Re: [time-nuts] LNA and Alias
Hi The nature of a ADC with a built in digital front end filter is to have multiple passbands centered around the ADC clock and it’s harmonics. Put another way - the chip has zero rejection at clock * N where N = 1,2,3,4,5…. They may also have zero rejection at other frequencies (like clock / 2) depending on the sampling architecture. If you are trying to look for spurs in the -120 dbc range, RF at clock * N will have to be ~ 120 db down. In this case, “down” is relative to the roughly 1 V output from the mixer. That equates to less than one microvolt of RF running around at the input to the ADC if the preamp gain = 1. If your preamp is running at 60 db of gain, you would need to be below 1 mv of RF at the input to the ADC. Since conversion at the harmonics may or may not be perfect and since phase noise goes up by 20 log (N) the attenuation needed is only approximate. It’s likely within 10 or 20 db either way. - One simple solution is to move the ADC clock far enough away from 10 MHz * N so that the beat notes are outside the region you are looking at phase noise. If you are trying to go to 100 KHz, then a clock that is a bit over 1% high or low would do the trick. The beat note is there, you just don’t care about it as much. You can’t tolerate a note that overloads the ADC, so some filtering is still needed. You also need to be careful that the close in noise does not wrap around and mess up your 100 KHz numbers. Again the solution is to have some filtering. You can go to a much larger clock offset that 1%. The risk there is that something like the 10th harmonic gets you in trouble. ——— Another “simple” solution is to just lock the ADC clock to your DUT. You still need some filtering so that the noise from the higher frequency images does not mix down and add to much to the floor. The same issues with overload would still apply as well. —— Lots to play with. Bob > On Dec 26, 2014, at 8:37 AM, Loïc Moreau wrote: > > Hi, > > I have just found the culprit > > The AD7760 eval board in charge of the ADC task, is equipped with a little 40 > MHz clock witch seems to generate a beat note with my 10Mhz noise > measurements setup. I have discovered the origin of the problem after > disconnecting the LNA from the measurement chain and visualizing the output > of the LNA with a scope. > > The beat note was clearly visible, 10mV peak-peak . After shutting down every > 10Mhz sources in my lab (a huge task) and altering the frequency of my phase > noise measurement setup noise floor, I noticed that the low frequency noise > was in fact a beat note following the ref frequency adjustments. ( The ref > OCXO was replaced with a 33521A to alter amplitude and frequency at will) . > > As soon as I fire up the 7.5V power supply of the eval AD7760 (the eval card > is not connected to anything). the LNA give a steady 10mV beat note as the 40 > MHz clock seems mixing up somewhere a few herz apart from the regular source, > polluting the close in phase noise measurements . > > So after weeks of questioning, I was a bit disappointed to find out that the > AD eval board, the least suspected item was at the origin of the problem. > > Now, I have to address it . I have noticed the presence of smb connectors on > the eval card, so I may substitute the internal clock, and a shield enclosure > may cure that little beast. At last DC power supply wires may also be > substituted by shielded cable just in case. > > I am relieved > > Regards. > Loïc > ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] LNA and Alias
Hi, I have just found the culprit The AD7760 eval board in charge of the ADC task, is equipped with a little 40 MHz clock witch seems to generate a beat note with my 10Mhz noise measurements setup. I have discovered the origin of the problem after disconnecting the LNA from the measurement chain and visualizing the output of the LNA with a scope. The beat note was clearly visible, 10mV peak-peak . After shutting down every 10Mhz sources in my lab (a huge task) and altering the frequency of my phase noise measurement setup noise floor, I noticed that the low frequency noise was in fact a beat note following the ref frequency adjustments. ( The ref OCXO was replaced with a 33521A to alter amplitude and frequency at will) . As soon as I fire up the 7.5V power supply of the eval AD7760 (the eval card is not connected to anything). the LNA give a steady 10mV beat note as the 40 MHz clock seems mixing up somewhere a few herz apart from the regular source, polluting the close in phase noise measurements . So after weeks of questioning, I was a bit disappointed to find out that the AD eval board, the least suspected item was at the origin of the problem. Now, I have to address it . I have noticed the presence of smb connectors on the eval card, so I may substitute the internal clock, and a shield enclosure may cure that little beast. At last DC power supply wires may also be substituted by shielded cable just in case. I am relieved Regards. Loïc -Message d'origine- De : time-nuts [mailto:time-nuts-boun...