Any ideas how to get quadrature outputs from a single tank LC oscillator? - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: November 15, 2019, 09:09:25 am
content: I need an oscillator of a few MHz, that can do a voltage controlled frequency sweep of a few KHz at most, but it must have a single coil in the oscillator tank.Already searched online for single tank LC quadrature, and at most I found schematics with 2 anti-series coupled coils, in the arms of an RS flip-flop with 2 transistors.Any ideas or tricks of how to get quadrature outputs from an LC oscillator with a single L, please?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 15, 2019, 10:53:36 am
content: Start with 2F and divide:

author: radiolistener
author_rank: Super Contributor
timestamp: November 15, 2019, 11:57:01 am
content: you can connect two channels of si5351 to the dividers as proposed above and set inverse bit for second channel.In such way your quadrature generator will consists of one si5351 and two high speed dividersAnd you can reuse your inductor and capacitors for power supply filter

author: RoGeorge
author_rank: Super Contributor
timestamp: November 15, 2019, 12:11:05 pm
content: That would be great for a digital setup, thank you. However, here is needed an analog way of getting sinusoidal quadrature output, proportional in amplitude with the main LC oscillations. My bad for not specifying this in the OP, sorry.I am trying to come up with a new method of doing single coil CW-NMR (Continuous Wave Nuclear Magnetic Resonance).The idea in CW-NMR is to place a sample of a probed material in a uniform magnetic field, then to excite the material with RF. If the RF frequency matches the resonant frequency of the atoms of the probed material, then some of the RF energy will be absorbed by the sample. The spectrum of the absorbed energy will reveal details about the molecular structure of the sample material. NMR is a well known method to investigate molecular composition in chemistry labs. I am trying to do some DIY NMR experiments mostly out of curiosity, just for the fun of it.According to some papers found online, the expected amplitude variations caused by NMR are in the range of 1%, but I expect 10 to a 100 times less than that. This is the level of amplitude variations I need to detect in an oscillator while sweeping the frequency around the expected resonance frequencies.There are many published schematics and papers for CW-NMR oscillators. The two main techniques used to detect NMR absorbtion are:- a Q meter setup, where the Q factor of the probing coil is measured- a marginal oscillator, where a barely self sustained oscillator will be perturbed by the faint amount of energy absorbed by the probed sample, and thus a dip in amplitude will be seen.Many other details are to be taken into account, like making sure the sample is not saturated with RF, taking care of the natural relaxation times T1 and T2 (some NMR specific terms this T1 and T2), magnetic field homogenity, temperature, and so on. It's a very, very delicate measurement, to get an idea of how difficult it is, industrial/laboratory grade instruments are scanning the sample hundreds and hundreds of times just to get a decent result, from minutes to hours and sometimes days of averaging for a single measurement.I'm trying to come up with a new type of oscillator, and for that I want to experiment with these 2 ideas I didn't encountered yet in any of the published papers/schematics that I've read (somewhere around 50 or so):- implement an amplitude stabilization of the RF oscillator based on the idea that sin^2 + cos^2 = 1 at all times during the period of an oscillation. Thus if each quadrature (analog) output is squared with an analog multiplier and then summed together, we will get a DC feedback proportional with the amplitude of the oscillation. This DC proportionality will be much faster than the classical scheme of rectify + filter, or even than a peak detector.- a directional coupler to make a better distinction between the energy pumped into the coil and the energy received from the wiggling inside the sample, the plan is to use this later in a lock-in amplifier to detect the phase of the resonating nucleus in the sample, T1, T2, etc.

author: Ian.M
author_rank: Super Contributor
timestamp: November 15, 2019, 12:54:23 pm
content: The oscillating current in the tank circuit is 90 deg out of phase with the voltage across it and is proportional to the voltage. Sense the current and amplify. You'll want something better than the proof of concept in the attached LTspice sim.

author: radiolistener
author_rank: Super Contributor
timestamp: November 15, 2019, 12:57:14 pm
content: with analog circuit you will get big phase error and floating frequency. I'm not sure, but I think it may affect your measurements.What about si5351 + two dividers and LPF on the output?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2019, 03:06:16 pm
content: How accurate does the phase need to be?Any phase shift network will do. You may not be able to do it with a single inductor, but there will be solutions that are relatively stable over that narrow frequency range.If nothing else, you can tack on an all-pass phase shift network and buffer it.Tim

author: cncjerry
author_rank: Unknown
timestamp: November 15, 2019, 11:20:47 pm
content: twisted wire quadrature hybrid by Breed et al.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 16, 2019, 12:45:55 am
content: Quote from: cncjerry on November 15, 2019, 11:20:47 pmPerfect!I didn't know that trick, thank you.

