Electronics > Metrology

Analysis of some hobbyist oscillators

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dnessett:
For about a year, I have been creating a test setup to investigate "hobbyist oscillators". By this I mean oscillators that hobbyists might use in their projects and also oscillators that don't receive alot of attention from the professionals (and therefore warrant investigation by hobbyists). Candidates for study are retired oscillators bought on ebay (examples: used Morion MV89As and FEI-5650a rubidium oscillators), new oscillators bought on ebay that have little or no published specs (example: low cost GPSDOs, cheap XO can oscillators), and other devices that are not normally studied.

My objective is to create information about these oscillators so hobbyists can determine which are best for their particular projects. The journey to create the test setup was long and arduous and is not fully complete. However, I have enough test equipment in place to begin to look at these oscillators and provide some information about them.

The test setup is documented in this EEVblog topic. It is a record not only of the evolution of the test setup, but also what I learned while buiilding it. Few will be interested in reading the whole topic (which as of this writing, 6/4/2019, comprises 12 EEVblog pages). Those who want just the punchline should read: post 1; post 2, which is partially superceded by post 3; post 4 and post 5, which provide some of the mathematical background behind the test setup (just algebra is required to understand them); post 6, which describes in detail the necessary steps to use the test setup; and post 7, which provides test data showing that the signal from the test setup does not go below the noise floor of the spectrum analyzer used (a PicoScope 4262).

Reading this material is not a prerequisite for understanding or discussing the results of the oscillator tests. These references are provided only for those who might be interested.

The first oscillator I decided to study was the Morion MV89A, a low phase-noise oscillator that I picked up from ebay (actually I bought 3 of them). I built an enclosure for these, which is not remarkable in any way, but for the sake of completeness, is shown in figure 1.



Figure 1 - Enclosure for MV89A.

Note that there is a switch that selects "Ref" or "Adj". When in the "Ref" position, an internal reference voltage (2.5V) is connected to the frequency adjust pin of the oscillator, keeping the output frequency at 10 MHz. When in the "Adj" position, the frequency adjust pin is connectd to the BNC connector shown at the top of the image. This allows the adjustment of the oscillator's frequency so it can be used as a reference oscillator when measuring phase noise of a second oscillator. This feature is not used in the experiments documented in this post.

Also note in the image that the date on the oscillator is 05/06, which means it is 13 years old. This is mentioned for later discussion.

To test this oscillator, I used the frequency discriminator configuration of an HP11729C (see post 3). To ensure the signal exiting the HP11729C did not go below the noise floor of the spectrum analyzer used in the test setup, I ran an experiment comparing this signal with the SA noise floor. The results of this experiment are documented in post 7. The punchline is: the signal stayed above the noise floor.

I ran some experiments analyzing the phase noise of the MV89A. Before collecting data I let the oscillator and the rest of the test equipment warm up for over an hour. I monitored the frequency of the oscillator using an HP5335A frequency counter. Each test took about 25 minutes to run and during that time the oscillator had a stable frequency of 9,999,999.98 Hz. For those who have read the test setup description (others can ignore the rest of this sentence), the HP11729C input from the oscillator was 2.87 dBm, the IF output was 11.3 dBm and the output of the Delay Complex was 2.41 dBm.

The properties of the low-frequency spectrum analyzer (a PicoScope 4262) used for the experiment are shown in figure 2. Note that the gate time of the collected spectrum is 52.43 seconds. Consequently, the results documented here are for short-term phase noise, not for longer periods such as would be studied to characterize the oscillator's use in, for example, long-term time keeping applications.



Figure 2 - Properties of the Spectrum used for the test (on a PicoScope 4262)

One important parameter missing from figure 2 is the averaging scheme used (this is given in a different window on the PicoScope software). Specifically, it should be noted that 30 segments were power averaged in order to create the spectrum.

After correcting the data according to the steps described in the test setup posts, I plotted the phase noise measured, which is shown in figure 3.



Figure 3 - MV89A measured phase noise

The plot reveals two surprises. First, the phase noise at various offsets was significantly above that given in the (MV89A specification ). In particular, the specification asserts that (for a 5MHz oscillator) the MV89A has the following phase noise characteristics (dBc/Hz) - 1 Hz: -105; 10 Hz: -130 ; 100 Hz: -145 ; 1 KHz: -150 ; and 10 KHz: -155. The corresponding characteristics from the plot are: - 1 Hz: -4.27; 10 Hz: -28.52 ; 100 Hz: -46.80 ; 1 KHz: -65.15. This represents a difference of almost -100 dB. This is discussed below. (for an explanation of the strike through text see this post)

Second, there were significant spurs in the phase noise spectrum below 500 Hz. This is also discussed below.

