Understood. The objective of the delay line approach is to get an estimate of phase noise and phase noise bandwidth for each oscillator. I need to know if the short-term phase noise of the GPSDO (for which I have no specification) is sufficiently smaller than the other oscillators in order to use it as the reference in a two oscillator configuration. While I may not get phase noise close to the carrier for each oscillator, I should get enough information to reasonably conjecture that the GPSDO has (or does not have) sufficiently lower phase noise than the other oscillators to use it as a reference. The reason I am worried about this (at least for short-term stability characterization) is the GPSDO has an OCXO as the base oscillator that is corrected by the GPS signal periodically. Short-term its stability may be no better than the other OCXOs I have.
You're using the phrase "short term" a lot, yet it has different meanings in different contexts. Short term as it relates to a GPS disciplined oscillator is orders of magnitude longer than short term as it relates to delay line measurements.
I think you may find that none of the phase noise of the OCXO is "short term", as defined by the delay line. In other words, all the phase noise is close to the carrier and, therefore, not readily measured by the delay line method.
In regards to the delay line measurements, it is not my objective to establish the complete phase noise characterization of each oscillator using this technique. I just want to see if the GPSDO can be used as a reference in each type of experiment (i.e., short-term (seconds), medium-term (minutes) and long-term (hours)). While I may not be able to measure phase noise close the carrier using the delay line approach for any oscillator, if the GPSDO has less phase noise than another oscillator away from the carrier, then I can conjecture it will have less phase noise near the carrier than the other oscillator.
For example, suppose I measure the phase noise for the GPSDO using the delay line technique and come up with -100 dBc @ 10 Hz, -125 dBc @ 100 Hz and -145 dBc @ 1 KHz. I then measure another oscillator using the delay line technique and come up with -90 dBc @ 10 Hz, -110 dBc @100 Hz, and -120 dBc@ 1 KHz. It is then likely that the phase noise of the GPSDO will be better than the other oscillator for Fourier frequencies nearer to the carrier. This will give me confidence that I can use the GPSDO as the reference oscillator in the two oscillator test setup.
In the context of delay line measurements, 10 Hz - 10kHz from the carrier is near the carrier. You will not be able to measure this with a delay line that is a few hundred feet long.
The delay line will be almost 600 feet long.
Yes, I know. You might want to do a quick calculation of how much the amplitude of a 10 Hz sideband is attenuated when measured with a 600' delay line.
Here's another quick sanity check: Add 1 kHz sidebands to a 10MHz oscillator. Make a direct measurement of the modulation index with your spectrum analyzer. Then set up a delay line measurement and measure the apparent modulation index at various delay line lengths. Repeat the experiment with 10 kHz, 100 kHz, and 1 MHz sidebands and look for a trend.
For the first calculation, do you mean add a 10 Hz sideband in the frequency domain?
Sorry, my choice of of the term attenuation was unfortunate, as it led to confusion. I was referring to attenuation of the signal due to the measurement technique, not due to cable losses.
1) Here is the calculation: If you have a 10 MHz carrier phase modulated at 10 Hz, calculate how large the detected (10 Hz) signal will be if you measure phase with a delay line setup and a 600' delay line. (You can ignore cable losses for the calculation.) You don't actually have to do an exact calculation; a "back of the envelope" calculation will still be enlightening.
2) The sanity check is to actually measure some sidebands using both the spectrum analyzer and a delay line setup. For convenience, modulate the 10 MHz carrier at 1 kHz, 10 kHz, 100 kHz, and 1 MHz. The spectrum analyzer will easily resolve these sidebands and give you a confirmation of the modulation amplitude. Compare these results to those measured with the delay line.
I spent yesterday evening and this morning thinking about this and for the life of me, I cannot understand what you are getting at.
I plan to start out with a one oscillator test set up (using the delay line approach), to get an estimate of phase noise of each oscillator...
The delay line method is a perfectly good way to make measurements, but you will want to buy a giant spool of coax if you want to do it. Your (relatively) short delay line will only allow you to see higher frequency phase noise. A much longer delay line is needed if you want to measure phase noise near the carrier.
I think you may find that none of the phase noise of the OCXO is "short term", as defined by the delay line. In other words, all the phase noise is close to the carrier and, therefore, not readily measured by the delay line method.
In the context of delay line measurements, 10 Hz - 10kHz from the carrier is near the carrier. You will not be able to measure this with a delay line that is a few hundred feet long.
You might want to do a quick calculation of how much the amplitude of a 10 Hz sideband is attenuated when measured with a 600' delay line.
Here is the calculation: If you have a 10 MHz carrier phase modulated at 10 Hz, calculate how large the detected (10 Hz) signal will be if you measure phase with a delay line setup and a 600' delay line. (You can ignore cable losses for the calculation.)
Follow the ball ...
1) Here is the calculation: If you have a 10 MHz carrier phase modulated at 10 Hz, calculate how large the detected (10 Hz) signal will be if you measure phase with a delay line setup and a 600' delay line. (You can ignore cable losses for the calculation.) You don't actually have to do an exact calculation; a "back of the envelope" calculation will still be enlightening.
If you have a 10 MHz carrier phase modulated at 10 Hz, calculate how large the detected (10 Hz) signal will be if you measure phase with a delay line setup and a 600' delay line.
I think the ball you have launched bounces all over the place and I am having trouble following it. You write:1) Here is the calculation: If you have a 10 MHz carrier phase modulated at 10 Hz, calculate how large the detected (10 Hz) signal will be if you measure phase with a delay line setup and a 600' delay line. (You can ignore cable losses for the calculation.) You don't actually have to do an exact calculation; a "back of the envelope" calculation will still be enlightening.
