Need LNA for low-frequency low-level preamp - Page 1
Forum section: electronics / general

author: dnessett
author_rank: Regular Contributor
timestamp: August 03, 2019, 02:14:46 am
content: I'm not sure what is the proper forum board to ask this question, but as it relates to signal conditioning of the output from a phase noise detector, I imagine Metrology is as good a place as any.I have run up against a problem analyzing the output of a phase noise detector using a low-frequency spectrum analyzer (PicoScope 4262). The project I am working on is documented inthis EEVblog thread, which in total is quite long. For those unfamiliar with the project, the 3 posts beginning withthis messagesummarizes the approach using a HP11729C in Frequency Discriminator mode. While I am now trying to use the HP11729C in Phase Detector mode, the problem I am attempting to solve is applicable to both configurations.Specifically, this problem is the noise floor of the PicoScope 4262 is around -92 dBm, whereas the phase noise information in the output of the HP11729C is below that. For example, I was trying to analyze the phase noise of a Bliley NV47A1282, which has phase noise specs: (10Hz: -120 dBc/Hz ; 100 Hz: -130 dBc/Hz; 1KHz: -140 dBc/Hz). For the given oscillator carrier signal level, all of these values are below -92 dBm. (It is a bit more complicated than this, but the power spectrum information in the HP11729C output, which requires correction to get the phase noise spectrum, is also below the PicoScope 4262 noise floor.)This suggests using an LNA with suitable bandwidth to amplify the HP11729C signal before processing it with the PicoScope. The problem is I am having trouble finding an LNA suitable for this purpose. Specifically, I need an LNA with bandwidth 1Hz - 100KHz and gain around 30-40 dB. Actually, I need one with 60-80 dB gain. However, I could gang two LNAs with the lower gain together to obtain what I need. Furthermore, the LNA must amplify very low signal levels, as low as 10 nVRMS.I have searched extensively for such an amplifier. Most LNAs I have found have bandwidth in the 100 KHz to GHz range. An exception are audio LNAs, but the ones I have found generally have bandwidth 20Hz-20KHz. This would eliminate the low offset frequencies that are of significant interest in phase noise analysis. Furthermore, the audio LNAs I have found are not intended for the low signal levels that the application must handle.I did find one possible device, theAlphaLab Oscilloscope Preamplifier LNA10, that might work. It is claimed that the gain of this device is up to 1000X (60dB- clarification on 8-3-2019: 60 dB voltage gain; 30 dB power gainNope - dumb error. As long as input and output loads are equal, voltage and power gain are equal), but the device's noise spec is 4 nV/sqrt(Hz), which means it is probably useful in this application at low offset frequencies (say 1Hz-100Hz), but above that range may introduce significant noise that will mask the phase noise in the signal.If anyone has experience with this device or has suggestions for other devices that might work, I would appreciate their input.

author: awallin
author_rank: Frequent Contributor
timestamp: August 03, 2019, 01:13:39 pm
content: Hoffmann "lono" could be a starting point:http://www.hoffmann-hochfrequenz.de/downloads/lono.pdfit uses many (N) ADA4898 op-amps in parallel to get 1/sqrt(N) improved input-referred noise.Johnson noise from 50 Ohm at room-temp (~1 nV/sqrt(Hz)) should be doable, but 1/f noise in the op-amps is increasingly problematic if you want <<10Hz frequencies...

author: jhenderson0107
author_rank: Regular Contributor
timestamp: August 03, 2019, 03:15:27 pm
content: Perhaps the Standford Research SR560https://www.thinksrs.com/downloads/pdfs/catalog/SR560c.pdf?

author: splin
author_rank: Frequent Contributor
timestamp: August 03, 2019, 03:38:10 pm
content: What is your source impedance?

author: chuckb
author_rank: Frequent Contributor
timestamp: August 03, 2019, 03:51:21 pm
content: The older HP 461A preamps are nice but they roll off the gain below 1khz.The AlphaLab Preamp amp should definitely help your setup.Attached is a simple amplifier designed by Charles Wenzel with less than 1nV / rt Hz noise density. This is 4-6x less noise than the Alpha Amp.Another simple step to improve your sensitivity by 6dB is to remove the 600 ohm load on the output of the HP11729C test fixture.Good Luck!

