FFS there's an easy way to quantify this.
Did you not follow my link in reply #163 or not understand its content ?
FFS there's an easy way to quantify this.
Did you not follow my link in reply #163 or not understand its content ?
I had a lot of trouble understanding it, since the poster doesn't seem to be a native English speaker.
All the info and clues you need to perform the same measurement are contained in the screenshot.
Please study it again in depth......every little snippet of info.
Clue, look at the Stats box and the # count = 1000s, so with infinite persistence the jitter (ch4) WRT 1pps (ch1) is under 4ns for a 1000s of a 10 MHz ref signal !
Very clever use of standard features in a DSO.
All the info and clues you need to perform the same measurement are contained in the screenshot.
Please study it again in depth......every little snippet of info.
Clue, look at the Stats box and the # count = 1000s, so with infinite persistence the jitter (ch4) WRT 1pps (ch1) is under 4ns for a 1000s of a 10 MHz ref signal !
Very clever use of standard features in a DSO.
I looked at the Rigol 1104Z manual, but could find no way to measure the skew between channels. So, I don't think I can perform the procedure suggested by the screen shot.
Not that I think it important, but to get this red herring off the table, I found two 10' coax patch cables and attached them to each other to create a 20' length. I then ran the GPSDO delayed test. The results are not suprising. Here are the maximum and minimum phase differences.
max=57.6 degrees
min=35.4 degrees
variation = 22.2 degrees
Consequently, reducing the length of the delaying coax provides no insight into the problem.
If the phase difference between the signal from the same source traveling down two pieces of coax is not constant, then the experimental setup is flawed.
Fc=10000000;
Fsam=500000000;
Fnyq=Fsam/2;
[b,a]=butter(6, Fc/Fnyq);
output=filter(b,a,p);
pf=output;
pfn=pf(:,2);
pfn=pfn.-mean(pfn);
If the phase difference between the signal from the same source traveling down two pieces of coax is not constant, then the experimental setup is flawed.
That is what I thought. But, when I studied the data, another possibility arose, which I think is the real answer. (And it has nothing to do with the length of coax used for the delayed signal)
I couldn't figure out how the phase difference between a signal and its constantly delayed image could vary as much as the data indicated. Then I noticed an artifact. (see Figure 1)
Figure 1 -
Figure 1 is an image produced by plotting the (1st 20000 data points in the) result of the following Octave code (where "p" holds the GPSDO delayed phase difference data).Code: [Select]Fc=10000000;
Fsam=500000000;
Fnyq=Fsam/2;
[b,a]=butter(6, Fc/Fnyq);
output=filter(b,a,p);
pf=output;
pfn=pf(:,2);
pfn=pfn.-mean(pfn);
This code normalizes the data by first applying a 5th order 10 MHz low pass Butterworth filter to it (to eliminate the 20 MHz superimposed signal) and then normalizing it by subtracting the mean from each element. I have marked with red lines prominent spikes in the data.
A free running oscillator would not have such spikes, but neither the Rubidium oscillator nor the GPSDO are free running oscillators. They are disciplined oscillators comprising a crystal oscillator that is periodically corrected by a reference signal. The periodicity of this correction (technically, its reciprocal) is commonly referred to as the servo loop bandwidth. My current hypothesis is the spikes represent periodic corrections to the frequency/phase of the crystal oscillator.
The effect of this is the crystal oscillator free runs for a while and then experiences a movement in frequency/phase. Sometimes this movement is significant, which appears as a large change in the phase difference between the signal and its delayed image.
One question that presented itself is how could the free running oscillator drift so far in frequency as to require a significant correction? One possibility is a previous change overcorrected the error, which then requires a significant movement in the opposite direction. That is speculation, but it is at least plausible.
I eye-balled the distance between two spikes and it was about 1200 points apart. At 2 ns between data points, this represents about 2.4 usec separation. That would imply a servo loop bandwidth of ~417 KHz.
Since I do not have access to the circuit diagrams and design information for the GPSDO, this is still a working hypothesis. However, it is a plausible explanation for the significant differences in the phase difference data. I don't know any engineers who have designed either a GPSDO or a Rubidium oscillator, so I cannot ask them whether this hypothesis makes sense. If anyone reading this thread is such an engineer or knows someone with such experience, comments from them would be appreciated.
Feed the signals from the two different lengths of coax to your DSO. Place cursors at the zero crossings. Let it run for as long as you like. The phase relationship should not change unless the coax is bad or you have a reflection problem. Until you can get a reliable signal to the instrument there is no point in speculating about possible causes of artifacts in the phase measurements.
