how to generate a wideband frequency sweep with PLL with Integrated VCO?

Hi all,

I need to generate a wideband stepped frequency signal in 2GHz- 4 GHz frequency band with 2 MHz frequency steps for a short range FMCW radar front end.

I came across with two devices which is working in the required frequency bands,

1. Linear technology LTC6946 - Ultralow Noise and Spurious 0.37GHz to 5.7GHz Integer-N Synthesizer with Integrated VCO
2. TI TRF3765 - Integer-N/Fractional-N PLL with Integrated VCO

I'm wondering is it possible to generate a wideband frequency sweep (wideband stepped frequency signal) with in a bandwidth of 2 GHz - 4 GHz with these devices?

I need about 1 MHz - 2 MHz resolution and 1000 - 2000 frequency points.

As I can understand from the datasheets, the loop filter(the feedback loop of the synthesizer ) requires different component values for different frequencies.
If that is the case can I use this device to generate the frequency sweep from 2GHz - 4GHz?

If this method is not working at all, could you please tell me what is the next cheapest solution?

Could you please help me to figure out this problem?

Thanks,

per_lube

TRF3765 will do the job using just one IC package. LPF should use the same component values if the output frequency step is the same for entire range.
The phase noise of these wideband PLLs are not the best, and you have to take this in consideration in your design.

Hi Vfone,

Thanks a lot for your reply.

Since in this application 2 GHz - 4GHz frequency sweep (the step size is always same) I need to change the LO_DIV_SEL.
This changes the loop frequency output.

Therefore, do I need to use a different loop filter?
Since the loop filter component values are changing due to the frequency output, I thought that this IC can not be used in my application.

Could you please clarify it for me,

cheers,

The LPF components are only function of PFD frequency (or frequency step that I mentioned).
So the same LPF can be used for the entire frequency range (2GHz to 4GHz) if use the same PFD frequency.

Have you checked the Hittite HMC701LP6CE?. It hasn't the VCO integrated so you have to add one, but it integrates the sweeper.

Mazz

Hi Vfone,
Sorry to bother you again. I'm new to RF design & sorry for my poor RF knowledge.

I tried to use the software tool : Texas Instruments Loop Filter Calculator to calculate the loop filter components for the upper and lower bounds of the output frequency 2 GHz - 4 GHz.

It gave me two sets of component values which the resistors are doubled and caps have become half of the initial value.

@2 GHz : C1 = 68.02 pF, R2 = 6981 ohm, C2 = 1240.5 pF, R3 = 6981 ohm
@4 GHz : C1 = 34.01 pF, R2 = 13962 ohm, C2 = 620.27 ohm, R3 = 13962 ohm

Could you please tell me whether is it ok to use same component value or have I done a mistake when using the loop filter calculator tool?

I got another question, This chip is below $10, however Hittite HMC586LC4B 4 - 8 GHz VCO chip is around $100.
If we use a TRF3765 and a frequency doubler,a filter and power amp, then we could easily achieve 4 - 8 GHz sweep way less than $100.

As I can understand, both can be used for generating same frequency range.

Could anyone explain what are the advantages and disadvantages of these frequency synthesizers and wideband VCOs or how they differ from each other?

cheers,

---------- Post added at 00:06 ---------- Previous post was at 00:04 ----------

Hi Mazz,

Thanks for the reply,
I'm looking for a chip with integrated VCO to reduce the number of components and to keep the cost in a minimum level of the design.

cheers

Are there any requirements for sweep speed? In other words, how long does the radar have to dwell at each frequency?

As for your other question about loop filter design. It's pretty difficult to change loop filter components in the middle of a frequency sweep. One degree of freedom that you have is the ability to change the charge pump current. I would recommend that you take the following procedure. I haven't played with the TI Part, but Analog Devices has a similar part, and they have some nice PLL simulation SW to use with it.

1 - Design the loop filter for the geometric middle of the band (2800 MHz) and look at the loop response at each end. Pick a charge pump current toward the middle of the programmable range.
2 - Keep the Loop filter components the same, modify the charge pump current at each end of the band to keep the loop response about the same.
3 - When you're using this, you'll have to modify loop dividers as well as the charge pump current as you change channels.

Dave

Hi Dave,
Thanks for the reply.

There are no restrictions on the sweep speed. I can allow upto 2 - 3 ms per one frequency step.
It is okey to have the sweep within 3 ~4 sec. over the 2 GHz - 4 GHz band.

I'll try to go through the datasheets and some literature to figure out how to design it based on your technique.

cheers,

Since there are no restrictions on the sweep speed, you don't need the HMC component.
Use ADISimPLL from analog device, you can get loop component for 2-4G. The software can auto calculate for the whole range if you can import the VCO spec correctly.

Hi tony_lth,

Thanks for the reply.
I'll try to check the possibility of using this device & software for my design as well...

cheers,