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2019, 04:39:55 pm
content: Like awallin suggests, there are some operational amplifiers with low enough noise to be used directly but 1/f noise is really going to hurt performance. See below for how to handle this. Paralleling several of these parts for even lower noise will help. None of them include current feedback amplifiers but these can be cascaded later for gain stages.An alternative I would consider for the first stage is a discrete amplifier. Medium power bipolar transistors like the BC327/BC337 (and BC639/BC640?) are good as low impedance low noise amplifiers. FETs have higher 1/f noise but may be feasible as well if that is removed. Again, see below. Many integrated transistors like the MAT03 have well defined noise characteristics and lower base spreading resistance than discrete parts. I have never tested it, but ring emitter type transistors might be a discrete part with low base spreading resistance.Somewhere below 10 Hz, 1/f noise will become a real problem. Chopper stabilized amplifiers have flat 1/f noise because the chopping process removes it. So a parallel path amplifier with the chopper removing the 1/f noise of the main amplifier is feasible. Because of the low bandwidth involved, the frequency breakpoint for lowest noise can be found with just a DC voltmeter; adjust it for lowest low frequency noise measured using the standard deviation of the output. I am a little fuzzy if choppers can be used in parallel for better performance in this application but I would at least try it. Just make sure that they are synchronized which probably means using a dual or quad part. Some single parts allow synchronization however via a separate clock pin.With enough bipolar inputs in parallel, maybe 1/f noise will not be a problem above 1 Hz.Watch out for the construction of the gain cascade. It needs to be well constructed to avoid oscillation caused by feedback from the output back to the input. I would not count on more than 40dB in a single enclosure. With as much gain as you are asking for, I might convert the signal to differential before most of the amplification.Long ago I did something similar to what you are suggesting but without the need for such high performance although it was a differential amplifier complicating things. I used a pair of LT1028s in a differential x1000 configuration with a dual LTC1151 chopper stabilized amplifier to remove 1/f noise and drift by driving the adjustment pins of the LT1028s. Noise was low enough to measure low values of resistance via Johnson noise alone. Adjustment of the frequency breakpoint was done exactly as I described above and the results matched the datasheet noise specifications exactly.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2019, 04:53:10 pm
content: Quote from: chuckb on August 03, 2019, 03:51:21 pmGetting below 1nV/SqrtHz was pretty easy even 40 years ago. But getting that below 10 Hz or even 1000s of Hz if you try to use JFETs or especially MOSFETs is very difficult because of flicker noise. Modern CMOS RF processes are incredibly bad for this and RF bipolar transistors are not great.Usually it would not be a problem however because the close in phase noise of the oscillator would be greater than the flicker noise of the amplifier (1) so I am not sure Dnessett's requirements (2) are realistic. So I just described how I would tackle the flicker noise problem anyway.I suspect a precision low noise bipolar amplifier will handle it just fine without anything fancy. At most, parallel a bunch of precision low noise operational amplifiers at a relatively low gain for the first stage. Take advantage of that 25 ohm source impedance.(1) Low noise oscillators use low flicker noise transistors for this very reason which suggests medium frequency amplifiers using bipolar transistors will always be sufficient for this application.(2) "I need an LNA with bandwidth 1Hz - 100KHz ..." - Dnessett

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2019, 07:04:43 pm
content: The source impedance is an important parameter. With still some 600 Ohms output impedance, resulting from a series resistor, there is no more need to get < 1 nV/sqrt(Hz). A 600 Ohms resistor is already a little more than 3 nV/sqrt(Hz).With BJT based OPs, one has to keep an eye on current noise too. The 1 nV/sqrt(Hz) of the LT1028 and similar BJT based OPs is combined with a high current noise that can give more noise if the source is more than some 50 Ohms. In the low frequency range the current noise tends to be even more important.So paralleling BJT based amplifiers is tricky and may not be a good idea, unless the source is extremely low impedance.Paralleling JFET based amplifiers is possible and allows to reach 1 nV noise level with less current noise, at least for some 10-100 Hz and up.Oscillators tend to have more noise in the low frequency band - so some 1/f noise may be acceptable.

author: dnessett
author_rank: Regular Contributor
timestamp: August 03, 2019, 08:56:35 pm
content: Quote from: splin on August 03, 2019, 03:38:10 pm600 ohms.

