RF Sampler

Many pieces of test gear required quite low input levels, in many cases that is 0dBm (0.001W) or less. So what do you do then if you want to characterize the bandwidth of an amplifier that is putting out say a couple of volts peak to peak? You can hope like hell that you do not blow the front end out of your test gear like that expensive spectrum analyzer, you could use an attenuator, but that is going to be limiting as power levels go up beyond a few watts, or you can use a RF sampler of some kind.

Now there are some pretty fancy RF taps and samplers out there costing many hundreds or even thousands of dollars and quite frankly while not out of my budget, buying a second hand tap off an ebay seller for a few hundred bucks to use on my spectrum analyzer that cost a few hundred bucks, seems like overkill to me.

An RF sampler is not a very difficult item to construct at home, a bunch of resistors and maybe a DC blocking cap and Bob is your mothers sisters brother. Sure there are other methods and I know that the fan boys will be frothing at the mouth about using a ferrite toroid and how they are superior hams because one time they bought a resistor yada yada yada, you know the instant internet types.

Anyway, I settled on the resistor divider method with a bit of series and parallel magic and some 1 watt resistors and the magic of the resistor divider not dissipating all the power you get a pretty low cost way of sampling higher powers and the ability to set the amount of attenuation by paying with the resistor values.

 

 

Here is the circuit that i designed in my ecad to make a pcb board.

The astute will notice that I am not using the same value resistors on this board as in the schematic, turns out i used the footprint for the 1w resistors that I have and well had to use 1/2w 1500 ohms instead. The value is kind of non critical anyway it just changed the amount of attenuation.

And finally, here is a S21 gain plot for the HF ham bands. Not quite linear, but -45 to 47dB. Good enough for the kinds of girls I go out with. And its something I will be using quite a bit as I also have a test board I had made so I can test some parts out to see how they perform. First one will be an high bandwidth op amp that might be suitable to use an an IF Amp stage. More on that soon.

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