12k views

### Length Matching Differential Pairs

I am routing a PCB with an Ethernet connection and I am having a bit of trouble deciding on how best to route the TX and RX differential pairs. I have done the impedance calculations to figure out the ...
• 513
425 views

### Controlled Impedance Microstrip Frequency

Are you supposed to calculate the characteristic impedance for the frequency you are using or at 1GHz? I see pretty much everything calculated at 1GHz in all examples and papers. For example - for ...
• 811
488 views

### SMD LED Perimeter Lighting with PWM for Dimming on each colour

simulate this circuit – Schematic created using CircuitLab So from the start what Im trying to achieve is drive way lighting in which each colour is separately dimmable with a pulse width ...
1 vote
2k views

### Automatic impedance matching in microwave waveguides

I am trying to building a magnetron plasma using microwave magnetron. I now need to build magnetron to antenna link using rectangular waveguide, WR-340. I then found out that it needs to be tuned ...
• 507
1 vote
5k views

### Internal USB (FFC?) cable with 90 Ohm Differential impedance

I'm trying to run USB D+/D- signals and some unrelated 19V power between two PCBs, on the daughter PCB the USB D+/D- will terminate to a USB-A-F jack. I have no problem maintaining 90 ohm differential ...
• 11
17k views

### Why Characteristic Impedance must be 50 ohms?

Where does this number come from? For single ended it must be 50 ohm and for differential pairs 100 ohm. Why? For PCB with controlled-impedance these are that common numbers. Outside of the PCB you ...
• 1,412
399 views

### Can I slow a CMOS output through an RC filter?

Is it good practice to slow down the slew rate of a CMOS output by putting an RC filter on it? What happens with impedance matching after I do this? Or can I just set the RC filter with such a low cut-...
• 3,784
3k views

### Reasoning behind Ethernet pair order

An Ethernet connector seems to use a surprising choice of signal placement in its RJ45 connector. Does anyone know the reasoning behind: The decision to make pair 2 straddle pair 1? Why Ethernet uses ...
• 26.3k
976 views

### Signal reflections at the driver?

This question is related to How do I use directional couplers on a differential signal? I understand that if I have an impedance mismatch at the end of a signal cable, E.G. from a terminator which ...
• 26.3k
4k views

### DDR1 Layout Considerations - DOs and DONTs

I am novice to high speed design. Before getting in to DDR, I recently learned about impedance matching and how it is done, likewise I learned about length matching and how it is done.(Baby steps ...
• 1,116
1k views

### Controlled impedance in presence of vias and through-hole components (PTHs)

We have some controlled impedance traces on layer 4 of a board. Layer 3 is a GND plane. Layer 5 is a 3.3V plane. Both planes are unbroken (they occupy the entire layer), with the exception of vias and ...
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