author: dnessett
author_rank: Regular Contributor
timestamp: August 03, 2019, 09:08:13 pm
content: Thanks to awallin, David Hess and Kleinstein on the complication of 1/f flicker noise in the amp at frequencies below 10 Hz. I hadn't considered that.

author: dnessett
author_rank: Regular Contributor
timestamp: August 03, 2019, 09:21:04 pm
content: Quote from: awallin on August 03, 2019, 01:13:39 pmThis design seems promising. Is it available as a finished product or, perhaps, a kit?

author: dnessett
author_rank: Regular Contributor
timestamp: August 03, 2019, 09:29:00 pm
content: I am not an experienced amplifier designer and that is especially true of low-noise amplifiers with the requirements I am seeking. So, while I found the comments about how one would design such an amplifier interesting, I really am looking for something already designed, i.e., a product or, perhaps, a kit I can purchase and assemble. That isn't to say these comments are not appreciated, since, for example, they made it clear that I failed to consider 1/f flicker noise in the preamp as a design issue. But, actually, I am looking for something I can purchase, rather than something I might design.

author: chuckb
author_rank: Frequent Contributor
timestamp: August 03, 2019, 11:00:05 pm
content: The HP11729C has a bipolar (2N5943) preamp built in. Does this built in amplifier need more gain to drive the Picoscope or do you want a lower noise amplifier connected to the 600 ohm output?The AlphaLab Preamp should provide low noise and enough gain for either situation.

author: dnessett
author_rank: Regular Contributor
timestamp: August 04, 2019, 03:15:15 am
content: Quote from: chuckb on August 03, 2019, 11:00:05 pmThe HP11729C has a 40 dB LNA built in. However, when measuring 10 MHz, because of the way the instrument is architected, the LNA is driven into compression. John Miles pointed this out on a different forum (seethis post on the HP/Agilent/Keysight test equipment forum). Consequently, I was forced to use a phase noise output (the 1Hz-1Mhz output) that bypasses the LNA.Added 8-04-2019: It occurred to me after writing this post that I had actually addressed the LNA compression issue on the EEVblog. The post is locatedhere.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2019, 08:30:32 am
content: Quote from: dnessett on August 03, 2019, 09:21:04 pmThe design with paralleled ADA4898 is only suitable for really low impedance source - that is something like 40 Ohms / N.So with 4 amplifiers this would be something like < 10 Ohms and less if more OPs are in parallel.AFAIK there is a newer JFET based version, discussed somewhere in the forum.For just a 600 Ohms source a single BJT based OP like OPA209 would be more suitable. It's higher voltage noise from the OP, but due to less current noise lower overall noise.As someone noted already much of the 600 Ohms is due to a resistor in the instrument (to get the standard 600 Ohms values and maybe for protection too). For lowest noise the signal should be taped off before that resistor.The other Forum points to the idea of adding some filtering in front of the internal LNA, so that this could be used.

author: ap
author_rank: Frequent Contributor
timestamp: August 05, 2019, 06:09:04 am
content: You may e.g. wish to check with Femto if you need something finished. Not cheap though.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: August 05, 2019, 05:14:17 pm
content: Quote from: dnessett on August 03, 2019, 09:21:04 pmNot as a kit, but there are some leftover boards.Note that the LONO's noise rises faster than 1/f below 50 Hz or so. That comes fromthe fact that the input capacitor at 100 or 200 uF foil is not large enough to short the10K bias resistor through the source impedance.That amplifier makes only sense for low impedance signal sources. A 60 Ohm resistordelivers 1 nV/rt Hz ; the LONO is 15 or 16 dB better ( 200 pV really). But in a 50 Ohmsystem you could not take advantage of that.I have a 95% completed follow-up version that uses a wet slug tantal 4700u/25 in theinput. That removes the steep noise rise at the low frequency corner but requires a lotof discipline when connecting DUTs. I managed to kill a LED "reference" with theprecharged 4700u cap or when connecting a 12V source suddenly could put theop amp inputs into jeopardy. But then there are 20 of them. Shared grief is divided grief...The current version has protection relays against that.Best handled by computer over the opto-coupled SPI interface. Gain & bandwithcan also be set with analog switches.I'm also working on a FET version with a dozen BF862 or a IF3602 pair. Most amplifiersyou find on the internet are not really stable, even if they claim to.The eye opener is plotting the real part of (Vsource / Isource). For most amplifiers thatgoes negative at 50 KHz and/or up. Not acceptable for a measurement amplifier.May add "personality" for audio enthusiasts.The problem is the feedback loop back into the source. Stability or bandwidth,pick one.cheers, Gerhard