@febo.com] De la part de Bruce Griffiths Envoyé : lundi 22 décembre 2014 20:30 À : Discussion of precise time and frequency measurement Objet : Re: [time-nuts] LNA and Alias On Monday, December 22, 2014 09:26:15 AM Loïc wrote: > Bruce Griffiths writes: > > Are you sure that the setup is aligned to minimise AM response? > > > > Are you using cross-correlation? > > Bruce > > > > On Monday, 22 December 2014 5:08 PM, Loïc Moreau > > eai..fr> wrote: > > Hi all, > > > > My phase noise measurements system give erroneous results in close in > > phase noise measurements, I got > > > humps in the 1Hz-1000Hz area as high à 20 dB more than expected. > > > > The setup is using a mixer to compare reference and DUT witch drive an LNA > > http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is > > driving an AD7760 ADC and an op > > > amp circuity is connected to the VFC reference to achieve quadrature. The > > results are analyzed with an > > > homebrew FFT charting software > > > > After struggling with different configurations, switched different > > LNA, > > ADC , sound card. A scope > > > connected to the LNA output indicate steady 20MHz residuals just before > > the ADC ( around 10 mv peak-peak). > > > In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are > > entering the ADC and so theses alias give > > > erroneous results in the 1Hz-100Hz area, displaying unexpected > > artifacts. In order to fix the problem > > > I will probably include an analog filter just before the ADC input > > (same > > as LNA input 1nF 80µH), but I > > > want to know if some more sophisticated measures should be undertaken as > > an 5th Order Lowpass Filter. > > > I have not found many clue about alias problems in phase noise > > measurements literature so I may have > > > missed something ? > > > > Any advice > > > > Loïc > > > > ___ > > time-nuts mailing list -- time-nuts@... > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-> > nuts > > > and follow the instructions there. > > Hi, > > I have a simple setup to measure the noise floor, the mixer is a SYM-2 and > a divider is feeding RF and LO inputs ports for that purpose. > > a pi network give the 90° phase difference. > Have you measured the phase shift when connected to the mixer? I usually use a narrowband 90 degree hybrid. > > The test is running with an HP 10811 giving around 7dBm, but i have > the same results with any sources, especially a 33521A witch can be adjusted > in level and frequency. > > > I intend to use cross-correlation later when i will be confident with > my the setup, for now i run a simple FFT. > > I have no clue about AM noise problem, i suppose that a sufficient > input level on LO will put that problem aside. > The phase shift between the RF and LO ports needs to be adjusted to minimise the response to AM in the source. To do this an AM modulator with low incidental PM is
Re: [time-nuts] LNA and Alias
Bob, It may not have been Comcast. I had issues (cockpit error on my part) with the server this morning around the time you wrote your message. It should be OK now. However, the new setup is making it more difficult to attach links to documents because when you click on a document in the Manuals pages, you get a link that is associated with your IP address, so that link may not work for other people. The best way is to simply reference the page name and let people search for the page for themselves, so that they can get their own link and download the document. Overall it is a better method anyhow because I occasionally move things around to try and keep some organization, so any document is susceptible to change folder, which will break old links. I rarely change the documents names though (once they are moved out of the ManualUpload folder), so searching for the name should always work. Didier KO4BB On Mon, Dec 22, 2014 at 8:39 AM, Bob Camp wrote: > Hi > > At the moment Comcast and ko4bb.com don’t seem to like each other. I > can’t refer directly to the schematic or the page. > > What you *should* have (assuming 10 MHz inputs for clarity): > > 1) The RF inputs go into the mixer from what ever sources you happen to > want to test. At least one source needs to be high level. > > 2) The mixer output goes to an L/C lowpass filter. That filter serves > several purposes: > a) It resistively terminates the mixer at RF (both at 10 and 20 > MHz) in the