DSA815 10Hz RBW capability - Page 1
Forum section: electronics / testgear

author: johnny_canuck
author_rank: Contributor
timestamp: September 12, 2013, 12:11:16 am
content: I kept wondering why Rigol doesn't offer the 10Hz RBW option. So, I fed my SA with the attenuated output of a small QRPP CW transmitter that I had built some years ago, the ARRL Tuna Tin Two. It's a crystal controlled, 300 mW CW rig which I powered from a 12V lead-acid battery. You can see in the attached screen pic that the SA can't resolve anything for about 25 Hz either side of the centre frequency. Anything 10 Hz away would be swallowed up.I would prefer that someone try the same thing with a known to be spectrally pure source such as an HP 8640B [like the one that I foolishly sold :-( ].That would tell the tale for sure.My suspicion is that they just couldn't get for-real 10Hz RBW capability out of the hardware, so just omitted it as an option.Ken

author: JackOfVA
author_rank: Unknown
timestamp: September 12, 2013, 12:20:37 am
content: Looks like you have the vertical axis set for linear, not log.That being said, the -3 dB bandwidth is just a hair over 10 Hz - reading it as 70.7% I come up with maybe 12-13 Hz.Remember that a spectrum analyzer filter is not designed like a typical CW or SSB filter where the 6:60 dB shape factor is 1.5 or 2, with steep sides. Rather a spectrum analyzer filter is usually a Gaussian type filter with a very gentle frequency versus attenuation slope and a rounded peak response. This is so that the spectrum analyzer responds to signals as they pop in and out of the passband with a minimum of ringing or overshoot on the display.So, from your image, it's not at all clear to me that your spectrum analyzer is doing anything other than working as it is supposed to.

author: JackOfVA
author_rank: Unknown
timestamp: September 12, 2013, 12:32:11 am
content: I thought I could quickly find some plots done with my HP8568B spectrum analyzer in 10 Hz resolution, but can't locate them on the hard drive. But the attached 30 Hz bandwidth plot may provide an idea of how a properly design spectrum analyzer filter response appears.The plot overlays four signal sources - red and dark yellow are two HP8657A synthesized signal generators - while I like these generators, they are really poor when it comes to 60/120 Hz spectral contamination. The two other traces, cyan and blue, are from Chan 1 and Chan 2 of a Rigol DS-4102 DSP-based function generator.You will note the 8657A signal generator have appreciable phase noise as well as 60/120 Hz modulation compared with the DS-4102. And yes, the 8657A generators are within HP's specs as the unwanted signal level spec is for +/- 5 KHz from carrier as I recall it.

author: orin
author_rank: Frequent Contributor
timestamp: September 12, 2013, 03:10:32 am
content: Quote from: johnny_canuck on September 12, 2013, 12:11:16 amMy 8640B isn't great, but here is a plot using an 8568* at 10Hz RBW/VBW.https://picasaweb.google.com/lh/photo/v-Jhd8gsghYQP95wQ2pTXdMTjNZETYmyPJy0liipFm0?feat=directlinkAs far as I know, the 40 ish Hz sidebands are for real. They were at 35 Hz in this particular plot. I've seen other 8640B plots with similar sidebands.I should do a plot from my Si570 oscillator - it looked better last time I tried.Orin.*I don't remember whether it was the 8568A or 8568B RF unit. Doesn't matter as far as the RBW/VBW is concerned.

author: casinada
author_rank: Frequent Contributor
timestamp: September 12, 2013, 06:24:56 am
content: Ken,This is confusing. Did you enable the RBW 10Hz option?Thank you

author: Orange
author_rank: Frequent Contributor
timestamp: September 12, 2013, 07:00:36 am
content: 200Mhz 10% AM modulated with 400Hz, -30dBm at 10Hz RBWOn a DSA815_TGSignal gen is a HP8657B, both analyser and generator locked to a Rb sourceAdded the same signal, but with 100Hz RBW

author: Mr Simpleton
author_rank: Unknown
timestamp: September 12, 2013, 07:04:05 am
content: Even if the LO in DSA815 has too much phase noise to make use of the 10 Hz RBW to resolve close in signals it may come in handy to improve the sensitivity. Noise floor will drop 10 dB, makes it easier to find weaker signals.Someone did post a plot where one could easily see the benefits of having 10 Hz RBW vs 100 Hz.Olsenn did show some plots, look at #930:https://www.eevblog.com/forum/testgear/sniffing-the-rigol%27s-internal-i2c-bus/930/

author: gilbjd
author_rank: Contributor
timestamp: September 12, 2013, 07:57:52 am
content: As with Olsenn's report cited above, I can confirm that using 10 Hz RBW with my DSA815 easily resolves the close-in sidebands of signals with very low audio frequency AM and FM modulation (eg. modulation with 20 - 30 Hz audio) generated, in my case, with a Rigol DG1022 function generator. These sidebands disappear into the broader peak displayed at an RBW of 100Hz.

