clock generation with DOPPLER shift

Great,

thanks a lot for all your answers !

I'll try to downmix the analogue VCXO signal...

Bye, BEN


Ulrich Bangert wrote:
> Ben,
> 
> if you can afford to use a small amount of analogue circuitry around your
> fpga, i suggest you use a second 1000010.0 Hz oscillator (this can be a
> "detuned" 1 MHz oscillator) and mix the received signal with this this
> oscillator down to the audio domain using a double balanced mixer. A lowpass
> filter (or better a so called diplexer) must be used at the IF port of the
> mixer.
> 
> The frequency variation in the downmixed signal stays the same, so with this
> arrangement you have shifted the scenario from 1 MHz, where a 5 Hz variation
> is a difficult to detect 5*10E-6 effect to a 10 Hz audio signal where 5 Hz
> variation is a easy to detect 50 % effect. This scheme is the usual one for
> comparing ultra stable oscillators against each other, where variations
> smaller by some orders of magnitude have to be measured.
> 
> No matter what scheme you will use: Keep in mind that a "normal" x-tal
> oscillator has a tempco of app. 1*10E-6/K. So a normal x-tal oscillator will
> give you a 1  Hz variation per Kelvin @ 1MHz, well in the region of the
> Doppler that you want to measure. This is a indication that a "better"
> oscillator has to be used. Don't go for a TCXO, look out for a cheap OCXO.
> 
> Rgegards
> 
> Ulrich Bangert
> 
> "Ben Marpe" <Ben.Marpe@gmx.de> schrieb im Newsbeitrag
> news:1137597178.426492.225110@z14g2000cwz.googlegroups.com...
> 
>>Dear experts in this newsgroup,
>>
>>in my diploma thesis i'm using a FPGA for baseband signal generation.
>>I'm interested in generating and varying a clock of 1Mhz which is
>>DOPPLER shifted +/- 5Hz due to movements between receiver and
>>transmitter.
>>
>>The +/- 5Hz Doppler must be applied in a very "smooth" way, the step
>>resolution should be as fine as possible.
>>
>>Any ideas how to do this on a (Xilinx) FPGA ?
>>The sine output of Xilinx LogiCore DDS isn't necessary and the step
>>resolution might be even a little bit finer for my application.
>>
>>Thanks a lot for every single hint you can give to me !
>>
>>Greetings, BEN
>>
> 
> 
>