[time-nuts] Re: DIY Low offset Phase Noise Analyzer
On 7/13/22 5:24 AM, Dan Kemppainen via time-nuts wrote: Erik, Just a thought. The 7805 in your schematic appears to be a big part of the signal chain. I've run into issues with 78xx series regulators being noisy. Spent a few weeks chasing down noise issues in some equipment and the 78xx regulator was a part of the problem. Replaced it with LT3042 as part of the solution. Also note, not all 7805's are created equal. Different vendors may be better/worse than others. Good luck! Dan +1 for the 200 mA LT3042 (and the higher current 0.5A LT3045). There are now negative voltage versions, too, LT3094, also 0.5A. Noise from 10Hz to 100kHz is around 0.8 microvolts. At 10 kHz 2 nV/sqrt(Hz). But the real thing these things are good at is PSRR at higher frequencies. >70dB at 1 MHz They have enable inputs, can be paralleled willy-nilly, etc On 7/12/2022 9:45 PM, time-nuts-requ...@lists.febo.com wrote: I'm struggling with the noise floor. First tests where done with a 5nV/sqrt(Hz) opamp. Noise floor with shorted mixer output at 10kHz was -140dBc/Hz. Then I tried with 1nV/sqrt(Hz) opamp, but that made no difference, noise floor at 10kHz was still -140dBc/Hz The setup was simplified to this schematic: http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG The REF_buffer creates a virtual ground, the Audio_LNA amplifies into the differential audio output . Why did the lower noise opamp not make a difference? Also the setup is acting like a nice microphone. Tapping the housing is clearly audible. Which component may be causing the microphony? ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)
On 7/12/22 3:51 PM, ed breya via time-nuts wrote: I forgot to mention that you should also consider possible effects from the RF present, on the LNA. This can be more significant than SMPS frequencies getting where they don't belong, especially since the RF is intentionally right at the opamp's input. Your LPF only reduces, and does not eliminate, the 2F and harmonics, so there can be significant RF present on the LNA circuit. A simplistic view is that the RF is far beyond the opamp's GBW or closed loop gain and should have no response, but it's not at all beyond upsetting or altering the operation. This can result in extra DC offsets and noise due to RF rectification in the input circuits, which only remain "linear" at frequencies where the output and feedback can keep up with the input. This can be fixed if necessary, by adding extra RF filtering, particularly some built to low-pass at a higher cutoff frequency well above the analysis frequency, and well below the expected f and 2f. For instance, in your circuit it looks like L1 is 1 mH, with 100 nF caps, which ideally cuts off quite low. However, 1 mH is a pretty big choke, and will tend to have a lot of inter-winding capacitance (and high resistance - don't forget to include it in noise), making it less effective at the higher frequencies. Adding an LC section in front of it, but set up for something in the MHz region, will give much greater rejection of the f and 2f, due to having more appropriate smaller L and C. Anyway, if it works fine as is, then no problem, but it's something to be aware of if you get strange effects down the road. Ed and a single LC is only a single pole, so the roll off isn't all that great in a dB/decade sense. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)
On 7/12/22 8:53 AM, Erik Kaashoek via time-nuts wrote: I'm struggling with the noise floor. First tests where done with a 5nV/sqrt(Hz) opamp. Noise floor with shorted mixer output at 10kHz was -140dBc/Hz. Then I tried with 1nV/sqrt(Hz) opamp, but that made no difference, noise floor at 10kHz was still -140dBc/Hz The setup was simplified to this schematic: http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG What's the noise contribution of the resistors? V = sqrt(4*k*T*R*B) 51 ohms is sqrt (4 * 1.38E-23 * 300 * 51) = 0.9 nV/sqrt(Hz) 100 ohms is ~1.4 nV/sqrt(Hz) What kind of op amp? what's the current noise vs the voltage noise? - you might low voltage noise, but high current noise, and that current noise across the input impedance can turn into surprisingly high voltage noise at the output. The REF_buffer creates a virtual ground, the Audio_LNA amplifies into the differential audio output . Why did the lower noise opamp not make a difference? Also the setup is acting like a nice microphone. Tapping the housing is clearly audible. Which component may be causing the microphony? My guess would be a parasitic capacitance between circuit and housing. changing the distance changes the C. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: GPS failed