Beginning with the differences between the published and measured phase noise specs, at first I suspected my test setup equipment or its procedures were either faulty or I was not following the latter precisely. This is still a possibility, but I decided to do a "back of the envelope" check on the results by connecting the MV89A to my Siglent 3032X and look at the spectrum in the immediate vicinity of 10 MHz. This should give a ball-park check on the phase noise data I generated from the test setup. The Siglent was configured according to the following parameters:


* Start Frequency: 9.995 MHz
* Stop Frequency: 10.005 MHz
* RBW : 10 Hz
* Detect type" Average Power"
* Sweep time: auto
* Sweep: Single
* Sweep Mode: FFT
I captured the spectrum results in a CSV file and moved it to my analysis computer. Using Octave, I deleted the spectrum to the left-hand side of the Carrier and then normalized the power in each spectrum bin by subtracting the carrier power (in dBm) from all other bins (this yielded dBc units) and then subtracted 10 dB from each bin to convert the values to dBc/Hz (remember that each bin was 10 Hz wide and that to get power values normalized to 1 Hz, it is necessary to divide by 10 or in the log world to subtract 10 dB). The results are shown in figure 4.



Figure 4 - Phase Noise from Frequency Discriminator versus side bands on Siglen 3032X

It is evident that the two plots converge around 5 KHz. It is important to keep in mind that the Siglent data was collected from a sweep of about 1-2 seconds, so averaging does not occur over the same time period as the frequency discriminator experiments. Figure 5 shows the comparison in the offset frequency range 0-120 Hz.



Figure 5 - Data in Figure 4 zoomed in to 0 - 120 Hz

Two conclusions arise from these two images. First, it is plausible that the Frequency Disciminator analysis of phase noise is correct (but see below for a discussion why the data may be corrupted by instrument noise floors). The convergence at 5 KHz supports such a conclusion. Second, below ~40 Hz the Frequency Discriminator results are questionable. This agrees with the conclusion from the noise floor analysis in post 7 and with the property that the frequency discriminator configuration is not suitable for analyzing phase noise close to the carrier.

How is the discrepancy between the values of the Frequency Disciminator analysis of phase noise and those of the published specs explained? After all, -100 dB is an enormous difference, representing a ratio of 10^-10. Several possible factors may be involved:


* It isn't clear from the spec what was the length of time over which was collected the data leading to the spec. As stated above, the time period used for this experiment was 52.43 seconds. 30 data segments with this gate time were collected and power averaged to generate the spectrum. If the spec data is based on much shorter or longer segment gate times, then it is possible some of the disagreement is explained by this fact. (NB: the spec also does not indicate whether averaging was used or, if so, how many segments were averaged.)
* The published phase noise specs for the MV89A are for a 5 MHz oscillator. The MV89A is a 10 MHz oscillator, which suggests it is frequency doubling 5 MHz. This would degrade the phase noise characteristics of the source oscillator. However, doubling the frequency should impose a degradation of about -6 dB (in theory, although in practice it may be more). See this article.
* One factor is the age of the MV89A - 13 years. One would not expect the phase noise characteristics to degrade by -100 dB in this time period, but some degradation wouldn't be surprising.
* An important consideration is the noise floor characteristics of the HP11729C and the Siglent 3032X. In the latter case, the Siglent 3032X datasheet specifies phase noise of <-95dBc/Hz, <-98dBc/Hz typical at 10 KHz offset for a 1 GHz carrier. So, the comparison of the Frequency Discriminator result with the Siglent data may not be valid as any phase noise <-100 dBc/Hz may be in the phase noise floor of the Siglent (there is no spec data for lower offset or carrier frequencies). Also, the HP11729C has the following residual noise specs (dBc/Hz @ <1.28 GHz carrier): 10 Hz: -115; 100 Hz: -126 Hz; 1KHz: -135; 10 KHz: -145. This is much better than the Siglent, but still above the specs for the MV89A (5Mhz). Note: these noise figures are for residual noise, not phase noise.

The spec data isn't sufficiently precise or complete to demonstrate that the results of the experiment are corrupted by noise floor limits. The offset values generated by the experiment are significantly above the residual noise spec values, so it isn't clear the experiment hit the HP11729C residual noise floor, in which case the difference between the MV89A phase noise spec and the results remain unexplained.
The bottom line is the results are inconclusive. More work is required to determine the correct phase noise spectrum for a 13 year-old MV89A. If noise floor problems are the cause, then it will not be possible to resolve this issue using the Frequency Discriminator configuration of the HP11729C.

The second surprise from the experimental results were the spurs observed in the phase noise spectrum. Figure 6 shows the spectrum zoomed in to 0-500 Hz.



Figure 6 - Phase noise spectrum zoomed to 0-500 Hz.

