I2C Routing Guidelines: How to Layout These Common. Read Article UART vs. Although that is a simple example, there are a lot more rules that can help in the design of high speed and RF traces: Trace Lengths: This rule allows the user to set a target value. CSI signals should be. For example, if you require a 5mil trace to achieve 50Ω impedance and if you have also routed other signals with 5mils width, it will be impossible for the PCB manufacturer to determine which ones are the controlled impedance traces. I have done the impedance calculations to figure out the track geometry needed for 100 ohm differential impedance and confirmed it with the board house. Cables can be miles long but a PCB trace is likely to be no longer than a foot. Vendor may adjust trace widths, trace. Read Article UART vs. 35 mm − SR opening size: 0. PCB trace length matching is a crucial process in designing high-frequency digital circuits, designers can minimize signal integrity issues. The frequency of operation is about 10 MHz. High-speed USB signal pair traces should. Impedance profoundly impacts signal quality in high-speed PCBs. 6 inches must be routed as transmission line. 1mils or 4. 2. Trace Height (H) Figure 4. Signal distortion in a PCB is a major signal integrity issue. 005 inches wide, but you may have specific high speed nets that need 0. Every conductive element in a PCB has some parasitic inductance, and multiple conductors together have some parasitic. Here’s how length matching in. Would a 2-3 cm difference in lines beget problems? Critical length depends on the allowed impedance deviation between the line and its target impedance. As you know, there are two types of interfaces in PCB design and length tuning will be different for each of them. 4 Trace Length Matching PCIe signals have constraint s with respect to trace lengths and matching in order to meet jitter and loss. • An increase in the minimum clock frequency from 125 MHz to 300 MHz. Let’s dig into this further and get a sense for why you should not route a trace over a gap in a ground plane. 4. For 165 MHz signals, it's not unlikely that the signal is actually transported as low-voltage differential signal – thus, a single signal is not a single trace, but a pair of. It would be helpful to know the tolerance in length difference that is allowed while designing this PCB. The speeds will be up to 12. Trace stubs must be avoided. If the signal speed on different traces is the same, length matching will approximate propagation delay. Technologies DDR3 Routing Topology Page No #5 DQ/DQS/DM:If a transmission line has a 50 ohm impedance, then connecting it abruptly to a 1 V source will cause a 1 V voltage wave and a 20 mA current wave to start travelling along the line. Series Termination. Here’s how length matching in PCB design works. The answer to this question, Characteristic impedance of a trace, shows that a 120 mil trace is required to get this impedance. 2) It will be vise to match the PCB trace impedance to the cable impedance, or you may get reflections. A 3cm of trace-length would get 181ps of delay. Special care needs to be made to match length in all these lines. This is also done to avoid under or over-etching. ImpedanceOne of these design aspects is the match between PCB via size and pad size. 50R is not a bad number to use. How to do PCB Trace Length Matching vs. Laser direct Imaging equipment eliminates variances in trace width. In the pair with smaller spacing (5 mil), the small traces in our 21 mil amplitude length tuning section have odd-mode impedance of 58. In the case of (2), Altium Designer (based on your screenshots) offers several ways to. PCB traces must be very short. I2C Routing Guidelines: How to Layout These Common. . Here’s how length matching in PCB design works. The length of traces can cause problems with loss and jitter for LVDS signals. Running through a number of calculations it’s obvious that the only case where the length of the PCB trace doesn’t matter is when trace and load impedance are matched. frequency calculator that. 025, the frequency as 10 GHz, the surface roughness as 6 μm, and the length of the trace as 1 inch. The board thickness and trace width and thickness should be adjusted to match the impedance. 5 cm Any PCB trace length greater than 1. This creates several effects in PCBs on FR4 that are especially important in high-speed or high-frequency applications. This 6-layer PCB stackup can enable orthogonal routing on L1/L3 and on L4/L6. PCB trace length matching is exactly as its name suggests: you are matching the lengths of two or more PCB traces as they are routed across a board. Figure 12. $egingroup$ @Krish No, as Marcus Müller stated there are more effects except length which will affect the signals e. A trace has both self inductance and capacitance relative to its signal return path. Cite. This extra margin could be used to relax layout requirements on trace length matching and impedance control on cost sensitive PCBs. 3 High-Speed Signal Trace Length Matching Match the etch lengths of the relevant differential pair traces. Sorted by: 9. When adjusting the trace length, ensure you get the correct size for a given group of signals—generally, the higher the interface frequency, the higher the length-matching requirements. So for bottom traces there will be massive high-frequency signals underneath them on the motherboard within 1-2mm distance. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. 1. – The impedance mismatch between vias and signal traces can cause transmission-line reflections. Recommended values for decoupling are 0. 8 A, making it. With careful balun selection and impedance matching, the AD9081 and AD9082 DACs and ADCs have a useable bandwidth of 7. Here are the PCB layout guidelines for the KSZ9031RNX: 1. At an impedance mismatch, a portion of the transmitted signal isFigure 3. This variance makes Inside the length tuning section, we have something different. If the traces differ in electrical length, the signal on the shorter trace changes its state earlier than the one on the longer trace. The above example does not mean that the PCB traces less than 1. How to do PCB Trace Length Matching vs. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Here is how we can calculate the propagation delay from the trace length and vice versa: Where: Vis the signal speed in the transmission line; In a vacuum or through the air, it equals 85 picoseconds/inch (ps/in). SPI vs. Traces and their widths should be sized. 2. The impedance formula is usually represented by Z = R – j/ωC + jωL, where ω = 2πf. SPI vs. Use the results from #3 to calculate the width profile with the integral shown below. frequency can be reduced to a single metric using an Lp norm. SPI vs. 0). These equations show that attenuation occurs in the circuit due to the (RC + GL) term. 1. Routing between connectors on a board and. 393 mm, the required trace width for this particular inductance value is w = 0. 34 inches to not be considered high-speed. I am more interested in the impedance, reactance and resistance of traces in my question for given frequencies in pcbcad softwares for a given layer stackup than the antenna shapes. 0uF. Diorio: Transmission lines 12Track length matching is key when trying to maximise the performance of your PCB. 35 dB to 0. This unwanted radiation can couple to any adjacent trace or even to a cable existing in the. Klopfenstein trace taper return loss spectrum for a 50 to 40 Ohm transition. The narrow spacing and thin layer count will force traces in the pair to be thin as well. )Only Need One Side of Board to be Accessible. 2) It will be vise to match the PCB trace impedance to the cable impedance, or you may get reflections. 36 RF / Microwave Design - Line Types and Impedance (Zo) Coplanar Waveguide)CPW Allows Variation of Trace. How to do PCB Trace Length Matching vs. A wire trace becomes infinite impedance at infinite frequency and open gaps become short circuits. Read Article UART vs. The PCB trace may introduce 1 ps to 5 ps of jitter and 0. Note2. The maximum PCB track length is then calculated by multiplying tr by 2 inch/nanosecond. I2C Routing Guidelines: How to Layout These Common. To ensu re a robust interface, the designer must address both components. Proper interconnect design must account for the lower noise margins of. Intra-pair skew is the term used to define the difference between the etch length of the + and - lane of a differential pair. Are there guidelines as far as trace length vs frequency? I assume that ~3 inch traces are fine with 20MHz (15 meters), but what is the general case? As frequencies increase, how to prevent long traces from radiating? Are striplines and coax the way to go? What is the RF characteristic impedance of a typical microcontroller output stage, anyway? See full list on resources. I2C Routing Guidelines: How to Layout These Common. Here’s how length matching in. PCB Trace 100 Ω Differential Impedance Source SCOPE CAT5 Belden MediaTwist(tm) Figure 1. Follow asked Jul 24, 2015 at 2:20. 2. (TMDS) signal traces Ground plane Power plane Low-frequency, single-ended traces Layer 1: Layer 2: Layer 5: Layer 6: High-speed, differential signal traces Ground Vcc2 Low-frequency, single-ended traces Layer 4: Ground Layer 3: Vcc1 5 - 10 mils 20 - 40 mils 5 - 10 mils Fig. My shortest signal needs 71*3. Initially the single-ended trace had higher bandwidth, however this could be due to its larger width (8. This, in turn, enhances the signal quality and minimizes signal loss. The eleven inch trace length represents a maximum loss host design (PCB plus package). DKA DKA. 5cm and 5. How to do PCB Trace Length Matching vs. Trace Length Matching: Matching the lengths of the positive and negative traces helps preserve signal timing and minimize skew. Dispersion is sometimes overlooked for a number of reasons. Read Article UART vs. Here’s how length matching in PCB design works. There are many demands placed on PCB stackup design. 