On 7/10/22 4:19 PM, skipp Isaham via time-nuts wrote: Hello to the Group, I'd like to get some opinions and war stories regarding GPS reliability at high RF level and elevation locations. Background: Three different hill-top GPS receivers, all different types, using different antennas mounted on an outside fixiture, plain view of the open sky, all stopped working. Test antennas were brought in and placed on a fixture well away from the original antennas, the recevers went back in to capture and lock. From what I understand, the original antennas are what I would call straight preamp with no pre-selection / filtering. The ordered and now inbound replacements are said to contain a SAW filter system. It is the intent of the client to just place these "improved antennas" in to service and get on with life. I would suspect a GPS antenna (and receiver) could be subject to RF overload or blocking, however, we're assuming nothing major has changed at the site, nor any nearby location. One might think there are more GPS receivers being pushed out of reliable operation by the world around them, I'm just not hearing those stories from a lot of people using them (GPS receivers). yes, this happens. We used to have a Pendulum timing receiver with a typical "small white cone" type amplified antenna - if someone was on the roof with a cellphone, it lost lock, presumably from the (way out of band) emissions. As to where the interfering source is - it doesn't take much, and it could be some distance away. After all, this was the big deal with LightSquared - it was moderately high powered terrestrial broadcast transmitters in the satellite downlink band next to GNSS. Any new install GPS receiver antenna ordered will/should contain some pre-selection to potentially avoid a problem, even some years down the road? Seems like that's where things are going... no more off the shelf, wide band, (hot) preamplified GPS antennas in busy locations? Thank you in advance for any related comments and/or opions ... cheers, skipp skipp025 at jah who dot calm ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] dual supplies Re: Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)
On 7/10/22 9:07 AM, Bob kb8tq via time-nuts wrote: Hi Yes it is a pain to implement dual supplies. I ponder that issue every time I build one of these setups. I’ve built a lot of them …. If you are going to do a single supply, setting up a “virtual ground” is probably the best way to go. Do it with a drive circuit to provide very clean 15V off of a 30V supply then tack everything ( including *all* the mixer grounds to that 15V supply. Yeah, but that virtual ground brings with it it's own set of problems. For instance, it has to both sink and source current, so you can't just use a 3 terminal regulator to create the midpoint, although I've seen schemes with a resistor from virtual ground to negative supply, but that's not very power efficient - the resistor needs to see, say, 10x the maximum sink current. Now that I think about it, some sort of op amp driven appropriately (or a complementary PNP/NPN pair?) might work - but then you're concerned about the output Z of the op amp, and how it varies over frequency. And, of course, the bias current. We face this in designing low noise instruments for space. You're running off a DC bus of some sort, and DC/DC inverters tend to be noisier than straight out buck converters or linear regulators. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)
On 7/7/22 8:55 AM, Bob kb8tq via time-nuts wrote: Hi Yes, you do need to know the system gain. Since we are talking about gain at audio, measuring the gain directly is not a crazy thing to do. One of the things that makes audio spectrum analyzers a nice tool for this that they eliminate the “variable gain to the sound card” issue. Some sound card setups are a lot easier to work with than others. If you are restricted to the sound input on your motherboard things can get a bit crazy. It is not unusual for folks to dig up a “pro” (whatever that means on a sound card ) card that has better drivers and more access to this and that. Given how fast the PC world changes, the board that was a wonderful thing last time somebody dove in, likely is long out of production by now. The drivers that made it work so well may have been “improved” and it no longer gives you the control it once did. This makes for a bit of trial and error to get it all going. Bob Rather than a sound card, it might be better to pick a small singleboard like a Teensy that has a decent ADC, and make a "sampling engine" with a USB interface. Or, in general, going to a USB interface sound interface might be good. You can get them with a lot of channels (at least 8) and they sample simultaneously, so the uncertainty in USB latency won't bite you. Google for things like the Focusrite Scarlett I've not tried it for this kind of application, but it is likely to have better noise properties than a "inside the PC" card. Typically 24 bit converters and 192kHz sample rates. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Fixing PN degradation via ADEV measurement