proper IF impedance > b) It passes the phase noise information on to the LNA > c) It rejects all RF going to the LNA > > 3) The LNA can be just about anything provided: > a) It handles the signal levels without overload > b) It does have low enough noise (that depends a lot on the mixer > and sources) > c) It has enough gain > d) Terminates the mixer in a reasonable resistive impedance at > audio. > > 4) The LNA feeds two things: > a) Your FFT box > b) Your DC bias box > > It sounds like the FFT part is working so the DC bias may be an issue. The > bias box is used to force the two oscillators into quadrature. It forms a > PLL around the oscillator pair. With the two signals 90 degrees apart your > mixer has a ~ 0V output. That is the point it is most sensitive to phase > noise and the least sensitive to AM noise. > > There are lots of ways to do this. Since I can’t see the schematic. Here’s > one based on an RPD-1 (500 ohm out) mixer: > > One side of the mixer is grounded, the other feeds the filter. > > 500 ohms in series with 820 pf to ground as the input to the filter. Some > sort of coil in the vicinity of 100 uH as the first series lowpass element. > Next a 470 pf to ground. Then another 100 uH in series. Another 470 pf to > ground. Another 100 uH in series. Another 470 pf to ground. At this point > you have a three coil and five capacitor lowpass filter. You should poke it > into spice to make sure it’s not going to be a problem with your parts. The > issue is cutoff at the highest frequency you want to look at phase noise. > You may need to tweak values a bit. The final stage may be overkill > depending on the quality of your coils. You will always have a tradeoff > between highest phase noise frequency and lowest RF frequency with this > setup. > > LNA can be a good audio op-amp. Run it in positive gain mode. Termination > resistance for the filter is simply set with a resistor to ground. I prefer > to use 5K for the RPD-1’s. It gives you a bit more output voltage. It also > makes the cutoff of the lowpass a bit lower. > > The DC bias box is an op amp plus a pot, resistors and capacitors. You > need to set the output to the EFC voltage on the OCXO’s. That will vary > between different parts. One pot is for centering this up. You need to set > the loop gain, so feedback resistors on the op amp need to be adjusted. > Some sort of R/C may be used to roll off noise. The cutoff frequency of the > loop will determine the lowest phase noise frequency you can check unless > you measure loop dynamics and correct all your data. > > Now that that’s all working, you need to calibrate the setup. Two common > approaches. Both use a beat note formed when the bias box is shut off: > > 1) Measure an power or voltage at the LNA output and do math based on some > assumptions. > 2) Capture the full beat note and look at the actual slope as it crosses > zero. > > You pretty much have to do number 2 before you can use number 1. The LNA > needs to have low enough gain in this case to not distort passing the full > signal. The math for 2 is pretty simple. Each cycle is 2*PI radians. Phase > modulation is normalized to one radian (yes it’s phase … ). You get a > radians per volt number and move on. > > Wish I knew what Comcast was doing this morning …. > > Bob > > > > On Dec 22, 2014, at 4:26 AM, Loïc wrote: > > > > > > > > > > Bruce Griffiths writes: > > > >> > >> Are you sure that the setup is
Re: [time-nuts] LNA and Alias
On Monday, December 22, 2014 09:26:15 AM Loïc wrote: > Bruce Griffiths writes: > > Are you sure that the setup is aligned to minimise AM response? > > > > Are you using cross-correlation? > > Bruce > > > > On Monday, 22 December 2014 5:08 PM, Loïc Moreau > > eai..fr> wrote: > > Hi all, > > > > My phase noise measurements system give erroneous results in close in > > phase noise measurements, I got > > > humps in the 1Hz-1000Hz area as high à 20 dB more than expected. > > > > The setup is using a mixer to compare reference and DUT witch drive an LNA > > http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is > > driving an AD7760 ADC and an op > > > amp circuity is connected to the VFC reference to achieve quadrature. The > > results are analyzed with an > > > homebrew FFT charting software > > > > After struggling with different configurations, switched different LNA, > > ADC , sound card. A scope > > > connected to the LNA output indicate steady 20MHz residuals just before > > the ADC ( around 10 mv peak-peak). > > > In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are > > entering the ADC and so theses alias give > > > erroneous results in the 1Hz-100Hz area, displaying unexpected > > artifacts. In order to fix the problem > > > I will