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I do not have any experience designing RF circuits and I am finding many challenges for my project.

One of them is to match the 50 ohm impedance that some components demand. In the input of my PCB I will have a mixer (RF=10 to 15 GHz, LO=15 GHz and IF=DC to 5GHz), which needs these 50 ohm in the RF, LO and IF pins.

The inputs, RF and LO, will go connected to SMA connectors (is that enough for the required impedance or I have to design the PCB trace to match the 50 ohm?).

The output, IF, needs the load of 50 ohm, which I guess I can get with the PCB trace. The thing is that right after it I'll have a LC low-pass filter at 1 GHz cut-off frequency. Will this 50 ohm transform the filter into a RLC?

All these questions may sound a bit stupid for RF engineers, but for me is very confusing. Everything is so easy in the DC domain..

Thanks

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  • \$\begingroup\$ If the connections are less than 1/10 the wavelength you needn't bother about the track impedance. This is probably unlikely at 15 GHz, though. \$\endgroup\$ – Leon Heller Dec 8 '18 at 11:00
  • \$\begingroup\$ @LeonHeller the 1/10th of a wavelength is a guideline/rule of thumb (similar to '-10 dB reflection = a good match') \$\endgroup\$ – Joren Vaes Dec 8 '18 at 11:28
  • \$\begingroup\$ If you use KiCAD it has a built in calculator for doing transmission lines (what you need for proper impedance matching.) Otherwise, there are online calculators like this one that you can use. Look for "transmission line pcb calculator." \$\endgroup\$ – JRE Dec 8 '18 at 12:27
  • \$\begingroup\$ Make sure to adjust the permittivity for the laminate you're using (RO4000 series or equivalent). You should also call out the controlled impedance traces to the the PCB house so they can tweak the widths to match their current board stock. Not having any idea of what you're after in terms of performance, you're going to (by necessity) get generalized answers. \$\endgroup\$ – isdi Dec 8 '18 at 12:40
  • \$\begingroup\$ Related questions: How is xΩ impedance cable defined? and Should each trace carrying RF be 50Ohm in characteristic impedance? How?. It's worth saying that if you need to ask these questions then any project involving 10 and 15 GHz signals is... very ambitious, to say the least. \$\endgroup\$ – The Photon Dec 8 '18 at 17:36
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You have to match the impedance on the transmission line. The fact that you are operating at 10-15 GHz only makes this more the case.

Regarding the filter: you should design the filter such that it presents a 50 Ohm input impedance within your target band. Make sure your components behave up to high enough frequencies, as the self-resonance frequency of your components might make them behave very different at 5 GHz than at near-DC (your inductor might suddenly be a capacitor at a few GHz!).

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  • \$\begingroup\$ Thanks, then I don't take the SMA impedance into consideration for the design? \$\endgroup\$ – Martin Dec 8 '18 at 10:48
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    \$\begingroup\$ the SMA impedance should already be 50 Ohm. The manufacturer will probably have a recommended footprint to provide good matching performance. \$\endgroup\$ – Joren Vaes Dec 8 '18 at 10:49

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