Electronics > RF, Microwave, Ham Radio

RF Sampler Question

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vk4ffab:


Ok there are these rf samplers on ebay, apparently rated to 50w. The voltage divider i can work out the values for, but what would be the value of the in and out series resistors and what would their rating need to be to achieve 50w pass through?

TheMG:
Looks sort of like a T-pad attenuator with a sample port added.

Odd arrangement for an RF sampler but I guess it makes sense if the coupling factor is only a few dB in order to maintain the impedance as close as possible to 50 ohms.

With most RF samplers such as would be used to connect a spectrum analyzer to the output of a transmitter, the coupling factor would be 30dB or more in which case the sample port has so little effect on the input and output impedance the series resistors are normally omitted entirely.

Can you provide a link to these samplers you speak of? What is your intended use and frequency range anyways? Do you require this sampler to have a flat frequency response?

vk4ffab:
https://www.ebay.com.au/itm/NEW-DC-2GHz-40dB-ultra-wideband-RF-sampler-SMA-interface/283235697566?hash=item41f229c39e:g:F-IAAOSw48Vb2Blp

I am only really interested in in HF, as long as its flat to 30Mhz im happy, if its flat into the 2m band, winner winner, beyond that my test gear becomes kind of useless. If its just a resistor divider those resistors dissipate bugger power and so i was wondering where they get their 50W rating, as the pass though trace should be large enough to take much more than that.

TheMG:
Being a resistive sampler it should be flat from DC to a fairly high frequency. Its high frequency limit would be dependent on how much attention to detail has been put into controlling the impedance characteristics of the resistors and traces but I would expect 144MHz should be easily achieved even with a relatively crude design.

As for the diagram it might just be a standard diagram they use across a range of different samplers, and in this case the values of those two series resistors could very well be "zero". Guess you'd have to take one apart to find out.

A very common type of sampler used for LF through to HF frequencies is a current sampler, such as this:

http://www.collinsradio.org/wp-content/uploads/2015/05/Build-a-Quality-RF-Power-Sampler-Jackson.pdf

I built one a few years ago from scrap parts I had lying around and it works quite well up to quite high power levels.

vk4ffab:
Yeah I am starting to think that the series elements are just transmission line and not actual resistors.

I found this online, I think i will just copy it like all good professionals LOL

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