On 6/20/22 2:39 AM, Magnus Danielson via time-nuts wrote: So, a counter is really like an ADC for phase, with wide bandwidth input and a sub-sampling mechanism (trigger/time-base). Through processing frequency estimates can be provided. Aliasing occurrs in the sub-sampling. Modern counters can provided estimation filters than goes from a higher sub-sampling rate to a lower, which to some degree removes aliasing, but not fully. These frequency estimation methods form a form of decimation filter. Cheers, Magnus An intruiging thought as I drink my first cup of coffee (meaning it's not well thought out).. jumping off from "counter is similar to an ADC for phase" - is there a time domain equivalent for Nyquist criterion? Certainly there's the cycle ambiguity.. you know when the zerocrossing occurred, but not how many are in between (although a counter usually does). For everything else there is a frequency/time duality, so I suspect there is. The criterion is usually explained in terms of information - so there should be an equivalent "has all the information" statement for counters/gate widths/precisions. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Is SC the most stable cut for lowest phase noise?
On 6/12/22 6:30 PM, Bob kb8tq via time-nuts wrote: Hi Tear into some of your SC cut based OCXO’s. Take a look at the crystal package. For bonus points, open up the crystal package. If you have the gear to test it, take a look at what the gas *is* inside the package. ( Good luck with that :) :) :) ) If you had the gear and the willingness to scrap out OCXO’s you would find that a number of fast warmup OCXO’s have a *tiny* amount of He in the package. Measuring this would be tough ( it’s that small). Go through the thermal modeling and it’s *way* more conductive (thermal wise) than a *perfect* vacuum …… Bob And the worst thing is that if your vacuum sealed widget is in an atmosphere with more He around (like waiting for a launch, in a place that does more He leak tests, etc.), the He will diffuse into your package and it doesn't work like expected. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Is SC the most stable cut for lowest phase noise?
On 6/10/22 1:57 PM, Dr. David Kirkby wrote: On Fri, 10 Jun 2022 at 17:39, Lux, Jim via time-nuts wrote: On the subject of rapid warm up. I suppose if you had a need, one could dump as much power as you need into the heater. Turn on oscillator, lights in room dim for a few moments. Is that not likely to damage a crystal? Different parts of the crystal and likely to be at significantly different temperatures at the same time, putting a lot of stress on the crystal due to a thermal gradient. It's probably a bit academic, as nobody is going to make an oven that heats up in fractions of a second, but if one did, I suspect it might not do the crystal a lot of good. This is only an educated guess - I don't have anything to back it up. Oh, it would be disastrous, although quartz is pretty strong, all the rest of the mounting components might not be. At the other extreme, would there be any advantage in actually heating the crystal very slowly, over the course of an hour/day/week, so the temperature gradient across the crystal is very small? Of course, if an oven took ages to reach the correct temperature, it would be inconvenient for most applications, but for some applications, the advantages might outweigh the disadvantages. Of course, if one does this, I suspect one would have to cool the crystal slowly too to prevent a significant thermal gradient across the crystal. I know it's a bit different, but I have a 600 mm f4 Nikon camera lens. I was told that Nikon cools the front element over a period of 6 months to reduce stresses in the glass. Big glass mirrors for telescopes do the same. Dave ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Is SC the most stable cut for lowest phase noise?
