LMX2594: LMX2594 Classic Linear FMCW Waveform

Dear Richard,

thank you for reaching out. These are excellent questions.

LMX2594 support ramp indeed but with the following limitation. Since the potential frequencies for a given chirp or ramp are generated by multiple VCO/multiple coarse adjustment, the ramp mode cannot be continuous to produce a traditional high fidelity FMCW chirp on wide range of frequencies. The dynamic tuning (using the varactor tuning only) for a given VCO/coarse tuning will not exceed 50-70 MHz. One could consider uninterrupted chirp in such a range.

For larger frequency range chirp, I recommend to consider LMX2491, LMX2492 or LMX2485. These devices are PLL with chirp capability and must be paired with an external VCO. LMX2491 and LMX2492 includes a programmable state machine to generate more complex ramps.

Regards, Simon.

Simon,

The customer did send me a couple more questions:

Can you please advise me what is the widest bandwidth of a continuous chirp that we can get with these new products that you recommended?

How fast (short) can a chirp be with the recommended products?

Thanks for your help with this!

Richard Elmquist

Simon,

Have you been able to look at additional questions that the customer posed in the previous post?

Thanks for your help with this!

Richard Elmquist

Simon,

I have just returned from vacation, but I did not see a response to my questions.

Can you please look at them and respond?

Thanks for your help with this!

Richard Elmquist

Dear Richard,

my apology, I did not see the follow up.

continuous chirp is done on the tuning range of the VCO which will not exceed 50 or 60 MHz. This range with change depending on the VCO being used. Also, You will need to take into account temperature change as the tuning range is typically used to insure temperature drift without losing lock. You could use the tuning range for chirp but would lose the drifting capability.

As for the maximum chirp slope capability, it will depend on the loop BW you can live with and the potetnial distortion you will see as the chirp is progressing. Remember, the chirp is generated by changing the fraction (numerator) on each PFD cycle so that the loop is constantly chasing a steady state.

I hope this helps.

Regards, Simon.

Simon,

Do we have a device or a reference design with a similar device that would offer a faster chirp rate that you estimated for the LMX2594?

The customer understands the response, but is really looking for a device that would be similar but offer a larger bandwidth for the chirp function.

Do we have anything that we might be able to offer him?

Thanks for your help with this!

Richard Elmquist

Simon,

Were you able to look at the further questions from the customer?

Please let me know if we might have anything to offer the customer whether it is an EVM or another IC.

Thanks for your help with this!

Richard Elmquist

Hello Richard,

larger chirp can be obtained with external VCO as long as the VCO can cover the desired range. We have 3 nice PLL's to connect with your favorite VCO:

LMX2491 with 8 state machine for automatic frequency chirping.
LMX2492 with 8 state machine for automatic frequency chirping.
LMX2485


Regards, Simon.