0 reaching 32 Gb/s, and PAM4 pushing signal integrity and speeds to the limit. This practical experience is gained from processing thousands of designs and understanding the ramifications of placing a via too close to a trace,7. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Read Article UART vs. Everything from 8-bit to 32-bit MCUs will use at least one of these protocols alongside GPIOs for programmability and sending signals to simple peripherals. Eventually, the impedance of your power delivery network will. High-Speed PCBs vs. pcb-design; high-frequency; Share. Aside from this simple design choice, you may need to design an impedance matching network for your connector. How tightly should trace lengths be matched for a 1Gbps serial databus? It seems to me that 100ps (15mm) should be more than sufficient. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. FR4 is a standard. 1. PCB signals undergo signal integrity issues such as signal reflections, signal distortions, crosstalk, coupling, and ground bounce. If there are high-speed transition edges in the design, you must consider the problem of transmission line effects on the PCB. The propagation delay is the time taken by a signal to propagate over a unit length of the transmission line: Where: V is the signal speed in the transmission line. Designers need to begin treating interconnects as a transmission line when the trace length begins to approach or exceed 1/10 the wavelength of the signal’s highest frequency. This consists of maximum and minimum trace width, and length matching with other traces. (Ɛr), the thickness of the substrate and the layout of the traces on the PCB. Here’s how length matching in PCB design works. 66 mm between this traces and nearby traces? Which rules are stronger?How to do PCB Trace Length Matching vs. Because trace, source, and load impedance mismatches are a critical concern in high frequency design, you need a PCB trace length matching vs. The need for multiple lines between the microcontroller and peripheral makes component mounting more of an issue and they should be placed as close together as possible to minimize trace lengths. Would a 2-3 cm difference in lines beget problems?Critical length depends on the allowed impedance deviation between the line and its target impedance. Each end of a differential pair. There are many calculators available online, as well as built into your PCB design software. Inter-pair skew is used to Routing high-frequency traces close to each other can result in crosstalk and interference. I2C Routing Guidelines: How to Layout These Common. (5) (6) From the results above we can see that the setup and hold margin are both greater than 0 as desired. Keep the spacing between the pair consistent. Follow asked Nov 27, 2018 at 12:32. Digital information synchronizes to a clock signal. I'm designing a board which contains an LTE module on it. – The impedance mismatch between vias and signal traces can cause transmission-line reflections. How to do PCB Trace Length Matching vs. I2C Routing Guidelines: How to Layout These Common. A very common, but also effective, rule of thumb is to use a minimum spacing of "2W" (better still, a "3W. The RS-485 protocol standard allows up to 32 drivers in one system, supporting communications over distances of up to 1200 meters, and can keep baud rates from 110 Baud to 115200 Baud. 010 inches spacing between them. A PCB trace is a thin conductor on a printed circuit board (PCB) that carries electrical signals between components. If you use the 1/4 rise time/wavelength limit, then you are just guessing at the. The third trace has a solid reference plane beneath, and its length is identical to trace 2, 120mm. The goal is to minimize magnetic flux between traces. How to do PCB Trace Length Matching vs. t pd =𝟏/𝐯6 Length Matching Overview The following sections discuss considerations for length matching. Since my layer thickness is 0. There is another important point to consider, which is trace length matching for parallel buses. SPI vs. 3. How to do PCB Trace Length Matching vs. frequency response. Read Article UART vs. This high clock speed and large storage capacity ensured DDR3 remained a mainstay in modern computing, but it was eventually improved to DDR4. CBTL04083A/B also brings in extra insertion loss to the system. So is the PCB trace impedance an impedance or a resistance? It's both (short story). It has easy manufacturability and has the wireless range acceptable for a BLE application. In particular, the transit time of signals often needs to be synchronized by matching the copper length of the traces on the PCB. Critical