On 6/10/22 12:13 PM, Richard (Rick) Karlquist wrote: I think there is a severe misunderstanding of this issue. First of all, "rapid warmup" is a red herring. The real issue is "rapid frequency stabilization". Indeed. But to a certain extent, that's why oscillators don't have 5 second warm up times - it wouldn't help. So the "few minutes" is a good compromise between design simplicity and waiting for the internals to equilibrate. I suspect that those fancy USOs in vacuum bottles take a long, long time to come to equilibrium. And that is one of the claims to fame of the CSAC - from power on to "on frequency" is quite short. The time it takes for the oven to cut back (typically only a minute or two) is a very minor part of the time budget to get to frequency stabilization. You could have an AT cut oscillator that reached "oven warmup" in 1 second, but then you would have something like a 1 hour wait to get frequency stability, due to the thermal stresses. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Is SC the most stable cut for lowest phase noise?
On 6/9/22 8:53 PM, Bob kb8tq via time-nuts wrote: Hi There happen to be *some* AT cut based OCXO’s that beat the typical SC cut on warmup … just saying …. :) Bob On the subject of rapid warm up. I suppose if you had a need, one could dump as much power as you need into the heater. Turn on oscillator, lights in room dim for a few moments. But everything is a tradeoff, and I suspect that over time "standard designs" sort of migrate to particular ratios of things like peak vs average heater current, etc. Especially in applications driven by design rules for things like "maximum current per connector pin" or "component derating" - I suspect that drives things more than the fundamental physics, in most cases. Why are diodes rated at 1 or 3 Amps? And not 2? ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
On 5/31/22 3:23 AM, Carsten Andrich via time-nuts wrote: On 31.05.22 01:10, glen english LIST via time-nuts wrote: Be aware not to confuse the antenna ground plane (the patch will always have its own plane because the top metalization must be fed against a plane or counterpoise - and a ground plane behind the antenna. I can see the usefulness of the larger ground plane for any purchased patch antenna to reduce the likelihood of interference underneath (if the feed coax has a good RF contact with the plane), and if the plane is coupled well, it may improve the low angle response . The supplementary ground plane doesnt have to have a galvanic connection if the gap between the underside of the patch is low- IE use purely a capacitive coupling to tie the patch antenna ground to the large ground sheet- [...] That means reducing the gap to about 0.05mm OR increasing the area- probably means using a bigger patch. Hi Glen, thank you for the insight. I was referring to a ground plane behind the antenna. Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with IP67 housing and an external ground plane seem practically challenging to me. When it comes to stacked patch multi-band antennas like u-blox' ANN-MB [1], the gap between the top patch and the external ground plane is probably significantly higher. Yet, u-blox generally recommends the use of a symmetric ground plane for the RTK applications [1,2]. From my experience, the M8P and F9P RTK fix barely works without a ground plane under the u-blox antennas. While it's just an empirically educated guess, I'd assume that what is required for RTK will not hurt for timing. Could you share your expert opinion on this? My antenna expertise is admittedly limited to reading data sheets and picking the right one for the particular RF measurement requirements. Thanks and best regards, Carsten I would think that the large grounded sheet below the antenna helps more for making the pattern uniform, and, to a certain extent, suppressing some multipath coming from "below" the plane of the sheet. - not as good as a choke ring(s), but not bad. That is, the sheet is not intended to couple to the antenna's ground plane, but is there as a predictable surface (and, probably, to provide a magnetic material for a puck to stick to). As such, the distance from the antenna's ground plane is not particularly critical. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: measuring tiny devices
On 5/26/22 8:18 AM, Bob kb8tq via time-nuts wrote: Hi The real answer to the problem is to dig into the bowels of 1940’s electronic craft. There are various methods for setting up an L/C filter. You short this / open that sweep to find a dip or a peak. You move it to the “right” place. Just what you do depends very much on the filter design. Many L/C’s got done this way or that way simply because they would fit a known alignment method. While it all sounds very cumbersome and obscure it actually isn’t. Long ago I stumbled upon a gal setting up very complex L/C IF filters this way. The display gyrated this way and that way as she did this or that. I don’t think it took her more than a minute to get the whole thing set up….. to this day, I’m amazed by how fast she was. Do I have any useful links to actually read up on this magic? … sorry about that. Bob There are actually computer driven screwdrivers to do tuning on cavity filters. The operator puts the screwdriver to each cavity in turn. The filter is hooked up to a VNA with a computer that runs the scripts.. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: measuring tiny devices
