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How accurate is the calibration with the Bessel null tone? the audio level of my transmitter
seems higher than most stations.
when I do the calibration through the analog stl the audio level rise with a 1.5 up 2db
i measure with an HP spectrum analyzer + the build in 31187.6 Hz test toneIt should be dead-on accurate! Bessel null is the most accurate way there is.
What preset are you using? Are you sure you nulled it properly? Please post a photo of the spectrum analyzer.
Best,
///LeifI will make a picture a minute, I think it has to do with the stl, because the quick sweep is looking a little strange .
here i have the pictures http://home.planet.nl/~lind1489
the ‘Bessel Null STL’ picture is measured according to the settings on your explanation, the other pictures with changed settings40dB down doesn’t tell you enough on the first analyzer photo. One major problem I see is the two analyzer photos have different settings. The first photo has RBW/VBW’s of 3kHz, the second has RBW/VBW’s of 10kHz? You’re not consistant with your test setup comparisons to be meaningful. Peak amplitude accuracy comes at the expense of narrow RBW and VBW’s (and slow sweep speeds).
In order to get precision measurements you need to be measuring with a RBW of 300Hz, with VBW of 100Hz. The narrower the better. Just be prepared to spend some time waiting for the slow sweep speeds to write the trace to the screen. The reward for your patience is very accurate amplitude measurements of the carrier null. Remember you don’t need to see all the modulation sidebands during these measurements (hence your widebandwidth settings), just the energy right at the carrier center freq, +/-100Hz. So zoom into this region and wait.
i know about the different settings, with the following settings:
Center frequency (CF) = f0 (carrier frequency of the transmitter)
RBW 10 kHz or lower (IF filter)
VBW 10 kHz or lower (video filter)
Span: 340 kHzis the carrier null but with different settings i cant get the carrier lower (not null????)than show on the picture.
either when i set the audio higher or lower, the carrier comes up.The carrier should really be able to basically disappear. The point where it completely nulls, that’s where you’re at +/- 75 kHz with the 31185hz bessel null tone.
I’m not sure why you’re not able to get it to null.
Check and make sure you’re really seeing the first null. Start by turning the exciter modulation level all the way down. You should see just the carrier, no sidebands.
Then, as you turn the modulation up, you should see sidebands appear and grow outwards, and you should be able to find the first null. That’s where you want to be
///Leif
As I stated, at 10kHz RBW/VBW’s you will not see much change in the carrier null especially if your trying to see a few dB of difference. It takes only a very small amount of deviation change to go from 40dB down to 70dB + down. For example with my lab instruments I can suppress the carrier right down to the noise floor of the spectrum analyzer. But I did it using RBW of 500Hz and VBW of 100Hz (old Tek 492 analyzer) and 0.1kHz carrier deviation steps. A modulation amplitude variance of +/5mV of results in 20dB of null depth change when starting from a nulled condition.
You need to first set your analyzer on exact center frequency of your transmitter with no modulation using the narrow RBW/VBW settings. Once you have this centered, apply the modulation tone and make adjustments to the deviation. You will see it doesn’t take much adjustment range to move through the exact Bessel null point. Using high modulation indexes, the first null point is very sharp.
Other questions:
1. Have you verified the generated modulation tone is actually 31185kHz? A +/-1-2 kHz variance won’t matter very much.
2. Have you checked the spectral purity of the demodulated STL output? Additional receiver noise or STL TX or RX PLL spurii will mask your null measurement point with unwanted modulation artifacts preventing you from seeing the sharp null. This has the appearance of softening the null point leading you to believe there is no adjustment range.
3. Is there any audio agc circuits enabled in the STL link (or exciter) that can interfere with your measurements?the problem is found, there are actually 2. I put too much audio in the stl which he became inaudible deforms, clearly visible on the scope. The sine of the Bessel null was totally deformed. Also the transmitter mpx-level control is not precise enough. even with breakaway’s ‘mpx ref level’ it’s not precise enough.Maybe you can make mpx-level to fine-tunein bbp?
I’ve seen that it is indeed possible for the carrier to get exactly 0. but I can not precise enough control.Even though you may not be able to get an exact null, the 0.1dB steps on the MPX ref level control are fine enough for setting the MPX level. Each step is less than 1%, so you’ll easily be able to hit 100% modulation — and even if you have to set it 0.1dB quieter, that’s not an audible difference 🙂.
Best,
///Leifthat’s a way but it looks very nice on the spec as it is totally null
still a question if I have a breakaway gecalibreert Bessel null. the deviation is always 75KHz no matter what preset i toke or drive +1 or 2db?
Yes, as long as there are no overshoots induced by sound card, stl or exciter. The most common reason for overshoots is poor infrasonic low frequency response in poor exciters.
A good inexpensive exciter is Broadcast Warehouse PLL+ 1W.
More expensive exciters are usually good. Any audio processor (regardless of brand) has the same exciter requirements, so if it does well with other processors, it will do well with Breakaway.
///Leif
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