length is longer when the impedance deviation is larger. Trace lengths should be kept to a minimum. How to do PCB Trace Length Matching vs. g. The caveat is that any editing of the clock or the traces on the edge of the tolerance band is likely to upset. How to do PCB Trace Length Matching vs. Impedance mismatch: Impedance mismatches between the source, transmission line, and load can. These traces could be one of the following: Multiple single-ended traces routed in parallel. That limitation comes from their manufacturing (etching) processes and the target yield. SerDes PCB Layout Guidelines: This means we need the trace to be under 17. The higher the frequency, the shorter the wavelengthbecomes. SPI vs. Ethernet: Ethernet lines. 3. As the driving frequency increases, mutual inductance between circuits in your board will cause the impedance of your power delivery network to increase. , RF signals), it's okay if you only know the value of the dielectric constant at a single frequency. Relative Permittivity: 4. the TMDS lines. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. How to do PCB Trace Length Matching vs. Myth: consider the differential traces must rely on the close. Below ~5GBps not something to worry about at all. The allowed deviation in length matching depends on the rise/fall time for digital signals between these two elements, although it is generally recommended that any deviation be less than 10 mm as MII and RMII use TTL logic. Place high-speed signal traces away from noisy components. Note that the y-axis is on a logarithmic scale for clarity. The length and Z o affects path loss and special delays with frequency/length ratios like 1/4 wave impedance reflections (inversion) and all odd harmonics of same. Here’s how length matching in PCB design works. Signals can be reflected whenever there is a mismatch in characteristic impedance. If the chips themselves are able to do the de-skewing, of course you should use that feature rather than extend the traces to do length matching. If your chip pin (we call this the driving pin) turns its. 005 inches wide, but you may have specific high speed nets that need 0. Cite. In summary, we’ve shown that PCB trace length matching vs. Match the etch lengths of the relevant differential pair traces. 1 Ohms of resistance. As I understand, the camera max frequency is 720 mbps, or 1380 ps of unit interval. Do you guys agree to this? mode voltage noise, and cause EMI issues. The roughness courses this loss proportional to frequency. Microstrip Trace Impedance vs. 1 Internal Chip Trace Length Mismatch. In lower speed or lower frequency devices,. These specifications can be found in datasheets, and you should set your high speed design constraints to hold these length specifications. Use shorter trace lengths to reduce signal attenuation and propagation delay. PCB design software, like Altium Designer ®, has high-speed design functionality for routing and trace tuning built into it. To minimize PCB layer propagation. 4 High Speed USB Trace Length Matching High-speed USB signal pair traces should be trace-length matched. 5 mm with the clock straddling the difference. 0) or 85 Ohms (COMCDG Rev. You'll have a drop of about 0. cable length performance far exceeding IEEE specifications and features that provide lower cost solutions, for both 10BASE-Tand 100BASE-TXEthernet protocols, the devices ensure compatibility and inter-. That's 3. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. How to do PCB Trace Length Matching vs. Read Article UART vs. SSTL 15 IO Standard (1) FPGA Side on-board termination(2. Taking away variables makes the timing and impedance calculations simpler. At 90 degrees, smooth PCB etching is not guaranteed. Here’s how length matching in PCB design works. SPI vs. Instruct the PCB fabrication house to use smooth copper, if the frequency exceeds 2 Gbps. 5 ns, so a 7-inch or more track carrying this signal should be treated as a transmission line. On a high-speed PCB (> 100MHz) where wavelengths are shorter, any critical net (see figure 4a) is electrically long enough to make it an efficient radiator, especially when left exposed on the top or bottom layer. Here’s how length matching in PCB design works. Use a 100 Ω tightly differential routing on the main host PCB up to the connector pins if you are using option 2 in Figure 102 at the connector. How to do PCB Trace Length Matching vs. Trace width can also be set up for a particular net or a net class, controlled impedance traces, differential pairs, or other specific traces like clock signals. 