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# Tag Info

## Hot answers tagged microstrip

11

The short answer is you're going to be fine, route it above VCC and make sure you have some VCC to GND decoupling caps near your chip. Plus your route is pretty short at 600mil, I've seen some people do terrible things to USB routes that still end up working :) I think the best way to understand this is to consider where your return current will flow. ...

7

Generally, your assumptions are correct. When connecting a microstrip to a component, how closely you are able to match the physical size and shape of that microtrip plays the biggest role. Package size, termination style, and trim method for a series resistor are what matters. Your other assumption that you can pretty much ignore transmission line ...

6

Via impedance can be approximated by its capacitance and inductance. From pages 257 to 259 of High-Speed Digital Design: $$C_{\text{via}} \text{[pF]}=\frac{1.41 \epsilon_r T D_1}{D_2-D_1}$$ D1: diameter of pad surround via [in.] D2: diameter of clearance hold in ground plane(s) [in.] T: thickness of PCB [in.] $\epsilon_r$: relative electric ...

6

Matched RF traces means that they have a certain impedance at the relevant frequencies. What does this mean? Basically, the lines have to have a certain geometry (width, thickness, height of board) that provides a certain impedance. Why is this done? Because when you have impedance matched, the power transfer is at its maximum. You don't want to "send" power ...

5

I can't answer this authoritatively but my gut tells me your spec is going to be "very difficult". In particular, your transition band of 50 MHz is only 0.02 decades at 1 GHz, so you're looking for a drop of 714 dB/decade between your pass band edge and your rejection band. Which implies something like a 71-pole filter, requiring 71 active elements. For ...

5

When we talk about the "impedance" of a pcb trace, we are talking about the characteristic impedance of a uniform transmission line. The characteristic impedance depends mainly on H, W, and $\epsilon_r$ in the figure above. To get a 50 Ohm characteristic impedance, you just have to define your trace width in the proper proportion to the H of your ...

5

This is either co-planar waveguide, or co-planar waveguide above ground. There appear to be filled vias in the top ground, so these almost certainly go a ground in the next layer down. CPWaG is a nice construction that allows easy transition to microstrip, to connectors, and to surface mount components. This is a demo board for a component. Such a board may ...

5

Your assumption seems to be that filtering only one time is enough. That might be true but it might also not be true, it depends factors like: How much filtering outside your band of interest is needed? If there are strong signals present at frequencies you do not want, you might want to suppress these as much as possible before they end up (with too much ...

4

I ran the calculation for you in Cadence SigXplorer (my favorite tool for this and a lot more): This is a 10.25mil wide trace (sorry units on the image are in metric) to give 50R pretty closely. Always use a 2D field solver for this (as you noticed, formulas are not enough). Be aware that the SMA footprint may not be a smooth 50R without some great care ...

4

One alternative is to use coplanar waveguide instead of microstrip. According to the Microwaves101 coplanar waveguide calculator, if your board shop can do 4 mil feature spacing, you can make a 50 Ohm CPW using a 30 mil trace width: Notice there is no groundplane beneath the trace on the back side of the board for this type of waveguide. If you have a ...

4

Your best bet would be to create a new clearance rule (Design -> Rules -> right-click "Clearance" in the left pane, select "New Rule", open up the new rule you just created (it'll be titled "Clearance_X" where 'X' is the highest number you see) and in the section labeled "Where The First Object Matches", open the dropdown and select "Net". A new dropdown ...

4

The antenna resonates at a particular frequency Not true. A whip (monopole) antenna (for example) produces an optimum output level for frequencies that are close to one-quarter wavelength. It will also produce a decent output level for frequencies surrounding that optimum frequency and the "spread" of those frequencies from "centre" can be considerable and ...

3

ANT-MMCX means Antenna MMCX connector. It depends on your antenna, but basically its just a matched connector. A Microstrip line is a transmission lines structure which is easy to build on a pcb. It consists of a trace over a reference plane with a set width and distance to produce 50 ohm impedance. You can find plenty of calculators online to calculate ...

3

I would like to route my USB 2.0 (12MHz) signal on the bottom layer of a 4-layer PCB. However, the edge-coupled microstrips would then be referenced to the power plane, not the ground plane. Simple solution: redraw your schematic. simulate this circuit – Schematic created using CircuitLab Now you can feel better about making a microstrip relative to ...

