[time-nuts] Re: GPS failed
Sent from my Phone > On 13 Jul 2022, at 02:34, Bob kb8tq via time-nuts > wrote: > > Hi > > The “typical” Symmetricom cone shaped GPS antennas are targeted at > cell phone tower applications. Being mounted on the same structure > as multiple cell transmitters puts them in a significant RF environment. > They have a *lot* of filtering built into the antenna to try to prevent > overload > issues. > > As with a lot of things, Symmetricom simply rebrands antennas made by > others. Not all cone shaped antennas are identical. However it’s a pretty > good bet that most of them are very similar to what Symmetericom ( and > the other folks ) supply for cell applications. > > The “other end” of the range are the multi band saucer shaped “survey” > antennas. They tend to have a lot less filtering and be more focused at > allowing the user to access a wide range of frequencies ( both GNSS and > supplemental services) via a single device. Lots of filtering also tends to > mess up delay here or there, that’s not a great thing for high precision > work. > > Bob >> Bob, That was true 10-15 years ago. Good quality surveying antennas often has quite decent filtering on each band they are designed for. /Björn ___ 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
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, 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 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] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)
Ed, Thanks for the many good advice. I've tried to incorporate as much as possible, updated schematic can be found here: http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG For audio into the PC I'm using a professional balanced microphone to USB input with a noise level of -130dBc/Hz and no spurs. Using a 7805 and 1500uF capacitors I tried to create a solid reference instead of the buffer op amp output and that did make a difference. Further the input of the first opamp has been change to have identical resistors at both the + and - input to reduce common mode signals. None of the capacitors (5.6pF) I tried to reduce high frequency gain improved the results. Most of the time it got worse. Removal of a ceramic capacitors eliminated the microphony. I've added a switch to select between 0dB and 20dB gain so I can calibrate the level by offsetting the DUT frequency while keeping the drive to the mixer. To calibrate the effective noise BW of the FFT I create a test signal combining a -70dBm 10.001MHz signal with a -90dBm/Hz noise signal. The 20dB power ratio was confirmed using a calibrated spectrum analyzer. The FFT length and sample rate at the PC where then changed till the PC FFT showed the same power ratio. Noise level at 1kHz is now -150dBc/Hz and -155dBc/Hz at 10kHz which is the spec of the DOCXO used so no need to go any lower. Also the 50Hz spurs and its harmonics are greatly reduced. I'll have to invest in better coax cables as the current cable seem to leak RF. Erik. ___ 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)
Am 2022-07-13 4:10, schrieb glenlist via time-nuts: Oh and now LED lights overhead your bench which are driven at 5-50kHz are are next new coupling of noise into your open bench circuits !!! The LED ringlight on my microscope creates 57KHz noise peaks when I have an unshielded low noise amplifier under it. Immediately visible on the scope, let alone the FFT-analyzer. Gerhard ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