probably include an analog filter just before the ADC input (same > > as LNA input 1nF 80µH), but I > > > want to know if some more sophisticated measures should be undertaken as > > an 5th Order Lowpass Filter. > > > I have not found many clue about alias problems in phase noise > > measurements literature so I may have > > > missed something ? > > > > Any advice > > > > Loïc > > > > ___ > > time-nuts mailing list -- time-nuts@... > > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-> > nuts > > > and follow the instructions there. > > Hi, > > I have a simple setup to measure the noise floor, the mixer is a SYM-2 and > a divider is feeding RF and LO inputs ports for that purpose. > > a pi network give the 90° phase difference. > Have you measured the phase shift when connected to the mixer? I usually use a narrowband 90 degree hybrid. > > The test is running with an HP 10811 giving around 7dBm, but i have the > same results with any sources, especially a 33521A witch can be adjusted > in level and frequency. > > > I intend to use cross-correlation later when i will be confident with my > the setup, for now i run a simple FFT. > > I have no clue about AM noise problem, i suppose that a sufficient input > level on LO will put that problem aside. > The phase shift between the RF and LO ports needs to be adjusted to minimise the response to AM in the source. To do this an AM modulator with low incidental PM is required. The phase is adjusted to minimise the sideband amplitude in the mixer output. > > by the way, I was a bit surprised that nobody pay any attention to RF > leakage from multiplier as the low frequency level is order or magnitude > lower than 20Mhz product present at the input. > Sufficient attenuation is usually achieved by using a well balanced mixer and by the mixer IF port termination and the low pass filter network between the mixer IF port and the low noise preamp. Using a JFET preamp can be useful in minimising any RF rectification effects due to input stage nonlinearity at RF. Bruce > > regards > Loïc > > > > > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the > instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] LNA and Alias
Loic, If you have a single-pole low-pass filter at say 20 kHz you will have a 60 dB damping as you hit 20 MHz. No reason to get really fancy. Either just do a two-pole low-pass filter (inductance, capacitance) or a two-stage RC-link, or consider doing a LC-series link between signal and ground to swallow that 20 MHz, providing a notch. Do it near the mixer, no need to polute the amplifier with the signal. You can do another LC-link at the ADC end to make sure you swallow that 20 MHz. Consider adding additional such links for say 10 MHz or other reasonable likely mixer products. Cheers, Magnus On 12/21/2014 09:39 PM, Loïc Moreau wrote: Hi all, My phase noise measurements system give erroneous results in close in phase noise measurements, I got humps in the 1Hz-1000Hz area as high à 20 dB more than expected. The setup is using a mixer to compare reference and DUT witch drive an LNA http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is driving an AD7760 ADC and an op amp circuity is connected to the VFC reference to achieve quadrature. The results are analyzed with an homebrew FFT charting software After struggling with different configurations, switched different LNA, ADC , sound card. A scope connected to the LNA output indicate steady 20MHz residuals just before the ADC ( around 10 mv peak-peak). In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are entering the ADC and so theses alias give erroneous results in the 1Hz-100Hz area, displaying unexpected artifacts. In order to fix the problem I will probably include an analog filter just before the ADC input (same as LNA input 1nF 80µH), but I want to know if some more sophisticated measures should be undertaken as an 5th Order Lowpass Filter. I have not found many clue about alias problems in phase noise measurements literature so I may have missed something ? Any advice Loïc ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] LNA and Alias