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2019, 01:11:48 pm
content: Quote from: dnessett on August 03, 2019, 08:56:35 pmIs the actual impedance from the source (seen into the source terminals) actually 600 ohms, or is the source designed to work into a 600 ohm load? This makes a difference for the low-noise input stage.

author: guenthert
author_rank: Frequent Contributor
timestamp: August 06, 2019, 04:31:28 pm
content: Thanks for sharing notes and design.Quote from: Gerhard_dk4xp on August 05, 2019, 05:14:17 pmThe otherwise not-so-remarkable (and certainly not-so-low-noise at lowest frequencies) schematic of the SR560 addresses that by adjusting the current fed into the input FETs depending on the sum of their outputs. I thought that was quite nifty, even though it's more interesting for high-impedance sources.

author: splin
author_rank: Frequent Contributor
timestamp: August 06, 2019, 11:35:05 pm
content: How about this 700 ohm noise equivalent amplifier:http://www.emelectronics.co.uk/a29.htmlOk, not a lot of gain at 100kHz but enough to allow a much cheaper low noise amplifer for the rest of the gain. Not especially low noise at higher frequencies but no flicker noise and only around 9nVpp .1 to .33Hz - you'd be hard pushed to get much better from an off the shelf product for less than $200.How about using different amplifiers for different frequency bands? You could use the even lower noise, 25ohm equivalent, A23 < $500. LF noise is only 1.3nV pp but unity gain bandwidth is only 10kHz. Use it for say < 100Hz and an LT1028 for 10Hz to 100kHz, (.85nV/rtHz).If you want to go the route of paralleled opamps, bipolar amps have too much current noise but the JFET ADA4625-1 is quite impressive; 150nV pp 0.1 to 10Hz, 3.3nV/rt(Hz) @ 1kHz but only 4.9fA/rt(Hz) current noise so you can parallel as many as you can afford. Expensive at $7.8, @ 10off; the dual channel version is cheaper (per amp) but only seems to be available direct from Analog @ $5.99, MOQ 100.If you need to make the measurement in one step, you could use both amps with a crossover set at some frequency where any gain and phase errors aren't important.TiN has played with some A23s - take a look on his xdevs site. You could parallel lots of A29 amps - current noise doesn't dominate below about 10k ohms source impedance. A bit pricey mind.

author: dnessett
author_rank: Regular Contributor
timestamp: August 07, 2019, 08:00:42 pm
content: Quote from: ap on August 05, 2019, 06:09:04 amThanks. There are Femto products that would nicely meet the requirements. Perhaps the best is the DLVPA-100-BLN-S, therelevant specsof which are: 1) DC-100 KHz, 2) voltage noise 700 pV/rt(Hz), 3) variable gain of 40dB-100dB in 20 dB steps, 4) input impedance 1 Mohm. I contacted the North American distributer and was quoted a price of $2,170, which as you mention is not cheap; in fact too expensive for a hobbyist such as I. Maybe I can find a used one on ebay.

author: dnessett
author_rank: Regular Contributor
timestamp: August 07, 2019, 08:07:39 pm
content: Quote from: Gerhard_dk4xp on August 05, 2019, 05:14:17 pmThanks for your kind offer of making a surplus board available for purchase. That is very generous.However, after reading you comments, I think, perhaps, this is a project still under development. So, I probably will make do with a less capable product that is more mature.

author: dnessett
author_rank: Regular Contributor
timestamp: August 07, 2019, 08:10:06 pm
content: Quote from: TimFox on August 06, 2019, 01:11:48 pmThe HP11729C operators manual says the output impedance of the 1Hz-1Mhz noise output is 600 Ohms.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: August 07, 2019, 08:25:05 pm
content: 600 Ohms is 3.1nV/rt Hz alone; a single AD797 would be adequate then; it's voltage noise (0.9 nV/rtHz)would simply go under. It adds geometrically. Even an OPA140 could be justified if you want to go JFET.At $5 for 10 boards from PCBway, there is not much purchasing. :-)I have solved my phasenoise problem with a Timepod. No more delay lines, mixersand calibration guesswork.My own interpretation of of the Wenzel / the NIST design is this here:It has a 12 year old precursor.