On 5/26/22 8:24 AM, Richard (Rick) Karlquist wrote: The tweezers are really good *only* for single components - even if they come with test leads, that's for measuring something like a motor start capacitor. I tried using tweezers (cheap ones to be sure) to measure a moderately complex assembly (trying to figure out stray C). It was a gruesome failure. VNA's of any kind (no matter how small their size) don't work well on components that are too far away from 50 ohms, at least if you make a simple minded s11 smith chart measurement. There are complicated work-arounds for these measurements, but they require different configurations depending on what you are measuring, so there is no turn key or universal solution. https://www.mwrf.com/technologies/test-measurement/article/21849791/copper-mountain-technologies-make-accurate-impedance-measurements-using-a-vna describes the various approaches With the low price of available VNA's, anyone can afford to buy one, but that doesn't mean they know how to use it correctly. Oh man, is that ever true. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] measuring tiny devices
On 5/25/22 3:16 PM, ed breya via time-nuts wrote: Thanks Mike, for info on LCR alternatives. It's good to know of others out there, if needed. I have an HP4276A and HP4271A. The 4276A is the main workhorse for all part checking, since it has a wide range of LCZ, although limited frequency coverage (100 Hz - 20 kHz). The 4271A is 1 MHz only, and good for smaller and RF parts, but very limited upper LCR ranges. I think it works, so I can use it if needed, but would have to check it out and build an official lead set for it. I recall working on it a few years ago to fix some flakiness in the controls, so not 100% sure of its present condition. The main difficulty I've found in measuring small chokes is more of probing/connection problem rather than instrument limitation. For most things, I use a ground reference converter that I built for the 4276A many years ago. It allows ground-referenced measurements, so the DUT doesn't have to float inside the measuring bridge. The four-wire arrangement is extended (in modified form) all the way to a small alligator clip ground, and a probe tip, for DUT connection, so there is some residual L in the clip and the probe tip, which causes some variable error, especially in attaching to very small parts and leads. When you add in the variable contact resistance too, it gets worse. Imagine holding a small RF can (about a 1/2 inch cube) between your fingers, with a little clip sort of hanging from one lead, and pressing the end of the probe tip against the other lead. All the while, there's the variable contact forces, and effects from the relative positions of all the pieces and fingers, and the stray C from the coil to the can to the fingers. I have pretty good dexterity, and have managed to make these measurements holding all this stuff in one hand, while tweaking the tuning slug with the other. I had planned on making other accessories like another clip lead to go in place of the probe tip, but not yet built. I also have the official Kelvin-style lead set that came with the unit, so that's an option that would provide much better accuracy and consistency, but the clips are fairly large and hard to fit in tight situations, and the DUT must float. Anyway, I can make all sorts of improvements in holding parts and hookup, but usually I just clip and poke and try to get close enough - especially when I have to check a lot of parts, quickly. The other problem is that the 4276A is near its limit for getting measurements below 1 uH, with only two digits left for nH. The 4271A would be much better for this, with 1 nH vs 10 nH resolution. If I get in a situation where I need to do a lot of this (if I should get filter madness, for instance), then I'll have to improve the tools and methods, but I'm OK for now, having slogged through it this time. You might check out the NanoVNA - people have made a variety of novel fixtures for measuring small parts (i.e. 0604 SMTs) It certainly has the measurement frequency range you need. The trick is figuring out whether you want to do a series or shunt measurement, and that sort of depends on the reactance of your device at the frequency of interest. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