50R is not a bad number to use. the series termination resistor is chosen to match the trace characteristics imped-ance. With any PCB, the trace design or the materials used for the trace can cause impedance values to change. Impedance in your traces becomes a critical parameter to consider during stackup. For instance the minimum trace width on a design may be 0. Due to these and other concerns, the following guidelines should be followed when laying out out your PCBA with SGMII and SerDes connectivity. Figure 1: Insertion loss of FR4 PCB traces. The PCB trace to the flex cable 4. The typical method for matching timing in a differential pair is to match the lengths of the two lines at the source of the interconnect, also known as phase matching. 2 mm. Yes, trace length can affect impedance, especially for high-frequency signals. Individual byte lanes want to use the same routing layers so that all of the signal integrity problems are equalized. 0 dB to 1. And the specication says the GPIO clock for the PRU is 100MHz. trace loss at frequency. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Matching trace lengths at specific frequencies require understanding dispersion in your PCB substrate material. This will be specified as either a length or time. Microstrip Trace Impedance vs. The difference between a cable and a printed circuit board track is length. PCB routing for RF (radio frequency) and antenna design is essential to optimize the performance of wireless communication. 3 Length and length matching Trace length greatly affects the loss and jitter budgets of the interconnection. Therefore, the minimum length over which the signal must be routed as transmission line is given by ?/10 = 0. 5 cm should not be routed as transmission line. First, adhere to the absolute routed maximums to prevent signal integrity issues. 7. Dispersion in the PCB substrate causes the signal velocity to vary with frequency. – Any discontinuities that occur on one signal line of a differential pair should be mirrored on the otherUse the same trace widths throughout the length of the trace. PCB Trace Stubs and Discontinuities • If possible, avoid routing high-speed frequency traces through the vias. magnetic field tends to be stronger when traces are running along the PCB. For a stripline (inner layer) you divide the speed of light in vacuum by the square root of the relative dielectric constant (e_r). )No Plated Holes Needed,)Can Narrow Trace to Match Component Leads. Therefore, you should make the 50Ω impedance traces 5. Tip #1: Reference Planes. FR-4 is commonly used for the dielectric material. The trace length decided to match with Wavelength of the frequency Wavelength (Lambda) = Wave Velocity (v) / Frequency (f) =299792458 /700000000 =428. 6mm-thick board it'll be impractical. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. For length-matched parallel buses, you'll usually use a mixture of the two. I2C Routing Guidelines: How to Layout These Common. Trace length matching; To know more about PCB routing read our article 11 Best High-Speed PCB Routing Practices. Once upon a time, length matching guidelines for high-speed signals required a designer with enough skill to remain productive when manually applying different trace-length turning schemes. On a real substrate, say FR4, the impedance of a real PCB trace will vary with frequency due to the dielectric constant and loss of the dielectric varying, and the resistance of. The stub length must not exceed 40 mils for 5 Gbps data rate. However: The Raspberry Pi Computer Module 4 (CM4) datasheet states: 2. ) of FR4 PCB trace (dielectric constant Er = 4. The ‘3W’ Rule (s) This actually refers to three rules. Signal distortions in the form of signal losses are common in long PCB traces. Cadence Orcad Guide OrCAD - PCB Solutions | PCB Design Software EDA Tools and IP for Intelligent System Design |. This question (paraphrased) goes as follows: Do length-tuning structures create an impedance discontinuity? The answer is an unequivocal “yes”, but it might not. What makes it distinct are parameters like impedance matching, type of traces (preferably co-planar), elimination of via stubs (to avoid reflection), ground planes, vias, and power supply decoupling. As replied above my trace length varies between 35 and 57mm. • Narrower DDR3 output drive ranges that can be recalibrated to adjust for voltage and temperature variations. We only ever have perfect matching at specific frequencies, but there are mid-range frequencies where the return loss spectrum is flat. except for W, the width of the signal trace. Sudden changes in trace direction cause changes in impedance. How to do PCB Trace Length Matching vs. It is performed by placing a terminating resistor in between the driver and the receiver. How to do PCB Trace Length Matching vs. Common impedance values are between 25 and 120. Ensuring that signals arrive in time to process means that trace lengths may need to match. A 3cm of trace-length would get 181ps of delay. Mainly because, 1, you're actually doing the length matching, and 2, you're using arcs. When it comes to high-speed designs, we are typically concerned with two areas. Quadrature coupler design can use discrete components or quarter-wavelength tuned traces to split or combine inputs and produce outputs with a 90°. Impedance matching for PCB traces is not an issue until total trace length between 75 Ohms input connector and MAX2015 input is below 5-7 mm. 5 inch (14 mm). Why insertion loss hurts signal quality. Some possible changes include the addition of termination components, careful design of impedance matching networks, or redesigning traces to adjust their impedance. And the 100ps would be equal to 15-20 mm in trace length difference, which is huge. Guide on PCB Trace Length Matching vs Frequency. How to do PCB Trace Length Matching vs. 7563 mm (~30 mils). Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Speed ≡ Clock frequency and/or edge rates. So I think both needs to be matched if you want to work at rated high frequency. I then redesigned the board with length matched traces and it worked. Why FR4 Dispersion Matters. As a thumb rule At what trace lenths should i used differential drivers (LVDS,RS485) etc for SPI interface. 5in, ~4cm) for a trace on a PCB with a dielectric constant of 4. TX traces can be a different length from RX traces. By the same token, each trace has capacitance distributed along the trace and the. Based on simulations and. Therefore, you must adjust the trace length for all parallel interfaces. 3. This is valid up to tens of THz for a typical PCB trace. Specialized calculators and. I2C Routing Guidelines: How to Layout These Common. The PCB trace on board 3. Here’s how length matching in PCB design works. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. SPI vs. Fast rise/fall times alone doen't need length matching. How To Work With Jumper Pads And. This allows you to automatically calculate and compensate propagation delay in your PCB without manually measuring traces with. Try running a 10 GHz signal through that path and you will see loss. The basic idea of this length matching is that the shorter trace follows a detour or meander in order to lengthen it to match the length of the longer trace. In this PCB, we have three straight traces. 92445. Alternatively, in terms of length, the matching translates to +/-60 mils using 160 ps per inch of trace length. Three important points in bus routing are designing for consistent trace impedance, proper termination, and a tight ground return path to minimize loop inductance. More important will be to avoid longer stubs. b. Multiple differential pairs routed in parallel. Maximum net length. A more. This puts the emphasis on smart component placement in the PCB layout, especially of connectors. Tightly coupled traces saves routing space but can be difficult to control impedance. If these traces are carrying signals which have a spectral content which includes any frequency greater than (speed of light) / (10 x trace length), then do 45 degree traces. 5 GHz. The traces are 0. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Here’s how length. Matching trace lengths at specific frequencies require. For the other points, the reflections are a result of impedance mismatching. 2. The PCB trace width and the spacing to the grounded copper regions need to be designed to set the designed impedance to the. •The physical length of each trace between the connector and the receiver inputs should be. 1V drop, you need to obviously widen the trace or thicken the copper. I2C Routing Guidelines: How to Layout These Common. Detangling the hair of a 9-year old doesn’t take as long as routing PCB traces, but the results are just as painful if not done correctly. The Altium auto router helps PCB designers with the difficult-to-master process of dense trace routing on a PCB. Determine best routing placement for maintaining frequency. However, you should be aware. Read Article UART vs. character as the physical length of traces becomethe s aconsiderable fraction of the signal wavelength. I2C Routing Guidelines: How to Layout These Common. The resistance of these conductive elements is low enough to be negligible in most situations. If your PCB has the space, why not match the lengths? It's good to practice length-matching any time you have the chance. Those familiar with high-speed design know that trace geometry, trace location, and board substrate all affect signal speed, impedance matching, and propagation delay. SPI vs. . How to do PCB Trace Length Matching vs. Other aspects such as stack-up and material selection also play crucial roles. Frequency Keeping high speed signals properly timed and synchronized requires PCB trace length matching vs frequency. Keeping traces short is another way to combat reflections and ringing. During that time both traces drive currents into the same direction. Each variance affects the characteristic impedance of an RF circuit. 2 dB of loss per inch (2. How to do PCB Trace Length Matching vs.