I did this on my IC-7100. I didn't use a dongle but a SoftRock IF plugged into my Elecraft K3 for receive with HDSDR software. I also used the "Data In" pin on the mini-DIN connector with the rig set to "9600 baud" mode. It isn't 9600 at all but a wide bandwidth audio input without preemphasis or other equalization. The first thing I tried was setting the audio frequency to 1250 and increasing power gradually. Before I could see the null, the 7100 stopped transmitting due to excessive audio level, above the nominal 100-300mVp-p. The method I ended up using was to set the audio voltage to 300mVp-p and lower the frequency until I saw the null. That worked and showed the maximum deviation to be 1680Hz. Further lowering the frequency revealed a 2nd and third null, and by looking up those modulation indices in Wikipedia, determined that the resulting deviation estimate was very close to 1680Hz as well. I haven't tried the "1200 baud" port yet but it may well show that it is 3Khz. I think the deviation for 9600 is limited due to the bandwidth required to handle audio in the 3-6kHz range. Thanks again for your informative video.

The peek you see on your tuned frequency is a DC component from the SDR. I have it too and also on the HackRF this is also why you usually tune little up or down to get the best result -- It really has boggled me for the past few weeks since I started working with SDR :D

Really enjoyed this, thanks!

At 16:20 you are adjusting the voltage while calling it out in kHz. The baseband frequency domain display is growing in amplitude but the frequency is constant. You switched back to volts once you get to the Bessel zero. I liked how you used GNU radio to put everything on one display. It's good for us hams who don't have those $20K mixed domain oscilloscopes with built in frequency analyzers.

@MrCircuitMatt

2 жыл бұрын

I totally agree. Thank you!

+Raymond Doetjes I cannot reply to your comment directly, I hope you'll see this. Yes you are right. Among other things, the LO will mix with itself resulting in a 0 Hz difference product which is DC. The approach you suggest is probably the easiest way to get around it. Thanks for commenting :)

Hi there; I can't wait to watch your next video. It just so happens that in the last few weeks I have been searching info exactly about the very subjects that you are touching in this video. Thank you very much; which wireless protocol did you use to read my mind remotely :-) ???? So, seriously, you are basically saying that given that the Mod. Index is related to the deviation and the modulating freq., if my constrains are deviation and M.I., then I can calculate the max audio freq. I can transmit and receive. So, given my constrains are Dev. = 5 Khz and max M.I. = 1 (NBFM, right?) I can transmit an audio signal up to 5 Khz (audio), am I correct? And ..... you mean Deviation 5 Khz total or 5 khz per each side band, so total deviation = 10 Khz (5 LSB + 5 USB)?

@MrCircuitMatt

9 жыл бұрын

Stefano Mollo Hello and thank you. The new video is out :). The relation between M.I, frequency and deviation is as you say and your math seems sound. When I say deviation I actually mean peak deviation, which is the largest excursion of the instantaneous frequency from the center frequency. As for the sidebands, this is where it gets really tricky. If you are feeding a pure sine wave into the modulator, the spacing of the sidebands will be the frequency of that sine. The spacing is not the frequency of the deviation. This is confusing as both are in kHz, but try to picture this: If you increase the deviation by increasing the amplitude of the modulation (just like I do in the video), the spacing of the sidebands does not change. You get more and more sidebands, but the spacing does not change. So it is correct to say that you have a peak-to-peak deviation of 10 KHz with a peak-deviation of 5 KHz, this does not translate into bandwidth. The reason for that is that in theory, FM has an infinite number of sidebansd -- just as there are infinitely many of those Bessel curves in my sketch. However, most of those sidebands have a really low amplitude, so they are sort of ignored when we talk about the bandwidth (not the deviation) of FM. The technical term for this is Carson's Rule and the formula works such that the bandwidth is considered to be whatever is necessary to contain 98% of the signal. I conveniently did not discuss this in the video though :-). Now on a side note, most transmitters and receivers filter out anything that is outside of that Carson Bandwidth. This is why when you are transmitting with too high a deviation (= too large audio amplitude), not only do you potentially interfere with neighbouring stations, but a receiver set up for the assumed deviation will filter out whatever you transmit outside of the corresponding bandwidth and this will not work properly. Another essay written right there. Thank you for this interesting question and thanks for watching! Matt

ThumbNet sold some dongles where basically the oscillator was replaced by a quite ok tcxo, which can easily be done on your own, and it is working so much better that I would say it is worth the effort than all the time being lead to nowhere. And yes, the peak is the LO (it is leaking everywhere in these things) and I usually tune center to quite a bit outside of the interested things, as long as I don't need to process MHz of bandwidth. Programs like SDRSharp have a correction for this already built in and can silence that tone.

