DDS with wide sweep frequency

[bibo1978]

I want to do a chirp signal that sweeps from 200-800 MHz but i can not find a DDS that work in this frequency range the faster DDS can workwith 1 GSPS which gives a frequency less than 500 MHz "Nyquest rate". This was the AD9858, so I was thinking of using this DDS and extending its range through external circuitry but I don't know how to do this, does anyone have any ideas ?

 

[Circuit_seller]

Hi

The way you will choose for implemnting this sweep range depends on the frequency resolution(step size) in this range.
For high resolution you should used complex techniques such a hybrid PLL /DDS and for low resolution you need a PLL Synthesizer and a mixer.

Regards

 

[radiohead]

I agree. The choice between these architectures is a compromise between switching speed and frequency resolution. Tell me what the sweep time is and we can give you more explicit ideas!

 

[VSWR]

You can add a Mini-Circuits RK-2 Frequency Doubler ($17.95 each) at the output of the DDS. It requires 1 to 15 dBm so maybe you have to add an amplifier between the DDS and the doubler. It has about 15 dB loss, so may have to add a second amplifier after the doubler as well. A suitable amp (both locations) is Mini-Circuits MAV-11BSM 50-1000 MHz ($1.50 each) or similar.

 

[radiohead]

I disagree with the RK-2 doubler solution. The loss is 15dB, but the suppression of the first harmonic is only 20dB! This means only a 5dB difference between the doubled frequency tone and the fundamental! This is a very distorded signal. And you cannot filter due to the high relative bandwidth.

 

[bibo1978]

Thanks VSWR, I already had done a design "which was like VSWR suggested" but I used REMEC FD34SM-1 doubler, it has a fundemental Isolation of 35-40 db (typically) and an instertion loss of 11.5 db (typically) and a third harmonic suppression of 33dbc refernced to the second harmonic, it seems to me OK and I think I may use an amplifier. I want to know if there is another suggestions that may be more effecient. what do you think "radiohead"

 

[nima]

May be you can use a mixer so that the frequency resolution is not changed after using the frequency doubler.