Impedance control of PCB

Hello,

I have recently designed the PCB for a prototype UHF Receiver. The receiver operates from 450 - 458MHz. Anyway I requested 1 board from the PCB manufacturer and they asked me if I wanted impedance control. I have already designed for an impedance of 50 ohms. At these frequencies can I expect much variation in the impedance of my boards? Do you think for example a manufacturer of cheap Pagers would bother with impedance control on all the boards?

Thankyou

If you use RF substrate, you don't need the impedance control because you had designed.
If you use FR4 substrate, you need the impedance control by the PCB manufacturer because of the FR4 substrate various.

What frequency range can I rely on FR4 for relatively constant characterikstic impedance then?

Thanks

Less than 1GHz. I had used FR4 for 4GHz without the impedance control because my application is not so precise.
You can change the Er of FR4 and the width of the trace to check the tolerance.

The permittivity (eps_r) of FR4 varies, and this means that line impedance varies. This is independent of frequency.

However, you only care about line impedance when the line segments have a certain criticial length. For short line segments, the line impedance is not relevant because the impedance transformation caused by an electrically short line is very small. Transmission line effects start to be important when you line segments are longer than 1/20 wavelength or so.

Hello, you have already designed your board so you already know the ideal dielectric constant, substrate height etc... Even at that frequency I would try to keep it as close to 50 as possible. I would do a Monte Carlo on your PCB, Microstrip and Stripline including the tolerances specified in your board stackup for the substrate height etc.... and see if you can live with the results. If you have accesss to a RF simulation tool like Genesys or MWO you can do that really quickly.

Engineering also means to understand what is relevant and what is not. We already know that at 450MHz, the lines on the PCB are so short (compared to wavelength) that it makes no difference to the circuit function if they have 50 ohm exactly.

Impedance control in PCB manufacturing means to adjust widths of small signal traces according to substrate and process variations, either using previously determined parameters or test structures on the actual board.

In contrast to the signal traces of high speed digital boards transmission line structures of RF PCBs are typically much wider. They are mainly affected by Er and substrate thickness variations, but can't be easily adjusted by correction factors in PCB manufacturing. If you are intending a series production on a badly defined substrate like standard FR4, you should agree with the manufacturer to use a specific substrate product and adjust critical design structures like transmission line filters according to prototype measurements.

These properties can be hardly addressed by regular "impedance control".