Thank you for your comments and question. I will address them in reverse order.
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?
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 V
P-P/sqrt(2). So, the RMS voltage of the OCXO output is ~2.47V. Looking into a 50 ohm load its power is V
rms2/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, since its existence complicates the analysis of the termination problem. I designed the input circuit to the AD8302 board with the idea of comparing two signals, a reference oscillator signal and a device (oscillator) under test signal. I didn't design it for comparing a signal with its delayed image; a situation I was forced into when the Rubidium/GPSDO comparison test yielded counter-intuitive results. I will admit, I didn't give the delayed coax configuration enough thought. In my defense I only will say that I was hunting down a bug and in the heat of the chase was more interested in getting some hints about what was going on than engineering interface circuits. And it worked, so it had that advantage.
In any case, now is a good time to investigate how the input circuit might affect the results I seek. Figure 1 illustrates the termination topology for both cases.
Figure 1 -
![](https://www.eevblog.com/forum/metrology/an-advanced-question-sampling-an-oscillators-signal-for-analysis/?action=dlattach;attach=475133)
In the original setup, both inputs had the same configuration, which is shown in the Figure. I measured the through resistance and to-ground resistance of the pad with the following results: 1) through resistance = 94 ohms; 2) to-ground resistance = 50 ohms. The AD8302 has a 51 ohm resistor to-ground in front of each signal input. So, the effective resistance from the coax to ground is 25 ohms in series with 94 ohms = 119 ohms.
The length of each coax cable is 3'. The wavelength of a 10 MHz signal is
Corrected 7-13-18:98.4 feet 64.9 feet (velocity factor of RG-58=.66 and wavelength in vacuum of 10 MHz~=98.4 feet => wavelength in RG-58 is 98.4*.66=64.9 feet). So, each coax is much less than even a quarter wavelength and in practice
can be ignored as a source of standing waves need not be treated as a transmission line. This greatly simplifies the computation of termination resistances, since the resistor network at the AD8302 input is effectively directly connected to the terminating resistance at the scope. If you do the math, I should have an 86 ohm terminating resistor at the scope to achieve an effective to-ground resistance of 50 ohms to the coax. I don't have a BNC terminator with an 86 ohm resistor and I really didn't want to build one; but I may reconsider if turns out to be important. In any case I used a standard 50 ohm terminator, which gives an effective coax termination of 35 ohms. Given the short distances of the coax, I think this is probably OK.
The delay coax setup is significantly different. The total coax length between the oscillator and scope is about 89', which is very near the full wavelength of the 10 MHz signal. Consequently, it is likely that a standing wave will occur due to the mismatched terminations. Since analyzing complex termination toplogies on coax is a skill gained from experience and since I don't have that experience, I will leave the math to others. Instead, I decided to simply measure some factors and see the results, rather than compute them.
One thing to keep in mind: the AD8302 separates the amplitude and phase data. So, a standing wave (which generates a DC bias as a function of position along the coax) should not affect the measurement I am making. There may perhaps be other effects that do, but if so I don't know of them.
Figure 2 shows the spectrum of the signal presented to input 1 of the AD8302. This is after it comes from the oscillator and moves through the BNC T and 30 dB pad. As is evident, there are no significant spectral anomolies present. Figure 3 shows the spectrum of the signal presented to input 2 (after it travels through the 83' of coax and then the BNC T and pad). Again, there are no significant spectral features that suggest a problem.
Figure 2 -
![](https://www.eevblog.com/forum/metrology/an-advanced-question-sampling-an-oscillators-signal-for-analysis/?action=dlattach;attach=475139)
Figure 3 -
![](https://www.eevblog.com/forum/metrology/an-advanced-question-sampling-an-oscillators-signal-for-analysis/?action=dlattach;attach=475145)
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.