@MrCircuitMatt

9 жыл бұрын

Dennis Lubert Thanks for the hint. I indeed tried to source a 28.8 MHz TCXO but failed to do so. 14.4 MHz oscillators on the other hand are easier to get and I've been thinking about doubling that with a mixer. I'm not sure it's worth the effort though. Is the thumbnet dongle still available? Somehow all I can find are t-shirts. I should indeed have tuned somewhere off the center. I managed to do that by mistake (you could see the carrier next to the LO spur), but still not best practice I'd say. In any case I think that moving somewhere far off-center is the best approach, also better than the integration thing done by SDRSharp etc. By the way there will be a second video within a few days (already recorded) where I use this technique to pre-cal a repeater. It worked amazingly well. I hooked it to the service station afterwards and it was spot on! Thanks again for your comment.

@PlasmaHH

9 жыл бұрын

MrCircuitMatt They are currently not available anymore but on www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/ they are promising something similar. I don't think it would be worth the effort to double up the 14.4, for just a little more effort you can probably design a whole better pcb containing better abilities for heatsinking (that thing runs here at 54°C per default and getting that down even a little bit improves the snr a lot), and circuitry for going down below 30MHz or whatever the range was that you can not get down to with these (either by mixing it up, or by bypassing the tuner for these freuquencies and directly delivering). If you look at the pcb of such a dongle, there is really not much to it.

@MrCircuitMatt

9 жыл бұрын

Dennis Lubert That's a nice idea. Really worth considering!

So to visualize, would you connect a 1khz sine wave to the WFM or NFM block? In your example, the ampltude of the signal determines the 'deviation'.. and isn't the same as the bandwidth (i.e. Wide vs Narrow)? GRC doesn't seem to allow the a sine signal source to go to the transmit block.. how would you do that?

@MrCircuitMatt

4 жыл бұрын

Regarding your second question: You can connect the signal source to the transmit block as long as the data type of the source is set to complex (blue output port). If done so, whatever hardware you use to transmit will output a constant carrier signal without modulation at its center frequency plus/minus the frequency you set in the source. If you want to connect the signal source to an FM modulator, you have to set its type to float (orange output port). Regarding your first question: Yes, peak deviation and bandwidth is not the same thing. They are related. In theory, the bandwidth of any FM signal is infinite but in practice, the majority of the power is concentrated in some area around the carrier frequency. The Carson bandwidth formula is one way to quantify this. If your channel bandwidth increases, you want to also increase the peak deviation (and thus the amplitude of the signal input to the modulator) to make the best out of the situation. Bandwidth and peak deviation and signal amplitude are therefore related concepts but they are not the same thing -- the numbers are different. "wide" and "narrow" are somewhat looseley defined terms that we use to talk about systems of a particular bandwidth and peak deviation. In analog systems, they also affect e.g. the choice of IF filter used, and in digital systems they would affect the equivalent channel filter. I hope this makes some sense. Thanks for asking and watching my video! Matt

I tired this, but in the baseband FFT sink window, it shows the Frequency of the tone, NOT the deviation. Any ideas how to fix this?

@MrCircuitMatt

5 жыл бұрын

What you describe is the way it should be. The baseband FFT will show the tone and the channel FFT will have the sidebands and the carrier which exhibits the Bessel null by vanishing at a particular deviation. But of course, since there is a single tone and the amplitude of the modulation increases as you knock up the deviation, you can look at the power of the tone in the baseband FFT to asses the amount of deviation. If this is what you'd like to do, you have to look at how high the peak in the baseband FFT is (vertical axis in dB), not at what frequency it occurs (horizontal axis in Hz). Does that make sense?

@davidkrauss4757

5 жыл бұрын

MrCircuitMatt Yes, but I was hoping for some way for direct readout of deviation. Looking at the baseband level is like hooking an ac voltmeter up to the audio line. It works but it’s not referenced to anything.

Someone else made a video of this in 5 minutes.