3

Using formulas to calculate strip/mictrostrip impedance's used to be good enough, but with modern PCB technology, this does NOT work well. On a typical high speed multilayer board you may have something around 100um trace width and 100um dielectric thickness. This puts you outside of the range where the approximations in any of the formulas I have seen works ...

3

The issue I see if you're trying to create a matched termination, is that except for the one at upper-right, your terminations are all short circuits, not matched terminations. Since your frequency band is exactly one octave, it's possible that you could design the length of the CPW from your probe pads to the short-circuit to be approximately 1/8 ...

3

The field patterns on microstip and CPW are very different, and it is the field distribution that determine characteristic impedance. What you have shown is actually GCPW or grounded coplanar waveguide. CPW can be and often is used without a backside ground. *Note for the pedants - the diagram bottom right is slightly incorrect as the E fields should be ...

3

You will lose some power, but probably not an unacceptable amount - especially if the input of the receiver is in the 40 to 50 Ohm range. I get a gamma of -0.19 for coax to ms which is less than 0.2 dB power xfer loss - not the end of the world :). However if the receiver input were, say 100 ohms, you'd get a gamma of about 0.5 there and lose 1/4 of your ...

3

'The' quadrature coupler does not relate to microstrip transmission line. If you bring a second transmission line 'close' to a first, so that some field is shared, then you get two coupled transmission lines. You can do this with any sort of transmission line, stripline, microstrip, co-planar or slot-line. You can even use coaxial or waveguide for ...

3

This answer just covers the part of the question about the reason for gold plating. The gold finish provides a smoother surface than most other available finishes. Roughness increases the loss of microwave transmission lines. See this article on surface roughness for more details. The gold on the top of the lines helps some, but for coplanar waveguide in ...

3

Microstrip simply means a controlled-impedance trace on one of the surface layers. A stripline is the same, except on an internal layer. A Microstrip is just a regular track but you determine its required width in order to have a specific characteristic impedance (50 ohms in your case) based on the copper weight and distance between it and the ground plane ...

3

You can design your via to minimize the discontinuity it introduces in your transmission line. Basically this means balancing the inductance introduced by the via with the capacitance between the via and other nearby conductors (such as plane layer copper, etc). The ideal geometry won't just specify the via diameter, but also the keep-out diameter around ...

2

The reflection at the end of a transmission line is given by $\Gamma = \dfrac{Z_L - Z_0}{Z_L + Z_0}$ Where Z0 is the line's characteristic impedance and ZL is the load impedance. So if you have a 55 Ohm line and you terminate it with 50 Ohms, you're looking at about 5% reflection. If you have a 50 Ohm line, followed by a short length of 55 Ohm line, ...

2

My suggestion is to take information from your PCB manufacturer as primary, and the online calculators as a double check. If you specify controlled impedance, with a tolerance, then you'll normally have to pay something of a premium, depending on the tolerance required. 10% or 5% are common tolerances, though most high-end makers say they can do better (...

2

The Heaviside Condition $\frac{R}{L}$ = $\frac{G}{C}$ or $R C$ = $G L$ or $\frac{R}{G}$ = $\frac{L}{C}$ Why isn't the Heaviside Condition more widely applied? Heaviside getting no respect even after all these years. More likely just the practicalities (impracticalities) of making the equality true. Heaviside came up with this idea as a ...

2

Is this helpful? I'll delete this if it isn't etc... In all circumstances of signal the characteristic impedance for a transmission line is: - $Z_0 = \sqrt{\dfrac{R + j\omega L}{G + j\omega C}}$ At low frequencies (below 100 kHz) where R and C dominate characteristic impedance is: - $\sqrt{\dfrac{R}{j\omega C}}$ and this basically means the input ...

2

The whole point of the DC bias network (TL3 and C1) is to present a very high impedance at the junction of the three TLs, in order to perturb the RF signal flowing through TL1 and TL4 as little as possible. At these frequencies, the parasitic effects of every component must be considered. For example, even a tiny SMT capacitor has a certain amount of ...

2

Dielectric constant for most PCB materials fall with increasing frequency. This is a logarithmic curve, so in practice it's fairly flat from 1-2GHz and up. That is the reason I would calculate characteristic impedance at 1-2GHz and be happy with that even at much higher frequencies. Notice that you may easily have +/-10% production tolerance on ...

2

The skin depth doesn't really have much, if anything, to do with impedance. It simply defines a loss mechanism. Note that with a thicker track, you will have marginally less skin effect losses due to a larger surface area for the track. The impedance of microstrip is inversely proportional to the thickness of the signal track, but the difference for 0.5 oz ...

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