author: RolandK
author_rank: Regular Contributor
timestamp: August 07, 2019, 08:31:45 pm
content: Just look at the works of Bernd Kaa DG4RBF:- Low Noise amplifier with 0,4 nVeff/Hz^-0.5 from 10Hz to 50 kHz.- He uses it to design low noise powersupply with LM723- Phase Noise Test setHis articles are in german, but google is your translater-friend. The first 2 are available if you search him in google. For the third article are only build descriptions available, but the original article explains best the principles and the calculations. You can get them from "UKW Berichte" or perhaps from the author. Scan them to pdf, ocr them with pdf xchange viewer and translate with google.Good luck.

author: dnessett
author_rank: Regular Contributor
timestamp: August 07, 2019, 09:00:29 pm
content: Quote from: splin on August 06, 2019, 11:35:05 pmNot really what I am looking for. Minimum input levels appear to be 2 mV, whereas I need something that will amplify signals in the 10-100 uV range. Nevertheless, thanks for the info.Quote from: splin on August 06, 2019, 11:35:05 pmAll these ideas are interesting, but not suitable for my purpose. As I write elsewhere, I am not an experienced amp designer (and certainly not an experienced low-noise amp designer), so suggesting IC devices that might be included in a design isn't what I am looking for. But, hey, others might find this information useful, so kudos for forwarding it.

author: splin
author_rank: Frequent Contributor
timestamp: August 07, 2019, 10:59:07 pm
content: Quote from: dnessett on August 07, 2019, 09:00:29 pmYou misread the datasheet:QuoteIt's designed for amplifying nanovolt signals.QuoteFair enough; but can you split the measurments into two parts? Use an A23 for ultra low noise low frequency measurements, say up to 10Hz or even 100Hz, and a more conventional low noise amp for measurements in the band 10Hz to 100kHz?When I say a more conventional amp it could be the simplest opamp inverting or non-inverting amplifier circuit - a couple of resistors for setting the gain and a few capacitors for decoupling and stability - well within the capability of many beginners (preferably one with access to a scope to check it doesn't oscillate!).The difficult part of your problem is flicker noise (1/f noise) at low frequencies, hence the suggestion to use an off the shelf design. There's a good thread here about testing the A10 and A23 amplifiers. Reply #25 includes the 2017 EM Electronics price list.

author: dnessett
author_rank: Regular Contributor
timestamp: August 08, 2019, 08:11:19 pm
content: Quote from: splin on August 07, 2019, 10:59:07 pmI did indeed misread it. Thanks for pointing this out.Quote from: splin on August 07, 2019, 10:59:07 pmIt is a reasonable option that I will consider. However, I still would have to design and build the second amplifier, something that doesn't appeal to me at this point. But, I won't reject it out of hand.Quote from: splin on August 07, 2019, 10:59:07 pmYou seem to have left off the URL to the thread you reference.

author: Vgkid
author_rank: Super Contributor
timestamp: August 08, 2019, 08:33:11 pm
content: Here you gohttps://www.eevblog.com/forum/testgear/nanovoltmeters-performance/?action=dlattach;attach=453013

author: maat
author_rank: Regular Contributor
timestamp: August 08, 2019, 08:34:18 pm
content: I will chime in on amplifier designs. I you have a fairly low impedance source, you could try the amplifier used by Linear Tech/ADI to test some of their voltage regulators:https://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdfIt worked pretty well for me so far and I still do have about half a dozen PCBs left of that design if someone is interested. It is a fixed 10k gain amplifier with 10 Hz - 1 MHz Bandwidth. I usually use it in conjunction with an SR560 to add more gain and do some more filtering before passing the signal on.Below 10 Hz you fare better using some zero drift design.

author: maat
author_rank: Regular Contributor
timestamp: August 09, 2019, 08:05:46 pm
content: These are some noise plots I made to benchmark the amplifier. The bottom trace is with the input shorted and the upper one is with a 10 Ohm resistor across the input. The dashed line is the Johnson-noise of the resistor. The discontinuities result from my setup. Unfortunately my spectrum analyser starts at 7 kHz, so I sampled the data below 10 kHz with two other devices and calculated the noise density from that. I also used an SR560 to amplify and filter the data for each segment.The result agrees pretty well with the Linear Tech App note.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 10, 2019, 06:48:57 am
content: Quote from: maat on August 08, 2019, 08:34:18 pmThat amplifier is BJT based and has high current noise. So it is not suitable for a 600 ohms source. The noise level is good at 10 Ohms, but expected to be poor for more than 100 Ohms.

