[time-nuts] Modern signal generators

[Bob kb8tq]

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 <dk4xp at arcor.de> 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
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
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