author: cncjerry
author_rank: Unknown
timestamp: November 16, 2019, 05:01:35 am
content: I have used one twisted wire quadrature hybrid from 0-50Mhz. Look at the R2 receiver by Rick Campbell. He used it in the down converter. It is very easy to tune, usually just by squeezing the wires on the toroid, if at all. By the way, there are two versions and both work the same. One version uses two capacitors, IIRC 100pf, one on each side and the other version just used one 200pf. I can't remember why you would use one vs the other but of course there is a debate.The only thing you need to select correctly is the mix of the toroid. If you use the red toroids you will easily get from very low to 30Mhz. Lots of advantages of using it vs a Johnson counter. I often wondered why MCL didn't put one in a little can. Heck, they make about two dozen other power splitter/combiners, about 100 mixers, etc. For high frequencies, you can use those really small toroids with some very fine wire and with the proper loading, you will be amazed how nice and balanced the I/Q signals look.Oh, and remember to use twisted wire. Put two ends, one of each color, tied to a nail and the other ends in an electric drill. This will give you a very consistent twist. Not that it matters, but it looks better.Jerry

author: Bud
author_rank: Super Contributor
timestamp: November 19, 2019, 04:47:11 am
content: 2-way 90 degree power splitter, Minicircuits has a range of them for various frequencies.

author: OwO
author_rank: Super Contributor
timestamp: November 19, 2019, 06:02:11 am
content: Quote from: RoGeorge on November 15, 2019, 12:11:05 pmThere is always going to be amplitude and phase error and you will still see some ripple in the detector output, but if we ignore that there is still the issue of controlling the amplitude fast enough, which is hard to do over shorter than tens of cycles because you want a high Q tank for your oscillator. If all you want is a stable output why not just drive an amplifier into saturation?