You will get apparent jitter in a stable signal because the scope is interpolating the trigger point and the measurement points. The 40 pS pulser is far less stable than the 33622A or GPSDO, but it *appears* to have less jitter because it has a very fast edge. I don't know the jitter spec for Leo's GPSDO, but the 33622A is specified at less than 1 pS but the RTM3K indicated ~24 pS standard deviation for the time period. That's not real. It's a DSO artifact.
7042 shows the 33622A hooked up with what should be a good piece of coax, but there is an obvious mismatch. There is not 30 pS of jitter in the 33622A. The GPSDO has a faster rise time so the step is more pronounced as seen in 7037, but when I connected the GPSDO directly the step went away. So my "To Do" list got testing and culling BNC cables added.
Tomato is absolutely correct, though not very clear. The first requirement is to verify that you can get accurate signals to the test device. There is no reason to assume that the inputs to the AD board are actually 50 ohms. Or that anything else is 50 ohms. 10 MHz is not all that high, but it is still RF and can be confusing because of the speed. I makde the mistake of buying 10 Chinese BNC cables. They make great 50 MHz notch filters, but are useless for anything else.
I suggest you start by sweeping your cables on the spectrum analyzer. If at all possible do the cal with a known high quality N cable. My "To Do" list already has testing a bunch of Chinese adaptors of which I know at least one is bad and I suspect there are others.
I think it is best at this point to document the test setup and seek constructive criticism of it.
I think it is best at this point to document the test setup and seek constructive criticism of it.
1) You've got some termination issues. You can't just connect your signal to the AD chip with BNC tees, because the AD inputs are terminated with 51Ω resistors. You need to connect via splitters or directional couplers.
1) You've got some termination issues. You can't just connect your signal to the AD chip with BNC tees, because the AD inputs are terminated with 51Ω resistors. You need to connect via splitters or directional couplers.
2) Why in the world do you have 30dB attenuators on the inputs of the AD chip?
I think it is best at this point to document the test setup and seek constructive criticism of it.
1) You've got some termination issues. You can't just connect your signal to the AD chip with BNC tees, because the AD inputs are terminated with 51Ω resistors. You need to connect via splitters or directional couplers.
A mild understatement.
1) Tee + terminator != thru terminator. That little stub rings like mad, but because it's short you can't see it on the Rigol. Can't see it on my 200 MHz Instek either. But it's there.
#6 40 pS pulser to 50 ohm thru
#7 same but Tee + terminator
#8 Tee+terminator but with a short BNC cable between the Tee and the terminator
The white reference trace in #7 & #8 is the trace in #6
#9 pulser feeding a Tee and BNCs w/ thru terminators. One cable is a couple of inches longer. Again, the reference trace is #6. Note the apparent increase in gain as we now have approximately 25 ohms terminating the pulser rather than the 50 it needs.
To summarize: You cannot make meaningful measurements with things connected the way you have them. I suggest a quick review of transmission lines.
Thank you for your comments and question. I will address them in reverse order.
Since you are asking about attenuators on the AD8302 inputs, I presume you have read the device data sheet. If that presumption is correct, then you know the input power range is 0 dBm to -60 dBm (with respect to a 50 ohm load).
I have several oscillators I want to characterize using the test set-up. There are (among a larger set) the GPSDO, which outputs 1.25V P-P sine wave, the Rubidium, which outputs a 1 V P-P sine wave, and an OCXO, which outputs a 50% duty cycle square wave from 0 to 3.5V. The RMS voltage of a 50% duty cycle non-negative square wave is VP-P/sqrt(2). So, the RMS voltage of the OCXO output is ~2.47V. Looking into a 50 ohm load its power is Vrms2/R ~= 6.1/50 = .122 watt =~20.6 dBm. So, a 20 dBm attentuator just misses the mark, which implies the next common attenuator value of 30 dB. That is why I put them in front of the AD8302 inputs.
I wanted to address the pad issue first ... now is a good time to investigate how the input circuit might affect the results I seek
This leads me to believe the phase difference data should be uneffected by the termination issues you raise. I am, of course, open to clear arguments that suggest otherwise.
You're making things too complicated again. A properly designed attenuator terminated by 50Ω will appear as 50Ω at it's input.
The problem is that your signal sees 25Ω at the BNC tee, because it is split into two paths that are both 50Ω. You need a splitter or directional coupler instead of the BNC tee.
OK. I need some help finding a splitter that satisfies the requirements you think important. Will this one work?
Oscillator | Lower | Upper | Bandwidth |
GSPDO | 9.589 | 10.402 | 813 KHz |
Rubidium | 9.669 | 10.328 | 659 KHz |
Rigol | 9.688 | 10.309 | 621 KHz |
I think you're measuring the phase noise of the spectrum analyzer; a good oscillator has> 150 dBc at 1 kHz offset.