Hello and welcome to the latest article for the blog series, Timing 201. This is a sequel to the article Timing 201 #1: The Case of the Phase Noise That Wasn’t – Part 1. In this post, I follow-up on Part 1 and suggest that both a limiting amplifier and balun are useful accessories to minimize AM and make good phase noise measurements. I include some example data and close with a few rules of thumb.
A Quick Review from Part 1
You will recall the basic idea we last explored was that AM (Amplitude Modulation) noise can impact phase noise measurements. (AM noise is the apparent “phase noise that isn’t”.) A spectrum analyzer cannot distinguish between AM and PM (Phase Modulation) or phase noise. Purpose-built phase noise measurement instrumentation can suppress but not eliminate AM to PM conversion. The use of a limiting amplifier can further reduce the impact of AM noise on phase noise measurement. This can be a valuable troubleshooting tool when investigating system-level clock noise and spurs.
Example Limiting Amplifier AM Rejection
First, a few words about the particular limiting amplifier used for data collection in this article. The ideal limiting amplifier or LA would be high gain, low noise, wideband, and modestly priced. That combination is hard to find.
The sample LA used here is the Hittite (now Analog Devices) HMC750 which has a 3 dB bandwidth of 11 GHz. An evaluation board is available from distributors though it’s relatively expensive. There are other LAs available from other vendors, and if you buy units in quantity and/or roll your own PCB, you can certainly lower your costs.
Two suggestions if you do use the Hittite evaluation board:
As a reminder, the time domain operation of the LA is illustrated as follows. Below left is a scope capture for a 100 MHz single-ended sinusoidal clock. The AM depth was 5% and the scope is set to infinite persistence. Below right is the same single-ended clock through the example limiting amplifier.
While the time domain operation is suggestive, AM rejection is better measured in the frequency domain. In the following measurements, a Keysight E5052B was used both as the measurement instrument and as a stand-in clock receiver. I employed a Keysight 33600A Waveform Generator, operating single-endedly, to apply AM at several frequencies and measured both the AM and phase noise spur amplitude. The carrier was 100 MHz and the AM depth was set to 1%. Getting a consistent AM spur above the noise floor was what I was looking for.
I then inserted the cited LA and repeated the same measurements.
Note the new noise columns that had to be added. The LA effectively eliminated the AM spur completely, reducing it to the noise floor. There is still a corresponding phase noise spur, reduced also. (Subsequent testing indicated this was incidental phase modulation from the source.)
Here is a sample plot for the 100 kHz case with no intervening limiting amplifier. The phase noise window is displayed at the top and the AM noise window is displayed at the bottom. Marker 1 in each window is set at 100 kHz.
And here is a corresponding sample plot for the 100 kHz case when using the limiting amplifier. As I mentioned previously, the AM spur is eliminated which I was looking for. The corresponding phase noise spur dropped another 12 dB. For some reason, at least for this sample, the AM noise floor above 10 kHz is not flat as would be ideal.
In general, we should use a limiting amplifier if AM noise or spurs are significant with respect to phase noise. In a measurement system that can measure both, we typically want AM noise or spurs to be at least 10 dB below phase noise or spurs at the same offset frequencies. (We weren’t quite getting that in the first table above.)
Ideally, the same would be true for the clock receiver accounting for its AM rejection. For example, if both the input clock’s AM noise floor and phase noise floor were the same at a particular offset frequency, then we would want the clock receiver to reject or attenuate AM noise further by 10 dB. This performance is not always specified and may need to be measured. It is also often a function of the input clock slew rate or rise/fall time.
A limiting amplifier is useful for removing AM from both single-ended and differential clocks. However, differential clocks are themselves an important approach to combating AM noise.
One of the most important reasons for differential signaling on balanced lines is common mode (CM) noise rejection. Since most AM noise on differential signals is CM (e.g. due to power supplies) then differential receivers will tend to reject AM. This can be illustrated pictorially in the figures below.
First, consider amplitude noise impressed upon a single-ended clock.
Now, consider the same amplitude noise impressed upon a differential clock.
There are several aspects to the diagram worth noting:
All of these will help to prevent CM AM noise from impacting the measurement.
Unfortunately, most spectrum and phase noise analysis instruments have single-ended inputs and don’t directly support differential measurements. If we replace the receiver in the diagram with an instrument, what is the best approach?
