Hi As I said, just how rational using these parts in a radio …. not at all clear to me. Back when I went to school, stuff that was this noisy was not in the “greatest” category. That was a *very* long time ago.
Oddly enough best performance synthesizers have gotten better. (as the posted presentations very clearly show). Just why a “high end” radio uses a less than ideal synthesizer likely relates more to cost (even at a price of thousands of dollars) than to anything else. Indeed cost also drives things like GPSDO’s and GPS modules. We often are not very eager to acknowledge that fact. Bob > On Dec 11, 2018, at 5:24 PM, Gerhard Hoffmann <[email protected]> wrote: > > > Am 11.12.18 um 20:30 schrieb jimlux: >> On 12/11/18 10:23 AM, Richard (Rick) Karlquist wrote: >>> >>> >>> On 12/11/2018 9:13 AM, djl wrote: >>>> Rick: I've spent some time with the dds blocks. We found them to generate >>>> lots of low level spurs, making lots of "birdies" when used as local >>>> oscillators in receivers. >>>> We had better results using: >>>> https://www.silabs.com/products/timing/oscillators. >>>> >>> >>> I was talking about making a programmable frequency synthesizer >>> with a DDS, to use as a general purpose signal generator. >>> >>> A silabs part functions exactly the same as a crystal oscillator >>> once it receives its one-time programming at the factory, AFAIK. >>> >> >> Most of the Silabs parts are available in an I2C programmable version >> rather than the factory programmed flavor. >> > Yes, but they all have in common that their oh! so good jitter values > > exclude the first 12 or even 50 KHz from the carrier. With enough DSP > > you can shove a lot of dirt towards the first 12 KHz: look, ma, no birdies. > > Those without an E5052B or FSUP won't notice, and the OC-48 or OC-192 > > or other telecom target market won't care anyway. But the birdies are not > > magically gone, they were squashed under the DSP steam roller. > > > The nagging DDS birdies happen when you are close, but not exactly on > > a subharmonic of the clock frequency. In an avionics com transceiver I got > > easily rid of them by using 2 clock frequencies and switching as best > > for the channel. With this DO-178? stuff you are punished for oscillator > > birdies when you try to make the receiver more sensitive. :-( > > > Ulrich and DJ7VY and some others have shown us >40 years ago how to > > do shortwave/VHF receivers (hey, I was still in school then, and it gave me > > the kick towards RF engineering) and ring mixers still have their place, but > > not in the input of a ham rig. There should be a 16 bit 150MHz+ ADC after > > the tuned preselector, and the rest is digital. I have published a > > synthesiseable sine / cos table on opencores.org a decade ago. The test > > bed is a DDS. just fill in the resolution you want and buy an el cheapo Xilinx > > Spartan FPGA to give it a home, and then filter & decimate the hell out of > > your ADC data. No more analog LO. We now have things like AD9172, > > ADC12J4000 and AD9625 to play with. That's the new frontier. > > > Now that I have your attention.... I'm currently interested into 1/f noise, > > or, more precisely, in how to avoid it. Is there anything known on 1/f > > in FETs as used in switches, such as choppers? Is there more than thermal > > noise of the channel? There is a paper of the Univ of Twente that suggests > there > > is some time delay when fets are turned on until the trap locations turn > active. > > That could be a nice by-effect. > > Even van der Ziel and Cobbold are silent about that. > > > cheers, > > Gerhard, DK4XP > > > > > > > > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