author: dnessett
author_rank: Regular Contributor
timestamp: August 10, 2019, 09:48:09 pm
content: Thanks to everyone who comented on this topic. The contributions contained useful and germane information. Several commentors noted that oscillator phase noise increases significantly as offset frequency approaches 1 Hz and for this reason a good low noise preamp probably would suffice:Quote from: David Hess on August 03, 2019, 04:53:10 pmQuote from: Kleinstein on August 03, 2019, 07:04:43 pmQuote from: chuckb on August 03, 2019, 11:00:05 pmI decided to explore this line of reasoning using theAlphaLab Oscilloscope Preamplifier LNA10as an example. I welcome comments on the following argument.When using the HP11729C in Phase Detector mode, the lowest phase noise device should be the reference oscillator. Its phase noise should be less than either the DUT or the HP11729C itself. The device I am using as a reference oscillator is theWenzel HF-ONYX-IV (part 501-11578-04), which has typical phase noise of -135 dBm at 10 Hz (which is less than that of the HP11729C and likely that of the DUT). Since oscillator specs generally quote phase noise as power relative to a 50 ohm load, this equates to 3.976e-8 VRMS/sqrt(Hz) = 40 nV/sqrt(Hz). The input-referenced noise of the AlphaLab LNA10 is 6 nV/sqrt(Hz).To obtain the gain I need requires cascading two AlphaLab LNA10s. Since noise adds like power, the noise resulting from two cascaded LNA10s is sqrt(2*(6^2)) = 8.5 nV/sqrt(Hz). So the noise contributed by the cascaded amplifiers is about 20% of the noise contributed by the reference oscillator.It is desirable that the noise added by the preamp is no more that 10% that of the reference oscillator. Nevertheless, the reference oscillator phase noise should be much less than either the DUT or HP11729C phase noise, so the noise added by the preamp is likely less than 10% of the total system phase noise, which should suffice for my purposes.From 10-100 Hz, the phase noise of the AlphaLab LNA10 is 4.1nV/sqrt(Hz), meaning in a cascaded configuration the added phase noise would be approximately 5.7nV/sqrt(Hz). This is about 15% of the noise contributed by the reference oscillator. This is still not at the 10% desirable level, but again probably sufficient for my purposes.Given this analysis, I am leaning toward purchasing two AlphaLab LNA10s and using them as the preamp for the PicoScope 4262. If for some reason this approach doesn't work out, I may then take a closer look at theEM Electronics A29.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 10, 2019, 10:09:35 pm
content: Using 2 amplifiers in series does not increase the input noise very much. The noise of the 2 nd amplifier is attenuated by the gain of the 1 st. So with a gain of >=10 for the first stage, the second stage gives essentially no extra noise.I don't think it even needs a second amplifier. The pico-scope should be well sensitive enough with only moderate gain (e.g. 10 x, maybe 50 x) up front. I kind of doubt the extra amplifier is that much lower noise than the scope.Not only amplifiers have more noise at low frequency - also oscillator phase noise usually has an 1/f contribution. So one likely would not need the same noise level at very low frequencies. The LNA10 does not look that bad.The 4.x nV noise level is not that much higher than the noise of the 600 Ohms output impedance, which is sime 3.2 nV/SQRT(Hz). One number I am missing the the amplifiers data is the current noise or impedance for best noise figure. Chances are high the current noise is rather low, as a BJT based amplifier should have more like lower voltage noise. So the amplifier is likely JFET based.

author: dnessett
author_rank: Regular Contributor
timestamp: August 10, 2019, 11:14:39 pm
content: Quote from: Kleinstein on August 10, 2019, 10:09:35 pmThe noise floor of the PicoScope 4262 is -92dBm. As a lower bound say the phase noise of the system is the phase noise of the reference oscillator at 10 KHz (it will be higher than this, but this is a worst case assumption). This is -165 dBc\Hz.I have not yet posted the procedure for using the HP11729C in a Phase Detection configuration, but after calibration and corrections, the carrier suppressed signal is raised about 17 dB, so the effective target is -165 dBm + 17 dB = -148 dBm (The reference carrier input is set to 0 dBm during the set up procedure). To raise this level above -92 dBm requires about 56 dB gain. Since the LNA10 has a maximum gain of 30 dB, a single amp is insufficient. Cascading two together would give a maximum gain of 60 dB, which would just work.Now, I will admit the calibration and corrections algorithm is a bit complicated. So, I'm not completely confident in the above analysis. Nevertheless, each LNA10 costs$275, which means two can be purchased for$550. This seems a reasonable investment and if only one is required, I can use the other for a different project.Added 8-10-2019: I went to order the LNA10s and noticed I got the price wrong. Each costs $270, which means two cost $540.

