There is a huge range of quality in USB audio interfaces. I'm sure you can read the specs. Interestingly the price is not so much an indicator of quality. The best places to shop are the outfits that sell to audio recording engineers, the people who work in studios. The worst places are those who sell to gammers and Home Theater enthusiasts. Likely has something to do with the education level of the customers Good results with brads : Focusrite, MOTU, Apogee, Presonus. One of the better places to buy is Sweetwater.com. I also use these for audio spectrum analyzers and audio free ocilascope as well as there intended audio recording use.
When designing your PCB don't forget to take advantage of the #1 feature of a professional level USB interface: 600 Ohm balanced inputs. They all use XLR connectors into balanced inputs to dramatically reduce the noise. The 1/8" stereo jack on the cheap consumer stuff does not know the difference between a shield and a signal ground and use try and eliminate group loops when you have USB and 1/8" stereo both plugged into the same box. On Thu, Aug 11, 2016 at 10:29 PM, Bob Stewart <[email protected]> wrote: > Hi Bob, > Thanks. Well, I guess the next thing to do is to get a USB sound card, and > lay down some copper. I did manage to get some DBMs and power splitters from > Mini-circuits, and I'll use the LPF from the NIST DMTD and see what I can > come up with. Hmm, I'll have to look at the LPFs, won't I? No amps or ZCDs, > of course. I think I'll start with a single mixer to see what that gets me. > But I'll go ahead and layout the board for a DMTD with power splitter and two > LPFs for the most versatility. If I can get something better out of it than > the 5370, it does solve a lot of problems for me. The 8640B should be fine > as a heterodyne oscillator. Also, I can experiment with a pair of > FCC-1/FCC-2 DDS systems I have that Bob Okas designed and sold before he > passed away, as well to see if they have any value for this application. I > have choices for master reference oscillator, as well. > > Bob > ----------------------------------------------------------------- > AE6RV.com > > GFS GPSDO list: > groups.yahoo.com/neo/groups/GFS-GPSDOs/info > > From: Bob Camp <[email protected]> > To: Discussion of precise time and frequency measurement <[email protected]> > Sent: Thursday, August 11, 2016 9:41 PM > Subject: Re: [time-nuts] Low cost SDR suitable for phase noise measurement? > > Hi > > In a DMTD (or in the equally useful single mixer setup) the advantage for > frequency measurement > comes from the downconversion process. The further you downconvert (10 Hz vs > 100 Hz) the > greater your potential resolution. The design approach is to push the beat > note as low as you > can without running into other limits. With conventional setups that is in > the 1 to 10 Hz range. > > If you want to fiddle a bit, there is no particular reason why you must use a > limiter after the > mixer. An ADC after the mixer could be used to process the audio and estimate > it’s frequency. > The same sort of ADC setup might give you some phase noise data. > > The gotcha with all this is that a single mixer / quadrature approach will > give you *much* better > phase noise data than a single or dual mixer setup. If phase noise is the > objective, there is no > reason to run the mixer setup as a DMTD. With a DMTD you digitize the carrier > (audio beat note) > and then try to dig down 195 db to get to the phase noise. With a quadrature > approach you null out > the carrier. Then all you have to deal with is the noise. > > Bob > > >> On Aug 11, 2016, at 5:15 PM, Bob Stewart <[email protected]> wrote: >> >> Hi Bob, >> If I understand the question correctly, yes. The DMTDs that I've >> encountered so far use a zero-crossing detector and a TIC. I wanted to >> build sort of a hybrid system that has a DMTD front end and an SDR back end. >> It seemed to me that that would be sort of half way between a DMTD and a >> Timepod. But whereas the Timepod digitizes 4 RF signals, this would >> digitize 2 audio signals. It may be that it suffers all the disadvantages >> of both and has the advantages of neither, but I thought I'd build it and >> see where it leads. >> As to the 1 to 10 Hz range, doesn't that depend on the heterodyne frequency >> you choose? I mean, you don't have to use a 10 Hz offset, do you? >> >> Bob >> ----------------------------------------------------------------- >> AE6RV.com >> >> GFS GPSDO list: >> groups.yahoo.com/neo/groups/GFS-GPSDOs/info >> >> From: Bob Camp <[email protected]> >> To: Discussion of precise time and frequency measurement <[email protected]> >> Sent: Thursday, August 11, 2016 3:13 PM >> Subject: Re: [time-nuts] Low cost SDR suitable for phase noise measurement? >> >> Hi >> >> Just to clarify: >> >> You are looking for a way to digitize the output(s) of the mixer(s) in a >> DMTD. The >> “target signal” is in the 1 to 10 Hz range. >> >> Is that correct or are you looking for something to use instead of a DMTD? >> >> Bob >> >> >>> On Aug 11, 2016, at 3:29 PM, Bob Stewart <[email protected]> wrote: >>> >>> Hi Brooke, >>> It looks like I've asked a question poorly, yet again. So, let me try >>> again. I have this idea of combining a DMTD with an SDR. So, said that >>> way, it looks like what I really need is a 2-channel audio SDR. And having >>> said that, maybe what I'm looking for is an external USB sound card. Those >>> I can find. I can probably even figure out how to convert them to DC and >>> supply them with a disciplined oscillator, if needed. >>> >>> Bob >>> ----------------------------------------------------------------- >>> AE6RV.com >>> >>> GFS GPSDO list: >>> groups.yahoo.com/neo/groups/GFS-GPSDOs/info >>> >>> From: Brooke Clarke <[email protected]> >>> To: Discussion of precise time and frequency measurement >>> <[email protected]> >>> Sent: Thursday, August 11, 2016 1:36 PM >>> Subject: Re: [time-nuts] Low cost SDR suitable for phase noise measurement? >>> >>> Hi Bob: >>> >>> The SDR-IQ has a lower frequency limit of 500 Hz. >>> http://www.prc68.com/I/Bats.shtml#SDRIQ >>> >>> There is a mod that replaces the on board 66 MHz oscillator with one locked >>> to 10 MHz. >>> >>> -- >>> Have Fun, >>> >>> Brooke Clarke, N6GCE >>> http://www.PRC68.com >>> http://www.end2partygovernment.com/2012Issues.html >>> The lesser of evils is still evil. >>> >>> -------- Original Message -------- >>>> A number of posts have appeared about Zynq SDRs on the "Shera revisited" >>>> thread. So, I'd like to ask the more general question: Are there any low >>>> cost SDRs that are suitable for measuring phase noise when coupled with a >>>> DMTD? I believe what I'm looking for is something with enough bits to >>>> work in the audio range down to DC. Yes, I could use a modified sound >>>> card, but I'm really looking for something not sound card related. My >>>> thought was to use a DMTD with a heterodyne frequency of maybe 5KHz. >>>> >>>> Bob ----------------------------------------------------------------- >>>> AE6RV.com >>>> >>>> GFS GPSDO list: >>>> groups.yahoo.com/neo/groups/GFS-GPSDOs/info >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
