EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: dnessett on September 26, 2017, 11:07:46 pm
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I own a Siglent SSA3021x. I purchased a 10MHz OCXO Frequency Standard Module from Tindie (https://www.tindie.com/products/AnalysIR/10mhz-ocxo-frequency-standard-module-or-kit/?pt=full_prod_search (https://www.tindie.com/products/AnalysIR/10mhz-ocxo-frequency-standard-module-or-kit/?pt=full_prod_search)), which is designed and manufactured by Analysir. I decided to analyze the phase noise characteristics of the signal generated by this product using the SSA3021x.
I set the start freq to 9.5 MHz, stop freq to 10.5 MHz and RBW to 10 Hz. When I viewed the spectrum, there were dual sidebands at 117.333 KHz on either side of the 10 MHz fundamental. These sidebands were at low power (-66.47 db down from the fundamental, which was at 12.33 dbm). So they were ~54 dbm ~= 4 nanowatts. This is not a serious flaw, but I was curious about their cause. By the way, these sidebands disappear in the noise floor at RBW larger than 10 Hz.
So, I contacted Analysir and asked Chris, the engineer responsible for the product, if he had seen this result before. He hadn't. Over a couple of days, we tried to figure out what might be causing these spurs, but we got nowhere. Then Chris mentioned he had found something on the internet that indicated a particular ADC had a maximum throughput rate of 117 KHz. Maybe the SSA3021x used this ADC and the strange frequency offset of the spurs somehow was related to that.
I didn't have the schematics for the SSA3021x, but I figured a simple test of this thesis was to use the SA to analyze a 10 MHz sinewave from my signal generator (see first figure). Lo and behold, the spurious sidebands showed up. So, the OCXO wasn't the culprit, it was the SA.
In order to investigate other hypotheses, I ran some other tests. Perhaps, I thought, the internal 10 MHz oscillator in the SA was interacting with the 10 MHz input and creating these sidebands (this would be rather odd, but at this point I was grasping at staws). To test this, I analyzed a 5 MHz and 15 MHz sinewave. The sidebands showed up in these spectra as well (see 2nd and 3rd figures).
I then thought I should upgrade to the latest firmware. I did so, and the sidebands were still there. At this point, I figure it was time to see if someone else can reproduce this problem. That is why I am posting here.
For the record, my system is characterized by the following information:
HW version: 07.03.00
SW1 version: 1.2.8.5a
SW2 version: 20170214.3
SW3 version: 000000D1
Calibration: 2017.02.19
Another interesting test (which I can't conduct) would be for someone with access to both a SSA3021x and a high level SA to analyze the 10 MHz reference signal it produces to characterize its harmonic distortion. If there is an anomoly at 117.333 KHz, this might be a clue to what is going on.
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I see the same thing you do. I tried two different signal generators with the same result. What I did notice is the peaks reduced after about a 30 min warm up period. I also would see two peaks on each side, I see a hint of those in your traces as well. I watch for many scans and noticed the peaks ( all 4 ) would randomly come and go.
I'm not sure these concern me too much.
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Confirmed. FW: latest 5a.
Set to Avg 1000 and they become clearer.
The USB stick I saved a screenshot to just died and I'll have to do it again before reporting to Siglent.
They can be found after some hunt but as Joel says I don't see them interfering with normal usage.
Anyways, good find. :-+
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I agree with both of you that this should not affect normal use. However, there are rare cases when knowledge of these spurs might be useful. If someone is trying to evaluate the stability of a clock source (e.g., a rubidium standard) by examining its phase noise, then knowing the existence of these spurs in the SSA3021x would prevent waisted effort trying to figure out what is going on.
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It also is told in data sheet.
I have also measured it with my SSA3kX
Input 0.0dBm 10MHz and 100MHz from HP8642B
Mixer level -30dBm (because signal level is 0dBm this means Attenuator set 30dB)
Ref level 10dBm
RBW=VBW=10Hz
Detector Pos Peak
Trace MaxHold (duration over 10 sweeps)
10 MHz , input related spurs ~118kHz LSB and USB, -67dBc
100MHz, input related spurs ~118kHz LSB and USB, -69dBc
In these cases no violation with specifications. Claiming this as "bug" is incorrect "alternative truth".
just for fun, Keysight N9320B
Spurious response (continued)
Input related spurious < —60 dBc —30 dBm signal at input mixer, 20 to 30 °C
All you see in display is fake, more or less. This is good to remember. With what ever measurement equipment. Without any exeption.