author: chuckb
author_rank: Frequent Contributor
timestamp: August 11, 2019, 12:37:26 am
content: The LNA10 also has a 470 ohm output Z. So keep the Picoscope on High Z input.One preamp has selectable 1000x gain this is 60dB.

author: dnessett
author_rank: Regular Contributor
timestamp: August 11, 2019, 05:09:52 am
content: Quote from: chuckb on August 11, 2019, 12:37:26 amPower gain = 10 log( (Vin2/ Rin) / (Vout2/ Rout) ). Rin= 500 Kohms and Rout= 470 ohms. So, Power gain = 10 log( (Vin2) / Vout2) * (Rout/ Rin). Rout/ Rin~= 10-3, so Power gain ~= 10 log( (10002* 10-3) )= 10 log(103) = 30 dB.On 8-10-2019 (forward reference) Kleinstein wrote:Quote from: Kleinstein on August 11, 2019, 06:44:14 amKleinstein is right. I created the derivation right before I went to bed and didn't double-check my work (always a perilous practice). The problem (in the derivation) is Power gain isn't 10 log( (Vin2/ Rin) / (Vout2/ Rout) ), it is 10 log( (Vout2/ Rout) / (Vin2/ Rin) ). This inverts the ratio of resistances, which therefore equals 103not 10-3.Also, Vin2/ Vout2, isn't 10002, it is 1000-2. Vout2/ Vin2= 10002.The bottom line is power gain is 90 dB, as Kleinstein correctly points out.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2019, 06:44:14 am
content: Quote from: dnessett on August 11, 2019, 05:09:52 amThe low frequency connections between the instruments are not using impedance matching. So the use of dBm is more confusing than helpful. The relevant gain is only the voltage gain, not the power gain.In the calculation there is another mistake: the step from 500 K at the input to 470 Ohm at the output adds another 30 dB to the power gain. So looking at the power one would get 90 dB, not 30 dB.Anyway I have some doubt the HP11729C output noise is so low. Changes are there is quite some noise in the HP11729C, even if it is just the about 500 Ohms resistor used to make the output 600 Ohms impedance.

author: dnessett
author_rank: Regular Contributor
timestamp: August 11, 2019, 11:21:51 pm
content: Quote from: Kleinstein on August 11, 2019, 06:44:14 amI have corrected the error in the original post with an edit.

author: dnessett
author_rank: Regular Contributor
timestamp: August 11, 2019, 11:24:01 pm
content: Quote from: Kleinstein on August 11, 2019, 06:44:14 amI address the point made in the middle paragraph in the original post with an edit.In regards to the output noise of the HP11729C, at 10 Hz it is -115 dBc/Hz in the band 5 MHz-1280 MHz, which is much higher than the Wenzel Reference Oscillator. I used the figure for the Wenzel as a worst case scenario.The first paragraph in the response raises two new issues.1. The amplification by the LNA10 boosts the signal above the noise floor of the PicoScope 4262. However, its effect must be removed by a correction calculation on the data after the FFT is applied. Assuming I use the 1000X amplification setting of the LNA10, is the correction to subtract 90 dB from the values computed by the PicoScope? This seems correct to me, since the corrected PicoScope data will be in units of dBc/Hz, not V/sqrt(Hz). Comments?2. Figure 1 shows the configuration of the test setup from the HP11729C to the PicoScope.Figure 1 - Test Setup from HP11729C to PicoScopeMy concern is reflections. The output impedance of the HP11729C is 600 ohms and the input impedance of the LNA10 is470 ohms500 Kohms.For the short lengths of RG-58 used to connect the devices reflections can probably be ignored. (Modified 8-11-2019.)The output of the HP11729C contains useful information from 1Hz-200KHz. The LNA10 has output filtering that can be set to low-pass 200 KHz. The 1/4 wavelengh of 200 KHz is 1230 feet. The RG-58 connecting the LNA10 to the PicoScope will be several orders of magnitude less than this. The input impedance of the PicoScope 4262 is 1 Mohm. So, I think the 600 ohm resistor at the Tee (shown in red) is unnecessary (as is the Tee) and should be eliminated to reduce noise (this is what has been suggested by several commentators). Agreed?[Edits embedded in text. This has not been my weekend for clean arguments.]