author: RoGeorge
author_rank: Super Contributor
timestamp: November 20, 2019, 03:13:57 pm
content: There will be some ripple in the sum of the squared quadrature signals, indeed, caused by the phase errors and the amplitude errors. Will look at them, but first, this is the plan:- a sample placed in a uniform magnetic field, surrounded by a coil, as in the first draw. The coil is pumped with a very low RF oscillation (mV range) of a frequency swept around the resonance of the atoms in the sample, and the hope is to measure amplitude changes induced by the probed sample, which for start it will be the protons' resonance of the Hydrogen atoms in water.- an amplifier with a voltage controlled feedback loop, as in the second draw. Its purpose is to maintain the oscillations at a constant amplitude even during the time when the sample is in resonance and absorbing more energy from the coil. To keep the amplitude of the oscillations constant, there is an amplitude detection circuit as seen in the third draw.- the amplitude detection circuit in the 3rd draw is based on 2 analog multiplier followed by a voltage adder. The 2 multipliers are fed with the original signal, Acos(\$\omega\$t), and its corresponding quadrature signal Asin(\$\omega\$t). The potentiometer is to balance as much as possible any amplitude errors between the two squared signals.In practice there will be some amplitude error and some phase error in the quadrature signals, but in theory, if the quadrature signals would be perfect, then the amplitude detector circuit will output a constant DC value, A2.Note that the square of a sinusoidal waveform is still a sinusoidal waveform, except it has exactly double the frequency of the original signal, and will also have a DC component equal with the amplitude of the original signal. This is true for either sin or cos, and after summing the squared sin and cos, the output of the detector will be proportional with the amplitude of the original signal because:V2= A2sin2(ωt) + A2cos2(ωt) where A is the amplitude of the two outputs;V2= A2(sin2(ωt) + cos2(ωt))V = A due to the trigonometric identity sin2(θ) + cos2(θ) = 1.The big advantage is the amplitude is known at all times, even at the points where the original signal is passing through zero, thus the feedback loop will be much faster than classical amplitude detection e.g. a peak detector.However, any amplitude or phase errors in the quadrature signals will add some ripple in the extracted DC amplitude signal. The next charts show how much ripple is seen for the case of a big 10% amplitude error, than a big 5 degree phase error, then both 10 percent amplitude error and 5 degrees phase error. In the upper right corner, in green, is written the ripple as a percentage of the ideal DC Amplitude.Those were very big errors, to see how they impact the DC output. The next two examples are with more realistic values, based on datasheet errors specs for the multipliers and small phase errors from the quadrature splitter. Note that the phase errors must be kept very low, a 0.3 degrees phase error alone will be enough to produce a 1% ripple in the DC amplitude.Next 3D chart is for how much ripple is caused by amplitude and phase errors combined:I want to fool around with this particular idea of summed squared quadrature signals mostly because I only recently found out about it from another EEVblog post (https://www.eevblog.com/forum/projects/low-distortion-audio-oscillator-stabilized-via-trigonometric-identity/), and I'm curious how well it works in practice, then because I want to come with something new, since the project is just for the fun of it and I don't care about the risk of failure or about deadlines.- The multipliers will be AD835 (https://www.analog.com/media/en/technical-documentation/data-sheets/AD835.pdf), because I already have a few of them (bought very cheap from AliExpress)- The magnets will be from a $10 magnetic welding clampInstead of a dedicated mechanical support for the magnets, will try to use at first a vice's jaws.No idea if any of the above will work as expected.Quote from: OwO on November 19, 2019, 06:02:11 amThe amplitude control will most probably be implemented with yet another multiplier, used as a voltage controlled amplifier. I don't understand what are the speed concerns you mentioned in relation with the Q of the LC, and for now I know no other way to implement a faster control. If you see another way to do that, or noticed any errors, please explain them.About the stable output, there are two main ways of noticing the resonance in the sample for single coil CW-NMR:- feeding the sensing coil with a constant signal from another generator, then measuring any change in the coil voltage (a so called Q-meter)- using the sensing coil to form an oscillator, then measure the amplitude of the oscillations. This is the one I want to try first, and the expectation is that there will be a feedback signal proportional with the resonances in the sample.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 20, 2019, 03:36:39 pm
content: On-chip at higher frequencies, I see people using injection-locked oscillators sometimes. Similarly, take 4 differential amplifiers in a loop (add 180 degree shift by flipping connections of last stage back to first). Use the impedance of the loop to tune frequency. If you need very high frequency accuracy, injection-lock them. To get very high phase accuracy, you can use some gain tuning of each stage (just add small variable resistor in supply network or tune the bias of the current source of your differential amplifier) to compensate.Ofcourse, you can also just use an RC and CR filter in series with your output, which will generate +- 45 degrees, which together add up to 90 degrees total. With discretes all of them can be made tunable to get rid of errors.With regards to your idea for summing them: keep in mind that the phases of any quadrature generation method will also depend on the impedance they are loaded with. Because you are loading it with another AC source at the same frequency, it will appear as a very capacitive or inductive impedance. You might want to figure out how to have some kind of active isolated summing stage?

author: OwO
author_rank: Super Contributor
timestamp: November 20, 2019, 04:11:35 pm
content: Ok, I get what you are trying to do now. Your approach looks workable, but it may be challenging because it's hard to do analog multiplies accurately to many MHz. What you are trying to do is very similar to what a 1 port VNA does, so here is the approach I would take:1. Instead of stabilizing the output over short timescales, measure it and correct for amplitude error afterwards. Use two receiver channels, one "outgoing power" channel and one "reflected" channel. Use a resistive bridge directional coupler like the NanoVNA.2. You observed the fact that detecting the amplitude of a sinusoid "instantly" is impossible because squaring it (or multiplying it by a complex exponential) produces ripples at 2f. The trick to remove the 2f component is to simply integrate over an integer multiple of the period because a sinusoid integrated over one period is 0. In other words simply observing one or more periods of the sinusoid is good enough to observe its amplitude; no need to "filter" over a long time period or use quadrature signals. This is what is implemented in commercial network analyzers.3. To actually stabilize the output of an oscillator there is no need to quickly respond to amplitude changes. The amplifier should be isolated from the load by a buffer followed by an attenuator. The attenuator guarantees the load sees a "linear" signal source with a defined impedance, and also guarantees load changes do not change the forward power.4. The reference channel corrects for medium-term (several periods) amplitude stability, the high Q of the oscillator (and power supply filtering) guarantees short term amplitude stability, and oscillator pulling is prevented by the isolation buffer and attenuator.