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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My Little Spec An

So yesterday I got to designing a RF Sampler for my little spec an, and one thing led to another and so i connected it to the FeelTech function generator, fired up a sine wave and with the maximum amount of data points over the smallest span the software would allow I took a look at how clean the signal is. Not sure about those 2 spurs, but if they are real and not artifacts, that’s not a good thing.

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NANO VNA Calibration

So long story. I have had some trouble getting the Nano VNA to hold a calibration and display the correct information. I always had a -10db offset when using certain SMA leads i had there. It would display fine with semi rigid SMA leads, but, others would show -10db. Like the leads had loss in them.

Well anyway, I think i have resolved the issue and got the calibration right. 0.5db loss in my leads would be about right and that is what is showing now, they are after all cheapest crap leads from China, not high end leads you would use in a lab.

So anyway, popper calibration procedure.

Open: Open Load on S11 port.
Short: Short load on S11 port.
Load: 50 ohm load on S11 port.
Isolation: 50 ohm load on both S11 and S21 ports.
Through: Shortest high quality 50 ohm cable connecting S11 and S21 together.

Then save that. That is it, that should then give you fairly accurate, well as accurate as the NANO VNA is results. As you can see by the plot of a 40m bandpass filter above, it looks about what you would expect from a known design that has low insertion loss. A couple of dB, made up of the lead loss and the filter loss. This means my filter has about 1dB insertion loss. That is something I can live with.

Anyway, through no fault of my own, I broke the mini USB socket off the original NANO VNA i bought and tore the tracks off the board. Yeah not really happy with myself but it is what it is. So i bought another one, this time it seems to be one of the better clones, it even came with shielding and a battery. So anyway, i plan this time to ruggedise my NANO VNA and mount the whole think into an aluminium box, and have an panel mount USB port on the side that will take the abuse of me pulling and stretching and inserting a lead in and out on a regular basis. So i ordered one of these as well, yeah the new VNA came with USB type C not that stupid micro USB rubbish. Anyway, by the time i mount this in a box, i will never have to worry about breaking the damn socket off the board again.

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RF Coupler Testing

Did some dicking about with the VNA today and tested the rf coupler i built. Not sure how i interpret the data, but i have it LOL

 

The through port return loss. I figure what this means is from 1 to 100meg the through port has a quite flat frequency response.

Port Isolation. I figure what this means is i have a coupling factor of say -18db or near enough.  I might have to rebuild it with some more turns, I was looking for at least 30.

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Nano VNA

Ok, so these are my initial thoughts of the device. Firstly, the only issue I have so far is that Windows does not always detect the USB device. I am not sure exactly what the issue is, low voltage, USB drivers kind of flaky or what. But powering it from an external 5v supply the device turns on every time. So i think its windows and computer related, not the device.

On to testing. Not a lot of time today to do anything flash, so I ran a bunch of S11 measurements on things i had handy and the plots were as expected. I do not really have anything here to test in the upper frequency ranges, being a HF kind of guy, but on measurements up to say 100meg, where I have the ability already to compare against another tool, the measurements look legit to me.

First up, calibration is easy and it comes with open, short and 50ohm loads to calibrate to, apparently I have the 900meg firmware installed, if i can work out how to change that to use the 300meg firmware I will. I dont need to measure beyond a couple hundred meg at best anyway.

First up to be tested was the Tri-Band vertical. Yes it is poorly tuned LOL and the SWR plot is correct. LOL I need to fix that 😉

This is the smith chart for the tri bander.


And this is what the LCD screen is showing while testing the tri-bander.

Next cab off the rank was my home brew dummyload. I knew already it was good out to about 100meg, but did not think it would be fine out to 300. Guess i did ok with a handful of resistors LOL

Next up was a 70cm antenna off the MD380 DMR HT. Now i have no idea how accurate this is, but it looks like what you might expect to see with a dip where it is meant to be.

And this is what the device was displaying while testing.

All in all, so far for S11 measurements the nanovna has done exactly what it was designed to do. I was a little surprised that it is as accurate as it is. This is not a very expensive tool at all, like $90 Australian delivered from ebay. Add this in with the rest of my cheap test gear and I have a good selection of gear for not a lot of dollars. When i get some time I will do some 2 port measurements and see how it goes. I expect it will perform quiet well. Bring on the filter testing 🙂

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