Measure RF signal and power - Page 1
Forum section: electronics / beginners

author: dhiru90
author_rank: Newbie
timestamp: April 11, 2016, 05:17:47 am
content: Hello,I am learning SDR(Software Defined Radio) and also learning basics electronics to meet my SDR requirements.I would like to know some not so costly way to Measure RF signal(frequency,sine/square etc.) and their corresponding power so that i may not risk my SDRs or to watch what is flowing in my circuits.For Eg. GPSDO emits SINE WAVE,1Vrms (10-15dBm) and square wave,3.3/4.7Vpp signal. How can i measure/confirm the signal gpsdo claiming.Also pls help me in identifying/confirming the RF signal and power I may be transmitting by SDR.I may be studying with DC - 6/8 GHz and require reliabily accurate measurements.What all equipments like frequency counter/meter, oscilloscope or something else which can meet my requirements economically.Apart from SDR, Right now i can do basic soldering, very very small test PCB designings, working on breadboards with adruino etc.Thanks.

author: dhiru90
author_rank: Newbie
timestamp: April 12, 2016, 11:26:31 am
content: Thanks evb149 for detailed reply.It took me a more than a day to browse all the links and pdfs due to non-electronics background.But i got fairly good understanding to protect my SDRs.Now pls can someone help me how will i identify the input signal ie sine/square or cmos square etc. and at what voltage.

author: danadak
author_rank: Super Contributor
timestamp: April 12, 2016, 11:56:40 am
content: That would be done with Oscilloscope if you want to see the time domainsignal. Triggering can get tricky due to modulation unless you are justlooking at a simple AM carrier. Note also scope has have BW to handlesignals of interest. Depending on probe point in circuit loading effects ofprobe can be another issue. There are wideband FET probes available,or you can build one.https://www.eevblog.com/forum/suggestions/scope-probes-teardown-and-diff-probe-investigation/Regards, Dana.

author: tggzzz
author_rank: Super Contributor
timestamp: April 12, 2016, 01:19:02 pm
content: Quote from: dhiru90 on April 12, 2016, 11:26:31 amI think you will find that is an invalid question. You need to think more aboutwhatyou want to seeand why; then you will be in a better position to ask "how".Sanity checks... What does an FM audio signal look like? What does an AM audio signal look like? What does an FM data signal look like? And then ask whether or not it matters.As for an input voltage, nobody thinks in those terms.

author: dhiru90
author_rank: Newbie
timestamp: April 13, 2016, 02:00:43 am
content: For Eg: I would like to verify/confirm that GPSDO is emitting 10Mhz sine/square/cmos and at what V level.Also suppose if i change GPSDO output to other for Eg. 12 or 20Mhz reference signal by si5338 clock or something else then I just need to verify my output signal is same as expected after my newbie error-prone mods.Is it ok to believe on data-sheets and conforms to their working/outputs without any questionings.As a reverse engineer its not uncommon for me to see mismatch values in cpu registers even for very high end and tight softwares where 1bit flip leads to catastrophic unpredictable behaviour as in crypto or memory mappings. But then its common for me and so my nature to verify everything.

author: tggzzz
author_rank: Super Contributor
timestamp: April 13, 2016, 02:17:37 am
content: Quote from: dhiru90 on April 13, 2016, 02:00:43 amThat's not a sufficient specification.Nothinggenerates a 10MHz square wave. Many things generate a more-or-less accurate 10MHz square wave.What level of accuracy (and there aremanyways of defining that) interests you? Depending on your answer, there are many possible tools ranging from two LEDs to a very high-end spectrum analyser.QuoteNo. You have to use engineering experience and judgement to assess what is likely/unlikely to be a problem, and then to test that. Ensure you distinguish between design defects and manufacturing defects.

author: dhiru90
author_rank: Newbie
timestamp: April 13, 2016, 03:06:59 pm
content: Quote from: tggzzz on April 13, 2016, 02:17:37 amI would be requiring around 9 digits accuracy. As in first post i said i will go for the economical solution.QuoteWith due respect, I would like to use your engineering experience and judgement. So pls suggest me the economical method/device to identify the signal frequency and type of waveform. I have started feeling that observing complex waveform can be very demanding and expensive.No cheap solutions exists as of right now to play with them.But i would like to know of some good method to at-least understand first upto 10 Mhz signals and require good frequency counter with power measurement of upto 6/8 Ghz.Should i have to go for frequency counters and oscilloscope methods for my requirements.But i come from a place where importing equipments doubles your cost, so i have to make a wise choice, so that it can become my lucky calibrated equipment very first time. Even if someone will donate me his equipment here i will have to think twice as there are more than 99% chance it would be evaluated twice the actual price and will be charged very high for clearing.Thanks.

author: German_EE
author_rank: Super Contributor
timestamp: April 13, 2016, 03:38:41 pm
content: Try this:http://www.ab4oj.com/test/pwrmeas.html

author: orolo
author_rank: Frequent Contributor
timestamp: April 13, 2016, 04:51:57 pm
content: The diode detector is the classical easy tool to measure RF power. I have heard about an alternative, but never got the time/drive to test it: an infinite impedance detector. The idea is to bias a JFET just at its threshold voltage, and use it as a detector. I think the idea originated in with tubes, a long time ago. The advantages should be uniform, very high input impedance, good behavior at very low power, and a quadratic response. The reason I have never tried this is that the threshold voltage is heavily dependent on the specific JFET. Has anyone tried this?

author: tggzzz
author_rank: Super Contributor
timestamp: April 13, 2016, 08:46:34 pm
content: Quote from: dhiru90 on April 13, 2016, 03:06:59 pmLet me ask you a question: what is your heart rate in beats per minute? The answersto that will illuminate how poorly the question is formed.QuoteBasically, if you are thinking ofrequiringa GPSDO and 9 digit "accuracy", you must be making measurements for very specificpurposes. I have no idea what those are, so I cannot provide an answer.You need to formulateyour"touchstone" or key questions. The answers to those questions will determine whether you do X or Y. Hence you must first formulate X and Y.Thinking about the answers to the heartbeat question will help you formulate your ideas. By all means give answers here, for discussion.