Hi At the moment Comcast and ko4bb.com don’t seem to like each other. I can’t refer directly to the schematic or the page. What you *should* have (assuming 10 MHz inputs for clarity): 1) The RF inputs go into the mixer from what ever sources you happen to want to test. At least one source needs to be high level. 2) The mixer output goes to an L/C lowpass filter. That filter serves several purposes: a) It resistively terminates the mixer at RF (both at 10 and 20 MHz) in the proper IF impedance b) It passes the phase noise information on to the LNA c) It rejects all RF going to the LNA 3) The LNA can be just about anything provided: a) It handles the signal levels without overload b) It does have low enough noise (that depends a lot on the mixer and sources) c) It has enough gain d) Terminates the mixer in a reasonable resistive impedance at audio. 4) The LNA feeds two things: a) Your FFT box b) Your DC bias box It sounds like the FFT part is working so the DC bias may be an issue. The bias box is used to force the two oscillators into quadrature. It forms a PLL around the oscillator pair. With the two signals 90 degrees apart your mixer has a ~ 0V output. That is the point it is most sensitive to phase noise and the least sensitive to AM noise. There are lots of ways to do this. Since I can’t see the schematic. Here’s one based on an RPD-1 (500 ohm out) mixer: One side of the mixer is grounded, the other feeds the filter. 500 ohms in series with 820 pf to ground as the input to the filter. Some sort of coil in the vicinity of 100 uH as the first series lowpass element. Next a 470 pf to ground. Then another 100 uH in series. Another 470 pf to ground. Another 100 uH in series. Another 470 pf to ground. At this point you have a three coil and five capacitor lowpass filter. You should poke it into spice to make sure it’s not going to be a problem with your parts. The issue is cutoff at the highest frequency you want to look at phase noise. You may need to tweak values a bit. The final stage may be overkill depending on the quality of your coils. You will always have a tradeoff between highest phase noise frequency and lowest RF frequency with this setup. LNA can be a good audio op-amp. Run it in positive gain mode. Termination resistance for the filter is simply set with a resistor to ground. I prefer to use 5K for the RPD-1’s. It gives you a bit more output voltage. It also makes the cutoff of the lowpass a bit lower. The DC bias box is an op amp plus a pot, resistors and capacitors. You need to set the output to the EFC voltage on the OCXO’s. That will vary between different parts. One pot is for centering this up. You need to set the loop gain, so feedback resistors on the op amp need to be adjusted. Some sort of R/C may be used to roll off noise. The cutoff frequency of the loop will determine the lowest phase noise frequency you can check unless you measure loop dynamics and correct all your data. Now that that’s all working, you need to calibrate the setup. Two common approaches. Both use a beat note formed when the bias box is shut off: 1) Measure an power or voltage at the LNA output and do math based on some assumptions. 2) Capture the full beat note and look at the actual slope as it crosses zero. You pretty much have to do number 2 before you can use number 1. The LNA needs to have low enough gain in this case to not distort passing the full signal. The math for 2 is pretty simple. Each cycle is 2*PI radians. Phase modulation is normalized to one radian (yes it’s phase … ). You get a radians per volt number and move on. Wish I knew what Comcast was doing this morning …. Bob > On Dec 22, 2014, at 4:26 AM, Loïc wrote: > > > > > Bruce Griffiths writes: > >> >> Are you sure that the setup is aligned to minimise AM response? >> >> Are you using cross-correlation? >> Bruce >> >> On Monday, 22 December 2014 5:08 PM, Loïc Moreau > eai..fr> wrote: >> >> Hi all, >> >> My phase noise measurements system give erroneous results in close in > phase noise measurements, I got >> humps in the 1Hz-1000Hz area as high à 20 dB more than expected. >> >> The setup is using a mixer to compare reference and DUT witch drive an LNA >> http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is > driving an AD7760 ADC and an op >> amp circuity is connected to the VFC reference to achieve quadrature. The > results are analyzed with an >> homebrew FFT charting software >> >> After struggling with different configurations, switched different LNA, > ADC , sound card. A scope >> connected to the LNA output indicate steady 20MHz residuals just before > the ADC ( around 10 mv peak-peak). >> >> In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are > entering the ADC and so theses alias give >> erroneous results in the 1Hz-100Hz area, displaying u
Re: [time-nuts] LNA and Alias