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: August 12, 2019, 12:51:54 am
content: The tee & 600 Ohm termination can be safely removed at 200KHz max. Your lab isnot so large as that this could matter.The manual of my Agilent 89441A FFT analyzer recommends the Hann filter for noisemeasurements; Blackman-Harris is more for sine signals with its steep transition bands.I have finally completed my switchable delay line, but need a larger +24V bricksince I underestimated the current drawn by the coax relays.There are 3 ranks in series: coarse, mid and fine. Each rank consists of 2 pcs. 1P6Tcoax relays and the 6 semi rigid coax cables. (Nexans Quickform 3.5 mm o.d.)The 1:6 relays reduced the number of contacts in series.Making the cables was highly meditative, but those SMA connectors are surprisinglyeasy to install on SemiRigid. I could check the actual delay with a TDR.You need a lump of iron with a 3.5 mm bore hole abt. 5 mm deep. You insert the cableas far as possible and circumcise it with a sharp knife. When you bend the cable somewhat,the outer metal will easily break and can be removed. Then remove the Teflon flush withthe outer metal, solder on the center contact, shove it into the rest of the connector andsolder the outside.Those 1p6T coax relays are normally quite cheap on ham flea markets; everybodywants nothing but 1P2T. There are relays with built-in TTL drivers. Usually one ofof the drivers is defective, that's why they ended up on the flea market. 24/28V supplyand TTL inputs don't go together well. Since I need only the working coil, that couldbe healed, but it does not look good.I first wanted to use AirCell cable for its lower loss, but found out too late that thesespecial SMA connectors were too thick to fit onto the relays. :-(The delay line can bring anything from 4 to 200 MHz into phase quadrature, no matterwhat amplifiers or filters may be in the path.

author: chuckb
author_rank: Frequent Contributor
timestamp: August 12, 2019, 01:23:05 am
content: You do not worry about impedance matching anything in the mixer output signal chain. Adding load resistors only attenuates your voltage signal. Your goal is to maximize the voltage supplied by the mixer and getting to the voltage measuring device (Picoscope).When you calibrate the Mixer / delay line gain you will end up calibrating the Alpha Amplifier gain, and the PicoScope FFT gain all at the same time.As you have pointed out, you are looking for dBc. The PicoScpoe displays in dBm. The correction factor for for dBc for your setup will be determined during your Mixer/Delay line calibration.The mixer probably has a 50 ohm or less source impedance so the 500k input Z of the Alpha amp will reduce the signal less that 0.1%.The 470 ohm output Z of the Alpha amplifier will likewise not be loaded (attenuated) by the 1Meg ohm input Z of the PicoScope.The Alpha amp will need to be AC coupled to remove the DC offset voltage from the mixer.You will probably also want to use the 100kHz low pass filter in the Alpha amp In addition to the scope low pass filter to help reduce unwanted 20 MHz frequency mixer products.Mr Rubiola's web site teaches how to do all this. It seems like he has the most advanced techniques in the world.http://rubiola.org/

author: magic
author_rank: Super Contributor
timestamp: August 12, 2019, 08:15:40 am
content: Quote from: Kleinstein on August 04, 2019, 08:30:32 amThey recently released a 2210 with slightly better noise and DC specs.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: August 12, 2019, 11:42:29 am
content: It irritates me, that the 11729C is always drawn with an amplifier after the low pass.If there is one, noise behaviour of the external preamp should be not an issue.If there is none, the effective source would be just the ring mixer and the real sourceresistance would be smaller. The prescribed 600 Ohm load would only be thereto keep the high discrimination sensitivity and to keep the filter happy.But the investment into a better voltage noise spec than for 600 Ohm would still pay.

author: dnessett
author_rank: Regular Contributor
timestamp: September 11, 2019, 09:29:35 pm
content: For those interested, I have posted some notes on my experience with the LNA 10here.