From the octave data, I have inserted the frequency and power values of the spurs into the spectrum. They occur at 60 Hz, 180 Hz and 300 Hz. They obviously represent power line 60 Hz noise modulating the carrier. The fact that it is the odd harmonics of 60 Hz that appear in the spectrum may have an explanation, but I am not aware of it (perhaps someone with more extensive knowledge of modulation theory would know.)

A likely candidate for the source of the 60 Hz contamination is the supply powering the oscilllator. This was a Rigol DP832, which is a triple linear power supply. In the case of the MV89A, 12 volts drives the oscillator. I connected the output of the DP832 @ 12V to the PicoScope input, using AC coupling, which produced the spectrum shown in Figure 7.



Figure 7 - DP832 12V power supply spectrum.

At the top of each spur in this plot is its offset frequency (this is different than figure 6, which labels each spur by its power value). What is somewhat suprising is lack of correlation with the spurs in the phase noise plot. In particular:


* There is no significant spur at 180 Hz, rather there is a major spur at 168 Hz.
* There is a huge spur at 250 Hz, for which there is no corresponding spur in the phase noise spectrum.
* The spur at 300 Hz is evident, but there also are spurs at 328 Hz and 332 Hz  that have no corresponding spur in the phase noise spectrum.
* There are spurs at 41 Hz and 82 Hz that have no correspondents in the phase noise spectrum.
So, the power supply may not be the source of the power line noise that modulates the carrier, resulting in the spurs. This is something that requires further investigation.

Preliminary Conclusions

The experimental results presented in this post seem to raise more questions than they answer. Here is summarized the preliminary conclusions derived from the results.


* The phase noise of the MV89A published in the specs could not be reproduced. This may have occured because there were noise floor limitations that prevented an accurate measurement. However, the published noise floor specs of the measurement instruments do not supply any obvious support for this explanation. The residual noise floor of the HP11729C was considerably below that of the measured (and corrected) noise floor of the MV89A dervied by the experiment. The noise floor of the spectrum analyzer (a PicoScope 4262) was sufficiently below the signal output of the frequency discriminator to suggest it was not an influence. Why the measured noise floor is almost 100 dB above the published specs has not been explained; although this magnitude of difference strongly suggests something isn't right.
* The spurs in the phase noise measurements at 60 Hz and its odd harmonics could not be explained by corruption of the power supply voltage. However, one conclusion is these spurs would introduce significant frequency fluctuations at those offsets. So, when designing an oscillator circuit for use in an application for which phase noise is an issue, a designer should pay particular attention to low frequency noise to ensure it doesn't introduce deterministic adulteration of the oscillator output.
* The results of this experiment focuses on short-term phase noise (on the order of a minute). They should not be considered valid in applications for which phase noise over longer periods is important. Study of longer-term phase noise is a future objective.
* It is well-known that a frequency discriminator cannot analyze phase noise close to the carrier. The results of this experiment suggest that for offset frequencies below 40 Hz, the phase noise results cannot be considered valid.

SoundTech-LG:
Not only are these MV89A oscillators on Ebay retired, there are also many fakes being sold. Labels produced that look similar to Morion labels. China seems adept at this. The first one I got seemed to be the genuine article. I left it to burn-in for over a year. Disconnected it from power, let it cool. It never powered up again. The second one I bought was certainly a fake. Not even the same mechanical dimensions. It also had poor measured specs. So, not sure what is currently being offered there, but certainly the buyer should beware.

bob91343:
I think you are having a lot of fun with this.  (Please correct spelling, the word is hobbyist.)

The absence of even harmonics of the spurious 60 Hz frequncy implies that the source is symmetrical.  A symmetrical wave (positive excursion being a mirror image of negative excursion) has no even harmonics.  The presence of even harmonics indicaates a difference between the two polarities.

Your oddball frequency spurious signals may be internally generated within the test equipment.  Spectrum analyzers are notorious for this.  It takes an experienced eye to differentiate between internally generated signals and those that are contained within the input signal.

I am by no means an expert in these matters so I will stop guessing.

dnessett:

--- Quote from: bob91343 on June 04, 2019, 09:21:05 pm ---(Please correct spelling, the word is hobbyist.)

--- End quote ---

Done. I hope I found them all.

Thanks,

Dan

dnessett:

--- Quote from: SoundTech-LG on June 04, 2019, 09:18:39 pm ---Not only are these MV89A oscillators on Ebay retired, there are also many fakes being sold. Labels produced that look similar to Morion labels. China seems adept at this. The first one I got seemed to be the genuine article. I left it to burn-in for over a year. Disconnected it from power, let it cool. It never powered up again. The second one I bought was certainly a fake. Not even the same mechanical dimensions. It also had poor measured specs. So, not sure what is currently being offered there, but certainly the buyer should beware.

--- End quote ---

Any tell-tale external properties that would identify a fake?

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