Bruce Griffiths writes: > > Are you sure that the setup is aligned to minimise AM response? > > Are you using cross-correlation? > Bruce > > On Monday, 22 December 2014 5:08 PM, Loïc Moreau eai..fr> wrote: > > Hi all, > > My phase noise measurements system give erroneous results in close in phase noise measurements, I got > humps in the 1Hz-1000Hz area as high à 20 dB more than expected. > > The setup is using a mixer to compare reference and DUT witch drive an LNA > http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is driving an AD7760 ADC and an op > amp circuity is connected to the VFC reference to achieve quadrature. The results are analyzed with an > homebrew FFT charting software > > After struggling with different configurations, switched different LNA, ADC , sound card. A scope > connected to the LNA output indicate steady 20MHz residuals just before the ADC ( around 10 mv peak-peak). > > In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are entering the ADC and so theses alias give > erroneous results in the 1Hz-100Hz area, displaying unexpected artifacts. In order to fix the problem > I will probably include an analog filter just before the ADC input (same as LNA input 1nF 80µH), but I > want to know if some more sophisticated measures should be undertaken as an 5th Order Lowpass Filter. > > I have not found many clue about alias problems in phase noise measurements literature so I may have > missed something ? > > Any advice > > Loïc > > ___ > time-nuts mailing list -- time-nuts@... > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time- nuts > and follow the instructions there. > > > Hi, I have a simple setup to measure the noise floor, the mixer is a SYM-2 and a divider is feeding RF and LO inputs ports for that purpose. a pi network give the 90° phase difference. The test is running with an HP 10811 giving around 7dBm, but i have the same results with any sources, especially a 33521A witch can be adjusted in level and frequency. I intend to use cross-correlation later when i will be confident with my the setup, for now i run a simple FFT. I have no clue about AM noise problem, i suppose that a sufficient input level on LO will put that problem aside. by the way, I was a bit surprised that nobody pay any attention to RF leakage from multiplier as the low frequency level is order or magnitude lower than 20Mhz product present at the input. regards Loïc ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] LNA and Alias
Are you sure that the setup is aligned to minimise AM response? Are you using cross-correlation? Bruce On Monday, 22 December 2014 5:08 PM, Loïc Moreau wrote: Hi all, My phase noise measurements system give erroneous results in close in phase noise measurements, I got humps in the 1Hz-1000Hz area as high à 20 dB more than expected. The setup is using a mixer to compare reference and DUT witch drive an LNA http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is driving an AD7760 ADC and an op amp circuity is connected to the VFC reference to achieve quadrature. The results are analyzed with an homebrew FFT charting software After struggling with different configurations, switched different LNA, ADC , sound card. A scope connected to the LNA output indicate steady 20MHz residuals just before the ADC ( around 10 mv peak-peak). In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are entering the ADC and so theses alias give erroneous results in the 1Hz-100Hz area, displaying unexpected artifacts. In order to fix the problem I will probably include an analog filter just before the ADC input (same as LNA input 1nF 80µH), but I want to know if some more sophisticated measures should be undertaken as an 5th Order Lowpass Filter. I have not found many clue about alias problems in phase noise measurements literature so I may have missed something ? Any advice Loïc ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] LNA and Alias
Hi all, My phase noise measurements system give erroneous results in close in phase noise measurements, I got humps in the 1Hz-1000Hz area as high à 20 dB more than expected. The setup is using a mixer to compare reference and DUT witch drive an LNA http://www.ko4bb.com/~bruce/LowNoiseMixerPreamp.html , the output is driving an AD7760 ADC and an op amp circuity is connected to the VFC reference to achieve quadrature. The results are analyzed with an homebrew FFT charting software After struggling with different configurations, switched different LNA, ADC , sound card. A scope connected to the LNA output indicate steady 20MHz residuals just before the ADC ( around 10 mv peak-peak). In fact , it seems that the mixer 20Mhz residuals ( DUT + REF ) are entering the ADC and so theses alias give erroneous results in the 1Hz-100Hz area, displaying unexpected artifacts. In order to fix the problem I will probably include an analog filter just before the ADC input (same as LNA input 1nF 80µH), but I want to know if some more sophisticated measures should be undertaken as an 5th Order Lowpass Filter. I have not found many clue about alias problems in phase noise measurements literature so I may have missed something ? Any advice Loïc ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
