Well, you motivated me to do a simple test. I'm using the Baudline signal analysis program under Linux and clocking the sound card at (nominal) 96ksamples/second.
I am currently injecting a 100mv p-p signal at 11.025kHz (to simulate the SDR-1000 IF output) into one channel of my Delta 44 card. The signal is coming from an HP 3325A synthesizer locked to an HP 5065A Rubidium standard. I'm using Baudline to decimate and downconvert the signal to get a few Hz spread across the screen (decimate by 2048, with an FFT size of 4096). Baudline has a high-precision frequency measurement function that has the potential of uHz resolution (it uses phase measurements rather than just FFT bins). Bottom line -- when I started the test half an hour ago, the nominal 11,025Hz signal showed as 11,024.33235 and right now, it shows as 11,024.33228. While that doesn't say anything about the long term stability, it's consistent with what I've observed over a year or more with this card; frequencies consistently read a little bit low, but never by more than a fraction of a Hertz. Now, this card is in a computer that's powered up 7x24 and has been running for months, so it's in an electrically and thermally stable environment. Results from a cold start might be different. 73, John ecellison said the following on 11/22/2005 05:16 PM: > John > > Thanks for the input. I have not heard of anyone else using the external > reference, and you never noted any severe adverse effects. Is the 5065a > keeping the SDR dead on, which is the primary objective in this whole > discussion. Do you notice variation due to the Sound card clock? Perhaps all > the theory we have been gumming about is not that important in practice. > > If you have to measure the difference on a high quality counter.. er well. > Also thanks in advance for the tests. > > Thanks! > > -----Original Message----- > From: John Ackermann N8UR [mailto:[EMAIL PROTECTED] > Sent: Tuesday, November 22, 2005 8:35 AM > To: ecellison > Cc: 'Ross'; FlexRadio@flex-radio.biz > Subject: Re: [Flexradio] Frequency calibration etc > > ecellison wrote: > >>Ross >> >> >> >>Well, I think we are a ways away from a kit. I was sort of suggesting that >>we try just using the Rockwell - Jupiter board and it's 10 mhz oscillator >>'naked' and just see what we get. In 2 lengthy arounds on this thread > > mostly > >>what we have is theory and nothing tried at this point. I think that John > > - > >>N8UR is the only one I have heard about who is using an external 10 mhz >>reference to the SDR. >> >> >> >>Comment John? Anyone else using Geralds mod kit with external reference? >>Results? > > > Yes, I've been using my SDR-1000 with external reference from an HP > 5065A Rubidium frequency standard. I haven't noticed any horrid phase > noise problems from the 10MHz to 200MHz multiplication, but I also > haven't been looking very hard for them. > > I hope that fairly soon -- perhaps over the Xmas holidays -- I will get > temporary access to HP's latest super-duper phase noise measurement box, > and one of the tests will be to compare the SDR-1000 with original > crystal versus the external reference. I'll certainly post those > results when I have them. > > 73, > John > > >