author: gilbjd
author_rank: Contributor
timestamp: September 12, 2013, 09:00:54 am
content: I've exaggerated slightly in my post above. Try 50 Hz modulation rather than 20 - 30Hz.In the plots below:Rigol DG1022: Carrier at 7.0 MHz, amplitude 1.4 mV RMS. Modulation: AM, depth 10%, modulation frequency 50 Hz (sine)Rigol DSA815: Centre frequency at 7.0 MHz, span 1 kHz

author: johnny_canuck
author_rank: Contributor
timestamp: September 12, 2013, 07:44:18 pm
content: To Casinada: Yes, I entered the 10Hz RBW option. It is readily available here on EEVblog.I found the Agilent 'Spectrum Analysis Basics' booklet. It explains that "When measuring two signals of equal amplitude, the value of the selected RBW tells us how close together they can be and still be distinguishable from one another (by a 3 dB 'dip')."I guess that I was expecting to see the entirety of both signals spaced 10 Hz apart. That would probably require more like a 1Hz RBW(!). A tough act, indeed :-)Thanks for everyone's insight.Ken

author: casinada
author_rank: Frequent Contributor
timestamp: September 12, 2013, 08:45:56 pm
content: Thank you for the answer.I'm looking for a spectrum analyzer and it is hard to want to spend that much money on used equipment that I can't probably repair or calibrateIt would be nice if the 815 had a little bit more bandwidth otherwise seems to be the only choice for hobbyists that want a general purpose SA.

author: orin
author_rank: Frequent Contributor
timestamp: September 13, 2013, 04:05:17 am
content: Quote from: casinada on September 12, 2013, 08:45:56 pmSpeaking of calibration, whatisthe cost to calibrate an 815?I was about to agree that the 815 with the RBW hack would be the best bet these days, and... hmm... yes a calibrated HP 8568x should beat it, but calibrating the 8568x will be the best part of $1k and well... those 10Hz RBW plots looked pretty good...Well, I thought my 8568x was the better option, but now I'm not so sure. The equipment required to keep an 8568A/B going and somewhat calibrated is going to be much more than the cost of a new 815.Now I'd like to see a side-by-side comparison of an 815 with an HP 8568A/B - anyone in the Seattle area with an 815?Orin.

author: G0HZU
author_rank: Super Contributor
timestamp: September 13, 2013, 08:07:33 pm
content: I think it will be difficult to make a meaningful comparison between the two analysers.The Rigol is a low budget analyser aimed at keen hobbyists and its RF performance will be quite limited in most areas. It does offer the benefits of the digital IF and a nice display and light weight though. The major benefit of the digital IF will be very good log accuracy and steeper RBW filter responses and I'd expect it to beat the 8568B in this respect.But where I expect it to perform relatively poorly (compared to the 8568B) will be for making casual harmonic measurements or for measuring third order distortion terms and also it will suffer from inferior measurement uncertainty due to the basic design of the RF up/downconverter it uses ahead of the digital IF.The phase noise is also markedly worse on the 815. I'm not sure how much value a calibration will add to the 815 operation because it suffers from such high measurement uncertainty anyway. So I would see the calibration as a false economy unless it was needed to satisfy a business requirement.I've had an 8568B here for many years and I've never had to spend any maintenance money on it. I would expect it to be more accurate than a new 815 for most tests even though I have never formally sent it for calibration. I can check it here using a few bits of test gear that cost a fraction of the cost of a calibration.Most of the time a formal calibration is only needed to satisfy a business case and isn't needed for a home user. I definitely would NOT trust my 8568B to any calibration house to meddle with unless it went to Agilent/HP and they don't support this instrument any more anyway. So it stays here.Many years before I bought it from my workplace it was NEVER allowed to go anywhere but to HP for calibration etc because nobody outside HP was trusted to meddle with it.

author: johnny_canuck
author_rank: Contributor
timestamp: September 29, 2013, 04:25:22 pm
content: I made a 50 MHz, 0 dBm source using a Crystek CVSS-945 Ultra Low Noise Sine Wave VCXO and a -6 dB pad.It uses a 9V battery feeding a 78L05 regulator.My Rigol has the 10Hz RBW licence enabled. I fed the source into it and measured 12 Hz RBW using the Rigol's 'N dB BW' function set for -3dB. see page 2-86 in the User's Guide. See the attached screen view taken using a single sweep.All of the RBWs seem to be a bit wider than their nominal values.Still a darn good value.Ken