GRUPOS IO
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NanoVNA usable as spectrum analyzer?. Yes, if you know what you are doing
Erik Kaashoek 9/09/19 #2118 If you ignore the phase any signal presented at port 2 should be mixed with the CLK2 and create some response. As a test I applied a 10MHz -50dBm signal. This resulted in the first measurement.. You see the double peak,each about 2kHz wide, caused by the mixing with CLK2 and the single frequency FFT at +/-5kHz on top of the noise like hump of about 48kHz wide. The 48kHz is related to the sampling rate of the ADC. If you increase the sampling rate the hump widens proportionally When removing the test signal the noise floor is flat at -90dB, excellent clean signal! With a 0dBm test signal you get the second measurement. Basically the same picture and no compression underpinning the huge dynamic range of the SA612 and the ADC. But what is causing the "noise" hump under the two peaks? This becomes obvious when zooming in as can be seen in the third measurement. The emerging pattern is spectral bleeding in the FFT you get when you do not apply a good window function and the input signal is not perfectly aligned in a multiple of full cycles. So it is in reality a consequence of the test signal not being exactly matched with the mixer LO and the FFT size. Not a problem when doing regular VNA measurements because then the alignment is perfect by design. So all is understood now and we can test a 20MHz wide scan and see if we get a nice single peak at 10MHz. This resulted in the fourth measurement scanning with 1000 points so each "dot" is 10kHz apart The 10MHz peak is there, somewhat lower due to not perfectly fitting into one of the 2kHz wide frequency samples, scanning only 5MHz would have solved that problem. But there are many many more peaks around 30dB lower then the 10Mhz signal. Removing the input signal gives a nice clean noise floor with no peak above -75dBm. The peaks you get are all result of all kind of harmonics of CLK2 and the input signal mixing in various modes. a real spectrum analyzer does not have this problem because of the LO/IF choice and the various filters So, yes, you can use the NanoVNA as a spectrum analyzer but you have to know very well what you are doing and how to interpret the measurement.     |
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franciscm@... 9/09/19 #2138 Gracias. Me preguntaba lo mismo. Espero mi analizador para realizar pruebas. Un saludo. EA1AWY Javier Muriedas |
franciscm@... 9/10/19 #2155 Please connect the input to exterior antenna, scan the FM broadcast band and post the results with various spans. Thanks and sorry for my bad english! |
Erik Kaashoek 9/10/19 #2158 Well, that is what you should NOT use the NanoVNA for as without filtering the very diverse signals will swamp the SA612 and raise the noise floor. But here you have the result anyway, 1000 point scan with exponential integration. I did use a 15dB amplifier and a small whip antenna.  |
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franciscm@... 9/10/19 #2165 Very thanks Eric. Sorry for the apparent "imperative aspect" of my previous message. My english is basic (and bad). I know what this measure is bad with this instrument, what some carriers are vissibles. I have a spectrum analyzer AT5011 bank (small toy), what I have curiosity for the nanoVNA in this works (up togheter antenna, i. e.). Javier |
Hello again Eric and other possible readers. I received my nanoVNA a couple of weeks ago and I have been doing many tests with it. First I must say my unit came without shields and I added them, but there was no considerable improvement in the base noise level. The second thing I am going to say is that I have been trying to check the operation of the CH1 input as a spectrum analyzer and I have been able see under what circumstances it is more or less usable. First, the level measurements appear to be around 15 dB higher than the signal applied below 300 MHz and around 20 dB higher to over this frequency. I assume that it is because the nanoVNA accepts as 0dB the value applied in its CH1 in the calibration, which is that provided by CH0 and has those values (in the measurement frequency). Known that offset, is not a major problem. Secondly, below 1MHz of span two peaks appear instead of one, as the image frequency also appears separated twice the value of the IF (i.e. 10KHz). In a "real" spectrum analyzer this image frequency is separated at several GHz and is rejected by the input LPF. Of the two peaks, the "real" is the higher frequency (the one on the right). The third problem I have appreciated is that to be recognized, the signal must fall exactly on any of the sampled frequencies because otherwise it is strongly or completely attenuated. Properly choosing the span this inconvenience is soslayable. This makes signals with a significant frequency modulation look very bad on the screen. This contributes to the low refresh rate, which also prevents an AM signal from being displayed correctly and as an extreme case a telegraphic signal. As a summary of the above, to say that the signal of an RF generator without modulation is looked and measured "good" but "practical" signals (the carriers alone do not serve much in real life) look bad. Greetings and forgiveness for the extension and possible-probable inaccuracies and bad english. Javier Muriedas EA1AWY |
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mike watts 9/29/19 #3464 This is an interesting topic. How would I hook it up to use it as a spectrum analyzer? Run the signal into the CH0 and kook at logmag? Show quoted textMike WY6K "... somewhere in the distance, there's a tower and a light, broadcastin' the resistance, through the rain and through the night..." |
Don 9/29/19 #3490 No, you would input the signal into CH1. |