Help finding a cheap way to measure a <1Ghz signal - Page 1
Forum section: electronics / beginners

author: Atomillo
author_rank: Regular Contributor
timestamp: April 27, 2022, 10:57:43 am
content: Hello:As a part of a physics experiment, I'm trying to generate Barkhausen-Kurtz electron oscillations in normal electron valves, as done here:https://www.elektronik-labor.de/Notizen/Laufzeitoszillator.html.The problem is that I need a way to measure a signal up to 1Ghz (preferably a bit higher) and I don't know a priori the level of the signal (since it is not specified in the above mentioned website). I would like to keep the cost of the experiment under 100 euros (taking into account I already have all the needed valves).A quick look at Amazon revealed several cheap VNAs able to measure up to 1.5Ghz (like this one:https://www.amazon.es/AURSINC-Analizador-Vectorial-par%C3%A1metros-estacionaria/dp/B087G3ZX8W/ref=dp_prsubs_1?pd_rd_i=B087G3ZX8W&psc=1). I imagined I could use this as a spectrum analyzer by simply not connecting port 0 and feeding the signal to port 1.Would this work? Or would I need something different?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 27, 2022, 12:08:16 pm
content: That's a VNA (Vector Network Analyzer) sligtly different than a SA (Spectrum Analyzer). Might work, IDK, but a VNA is rather good for tuning antennas, reflections etc. don't know how high its transceiver goes. Some VNA are using less than 1Ghz, and do some tricks by applying square wave with lots of harmonics to cover higher bands.MaybeADALM-PLUTO, aka Pluto SDR will be closer, and can go up to 6GHz. It doesn't have any (pre)amplifier (1.5Vpp at 50ohms input/output RF IIRC, on either Rx or Tx). Open source design and programmable, has ready made Spectrum Analyzer software available, either from Analog Devices or third party, e.g.:http://www.albfer.com/en/2020/02/21/satsagen-2/https://www.eevblog.com/forum/rf-microwave/adalm-pluto-as-simple-spectrum-analyzerw-tracking/No idea if Pluto-SDR will be suitable for your experiment.

author: CaptDon
author_rank: Super Contributor
timestamp: April 27, 2022, 01:25:44 pm
content: Be aware that Barkhausen oscillations take on more of a narrow band noise signal than an actual stable frequency sinewave oscillation. An actual frequency counter would probably reveal an averaged signal and each update could be different by 1KHZ to 10KHZ or more depending on frequency stabilization components around the vacuum tube. There may also be spurious oscillations at 3 or more separate frequencies. A spectrum analyzer may be your best bet but you are only going to get a 'ballpark' value for frequency. My view on the digital frequency counter is that it will be trying to count 'noise' being disrupted by one or two other sources of noise. Cheers mate!

author: Atomillo
author_rank: Regular Contributor
timestamp: April 27, 2022, 01:39:42 pm
content: Quote from: RoGeorge on April 27, 2022, 12:08:16 pmI just found a supposedly good up to 1.5GHz VNA for 50 euros. If it turns out to be just a cheap mistake, I will look a lot closer to ADALM-PLUTO, which seems to be a much more serious board.Thanks for bringing it to my attention!

author: RoGeorge
author_rank: Super Contributor
timestamp: April 27, 2022, 02:30:08 pm
content: If you buy a small VNA, you may want to read about nanoVNA. Even if it doesn't fit your application, a nanoVNA is a very useful tool to have around the lab.There are some threads around about the nanoVNA and its versions (at least 1 and 2 AFAIK), and the designer is present here, on EEVblog, too. Some manufacturers are better than others.You maty want to google for "nanoVNA site:eevblog.com" if you want to find out more about it, but generally the nanoVNA is very appreciated by the RF enthusiasts.

author: Atomillo
author_rank: Regular Contributor
timestamp: April 27, 2022, 02:48:50 pm
content: Quote from: RoGeorge on April 27, 2022, 02:30:08 pmI was not aware of this way of searching.It is amaizing because one can also search for images inside the forum and also use the google interface.Many thanks!!

author: RoGeorge
author_rank: Super Contributor
timestamp: April 27, 2022, 02:57:24 pm
content: Glad it helps, there are many other operators for querying Google:https://www.googleguide.com/advanced_operators_reference.html

author: Atomillo
author_rank: Regular Contributor
timestamp: April 27, 2022, 05:58:20 pm
content: I just realized one thing: on top of the signal of interest there is the bias of the base. I have been reading about VNAs and it seems they really don't like DC voltages (or any significant power) at it's inputs. Even though I could not find a specific rating for my VNA (it's just labelled NanoVNA). Could just a capacitor and two diodes suffice?

author: Bicurico
author_rank: Super Contributor
timestamp: April 27, 2022, 06:49:17 pm
content: A VNA is not a SA. They measure different things.If you want o visualize a signal in frequency domain, then you use a spectrum analyser. It will allow measurements of bandwidth and signal level, but with less precision than other test equipment made for specific signal measurement.The only cheap SA I would recommend for something close to some precision is the TinySA.Next step in quality would be a Silent SSA/SVA.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 27, 2022, 10:43:12 pm
content: Quote from: Atomillo on April 27, 2022, 05:58:20 pmIt depends. I made a video that covers some of the basics on how to protect the front end. It's not intended to provide a solution as there are too many variables.If the signal is stable enough, you may be able to detect it by feeding the signal into the second port (what ever they labeled it as) and just setting the center frequency where you expect it to show up with a wide enough bandwidth. It's a poor choice but since you already have one, why not try it.

author: Atomillo
author_rank: Regular Contributor
timestamp: April 28, 2022, 05:11:27 pm
content: Quote from: Bicurico on April 27, 2022, 06:49:17 pmHi:I tried to implement something similar to what is done here:I was aware of the numerous limitations but because I wanted a VNA anyway decided to give it a try. But due to the results I very much suspect I will end up buying a spectrum analyzer

author: Atomillo
author_rank: Regular Contributor
timestamp: April 28, 2022, 05:17:45 pm
content: Quote from: joeqsmith on April 27, 2022, 10:43:12 pmHi:I found your video through YouTube and found it very helpful! However I was referecing to making a reading when the signal is superimposed on DC bias, and proposed the following schematic. Could this work? At the high frequencies present I would imagine the capacitor should have some special property.

author: Atomillo
author_rank: Regular Contributor
timestamp: April 28, 2022, 05:27:21 pm
content: The VNA arrived today and did a couple of tests. First of all, connect a low level signal from a FG directly to channel 1 and measure mod(S21) (after calibration of course). This is where I quickly found that in order to have any decent resolution I need to use a low span (for instance, to properly see a 40MHz 2mVpp sinewave the maximum span was about 100Mhz).I then did the simplest test I could think of: simply put a piece of open circuit wire connected to the channel 2 and got it close to the wire where oscillations should be present, like the schematic shown below.I tried various tubes used in the previously mentioned website (the PC86 and the PL504) but unfortunately could not see any peak in the spectrum.Has anyone tried producing Barkhausen oscillations with normal transmitting tubes? What is the expected power output?

author: joeqsmith
author_rank: Super Contributor
timestamp: April 29, 2022, 11:12:56 am
content: Quote from: Atomillo on April 28, 2022, 05:17:45 pmThe video talked about the reason for adding the DC block. Rather than have some DC voltage, I would clamp to the return path only and use a very low capacitance bidirectional TVS like I show but it all depends.I have attempted to use the low cost VNAs to make PDN measurements (see link below) which certainly do have a DC bias (pretty much the point of them). However, I am interested in working with low voltages.