A ready measurement would be to route one differential clock polarity, e.g. CLKP, to the instrument and terminate the unmeasured polarity output, e.g. CLKN, in to 50 Ω. Generally you will want to AC-couple these so as to present roughly the same impedance. The necessity for balanced termination is discussed in Timing 101 #10: The Case of the Half-terminated Differential Output Clock.
This is a valid measurement but is overly conservative as it neglects the advantage that would actually be experienced in a differential clock system. What we need is a device that will convert the differential signal to a single-ended signal. Such a device is known as a balun (taken from balanced to unbalanced).
This could be in the form of an amplifier (sometimes called an active balun) or more typically a passive component. As an example of the latter, a classic transformer balun or voltage balun has a center-tapped winding on one side for converting 100 Ω differential to single-ended 50 Ω.
The example balun used in these tests is the Tektronix (formerly Picosecond Pulse Labs) PSPL5310R. This is a phase matched balun which can operate from 4 MHz to 6.5 GHz. Unfortunately, for some reason, these are no longer available from Tektronix.
Other vendors which sell baluns include MACOM, Marki Microwave, and Mini-Circuits. (This is not meant to be an inclusive and exhaustive list. These are just companies I am familiar with.)
Look for a balun wideband enough to pass the necessary waveform with sufficient risetime and with good phase and amplitude balance. The old PSPL balun had anywhere from ± 0.5 to ± 2 degrees differential phase balance and ± 0.1 to +0.3/0.2 dB differential amplitude balance depending on the frequency range.
Example DUT with AM Noise
I have selected for a practical example one of my favorite “desert island” parts, the venerable Si570 which is an older generation programmable oscillator. It was the first 5 mm x 7 mm I2C programmable oscillator that could cover 10 MHz to 1.4 GHz. Radio amateurs may recognize this device as used in some “Softrock” SDR (Software Defined Radio) applications.
Not only can it support such a wide frequency range but it can also be ordered with “real” LVPECL drivers. By this I mean it can drive DC-coupled LVPECL loads defined as 50 Ω terminated in to VDD – 2V. (By contrast, clock devices typically provide LVPECL compatible modes where they can supply the right swing when AC-coupled.) There is some CM (Common Mode) AM noise using this mode. However, this is mitigated when used differentially as intended.
Below we will tabulate several measurement scenarios with and without a balun and LA. The DUT is an evaluation board with P/N 570AAA000121DG installed operating AC-coupled. This is a differential programmable XO with a 100 MHz starting frequency, output format 3.3V LVPECL.
Here are the phase jitter results for various configurations.
The biggest bang for the buck so to speak was using a balun. That got us to the 300fs range or less which is what we expect this device to yield. (The closest Si570 datasheet spec is 360 fs typical for Fout from 125 to 500 MHz.)
Here are a few plots to go with this table.
First, the single-ended configuration with the highest phase jitter, over 600 fs. Note the several spurs above 1 MHz in both the AM and phase noise measurement windows.
Next is the balun only configuration which eliminates some spurs and drops the phase jitter roughly in half, i.e. below 300 fs.
Finally, here is the phase noise plot combining the DUT driving the LA differentially which in turn drives the balun connected to the E5052B. This plot eliminates all the spurs above 1 MHz and still keeps the phase noise floor mostly below -150 dBc/Hz beyond this offset frequency.
Virtual Limiting Amplifier
Ignoring spurs, it is possible in principle to estimate the DUT’s post-LA phase noise performance by mathematically applying a “virtual limiting amplifier”, if you know what the LA AM noise floor will be and what the AM to PM conversion is for your instrument. However, that is a topic for another time.
In practice, as one peels the onion to achieve lower and lower phase noise measurements, spurs play an increasingly larger role. This can be problematic for estimation purposes since even if the location of spurs can be predicted, often their amplitudes cannot.
A Few Rules of Thumb
Based on this and the previous Timing 201 post, here are a few suggested rules of thumb when making phase noise measurements on sources that may have AM noise:
Having a good limiting amplifier and a good balun on your bench can help you make more accurate phase noise and jitter measurements, and help distinguish the type of noise contributors you are dealing with. If you are working with differential clocks, you really should use a balun even if AM noise is not that significant.
I hope you have enjoyed this Timing 201 article. If you have a favorite balun or limiting amplifier you would like to share with others, please let me know.
As always, if you have topic suggestions, or there are questions you would like answered, appropriate for this blog, please send them to firstname.lastname@example.org with the words “Timing 201” in the subject line. I will give them consideration and see if I can fit them in. Thanks for reading. Keep calm and clock on.