You suggest calculating the strength of a 10 Hz signal that modulates a 10 MHz carrier after transiting a 600 foot coax, but I am to ignore cable losses. What property of the coax am I to use to carry out this calculation? What effects the diminution of modulating signal strength over a coax other than attenuation due to its lumped elements (fundamentally, its resistance per unit length)?
I measured the diminution of a 1 KHz modulating signal on a 10 MHz carrier over 183 feet of coax and found that it looses strength at the same rate as the carrier. What makes 10 Hz modulating 10 MHz over 600 feet different?
What are you trying to get at in your proposal:If you have a 10 MHz carrier phase modulated at 10 Hz, calculate how large the detected (10 Hz) signal will be if you measure phase with a delay line setup and a 600' delay line.
If you mean the delayed signal amplitude will be lower than the non-delayed signal, that is pretty obvious (and is a result of cable losses you suggest I ignore). However, the AD8302 uses logarithmic amplifiers to ensure the two signals are at roughly the same amplitude before presenting them to the phase detector circuit. If you mean something else, just state it. Stop trying to mimic Aristotle.
So far, I am unconvinced that the delay line approach has any problem that the two oscillator approach doesn't have, other than a higher noise floor. In addition, I think digging the noise signal out of the modulated oscillator signal is by far the hardest problem to solve. This is true whether one uses the one or two oscillator setup.
Not to put too sharp a point on it, but:
sin(w*t) - sin(w*(t+dt)) = 2*cos(w*(2*t+dt)/2)*sin(w*dt/2)
So the result is a cosine wave at over twice the frequency and the maximum amplitude goes to *zero* at certain delays.
In the context of delay line measurements, 10 Hz - 10kHz from the carrier is near the carrier. You will not be able to measure this with a delay line that is a few hundred feet long.
I am going to go out on a limb and say you are wrong. There is no structural reason why the delay line (aka one oscillator) set up cannot measure phase noise arbitrarily close to the carrier.
According to Phase Noise and AM noise measurement in the Frequency Domain, the noise floor of the one oscillator set up is reduced compared to the two oscillator approach. The graph given in support of this shows the noise floor rising as the the Fourier frequency of the phase noise approaches that of the carrier. Unfortunately, the justification for this graph is another paper that I have not been able to acquire. So, there is no way to check the argument that led to that graph. However, the text makes no mention of a "structural problem" that leads to the result.
There are plenty of practical problems with measuring phase noise close to the carrier. However, these are not specific to the one oscillator set up. They apply equally to the two oscillator set up. I will describe them in a separate post.
So, on to the argument that the delay line/one oscillator measurement set up is not structurally deficient as a measurement technique. This argument follows your lead in assuming transmission lines are perfect (not lossy and linear) and it assumes all electronic circuits are perfect (e.g., filters have cutoff frequencies that are exact - they do not drop off over a range of frequencies). In this regard, the argument assumes a bandpass filter that passes only the carrier frequency and the carrier frequency plus 1 Hz. This filter is placed on the oscillator output before the signal enters one side of the mixer and the delay line. So, the signal presented to the double balanced mixer on both sides comprises a 2 Hz band limited to the carrier frequency and the carrier frequency plus 1 Hz. Since the delay line is perfect, the amplitudes of the generated and delayed signal are exactly equal.
If you disagree or find fault with this argument, I welcome you to provide a counter-argument or refutation. However, I am not interested in playing 20 questions with you. So, if you follow your recent habit of patronizating discourse, I probably will not respond.
But, in order to learn, I have to understand what I am doing and why... I don't want to make the same mistakes others have turned into knowledge.
... my point is that just telling someone new to the field to do something is useful, but limited. It is better to explain why they should do it - what is the experience on which the advise is based.
While I don't believe there are structural reasons why the single oscillator setup could not achieve this, there are plenty of practical reasons why this objective is outside the capabilities of a hobbiest whether the one or two oscillator set up is used.
YMMV with cheaper two-channel SDRs (like red-pitaya or similar). Noise floor should scale with bit-depth, so if you are just into frequency comparisons of Rb-clocks/GPSDOs an 8-bit two-channel SDR might be enough. If H-masers is more of a thing for you then look at 14-bit or 16-bit SDRs. It would be good to come up with common gnu-radio and UI software for this, so that time-nuts worldwide could evaluate and compare the bang-for-buck of different SDR setups.
YMMV with cheaper two-channel SDRs (like red-pitaya or similar). Noise floor should scale with bit-depth, so if you are just into frequency comparisons of Rb-clocks/GPSDOs an 8-bit two-channel SDR might be enough. If H-masers is more of a thing for you then look at 14-bit or 16-bit SDRs. It would be good to come up with common gnu-radio and UI software for this, so that time-nuts worldwide could evaluate and compare the bang-for-buck of different SDR setups.
Stumbled on this thread about a Fluke counter that just might give to the info you need for oscillator analysis:
https://www.eevblog.com/forum/testgear/fluke-pm6690-12-digits-frequency-counter/
Lots of examples of what it can do later in the thread.
Stumbled on this thread about a Fluke counter that just might give to the info you need for oscillator analysis:
https://www.eevblog.com/forum/testgear/fluke-pm6690-12-digits-frequency-counter/
Lots of examples of what it can do later in the thread.
The HP 5335a has 0.1deg phase resolution. It is 30Hz - 1MHz , though.