How to generate sawtooth voltage sweep in ns

[ABIRAMIM]

Hi there
I am a newbie working to generate chirp pulses for FMCW radars. I have a VCO to generate pulses in 5 - 6GHz frequency when Vtune of the VCO is varied from 2 to 12V. My understanding is I need to generate sawtooth voltage sweep in ns time duration to feed the Vtune so that the VCO will generate the required frequency shifts. Can someone help me how can I generate this voltage sweep to feed the vco. Because as far as I know, 555 timer ICs generate pulses in few hundred KHz only and I need the sawtooth sweep frequency to be in MHz (Ton = 13ns and Toff = 200ns)

Thanks

 

[dick_freebird]

Perhaps you need to think less, many regular logic families
will produce this kind of risetime just by loading.

Now if you want FM "modulation shape" fidelity you're on a
different sort of hook. But if all it wants is "there to here"
frequency sweep / chirp, maybe it's easier than you think.

The narrowest chirp you could get will be from an advanced
CMOS logic family, make a one-gate-delay positive-edge-only
"yip" with a quad NAND gate. Burden the output (incl the
varactor) with external C until you like the risetime (if too fast).
If too slow, pick another logic family.

A very linear ramp would need a fancier circuit. Like maybe a
current source charging a C, with a shunt switch to reset / hold
off driven by a suitably-matched one-shot.

 

[D.A.(Tony)Stewart]

Hi there
I am a newbie working to generate chirp pulses for FMCW radars. I have a VCO to generate pulses in 5 - 6GHz frequency when Vtune of the VCO is varied from 2 to 12V. My understanding is I need to generate sawtooth voltage sweep in ns time duration to feed the Vtune so that the VCO will generate the required frequency shifts. Can someone help me how can I generate this voltage sweep to feed the vco. Because as far as I know, 555 timer ICs generate pulses in few hundred KHz only and I need the sawtooth sweep frequency to be in MHz (Ton = 13ns and Toff = 200ns)

Thanks

Can you define any more what you expect?
Stepped ramp or linear ramp? When Ton << Toff that looks more like a 26 ns pulse with slew rate control of a triangular 13% pulse using the leading edge or 6.5% or 1/2 cycle of 38.46 MHz with a 5 MHz cycle. What Xtal and sync. counters be suitable for this in terms of stability (ppm) and phase noise). Do you need a Fractional-N Synth or can you adjust it to be an integer?
i.e. 12.5% instead of 13% for an integer of 8* f_rep or 16*f for on time duty factor.

This is usually created in a DSP and modeled by something like Visual System Simulator (VSS) software from AWR Corp https://kb.awr.com/display/awrfaq/Getting+Started+with+the+AWR+Software

Then you might need a $50k VNA to analyze it e.g. https://www.ni.com/en-ca/shop/model/pxie-5668.html

 

[vfone]

There is a confusion. In normal FMCW radars, the range of the period of the chirp is between few microseconds up to hundreds of microseconds.
I think your request for nanoseconds range is meaningless.

 

[D.A.(Tony)Stewart]

You have a duty to include design specs for range, BW .