CC1121: Using the CC1121 with a differential input front end

Hi,

CC1121 datasheet has impedance values listed in section 4.10. You can use these values to design a differential matching network between the RX ports.

I believe SKY66101 is a 100 ohm differential input device. You can also look up (google) app note from Skyworks for matching their differential devices to any impedance values. After getting theoretical values for the matching network some in-lab tuning will be required to optimize sensitivity.

Regards,

HI,  This is not something I am at all familiar with, do you know of any tools that will allow me to calculate the match circuit?

What about the requirement for the differential inputs of the CC1121 to be DC connected to GND, how should that be achieved?

Thanks

HI,  I have looked at the datasheet (section 4.10) and it is giving 'optimal source impedance' of 60+J60 Ohms.  I am assuming that this is not the actual termination impedance then.

Hi,

I cannot seem to find the Balun reference design on the CC1120 product page.  Can you please send me a link to it.

Thanks,

Ian

Hi,

You can refer to the CC1120SKY65313 Reference Design. The following is the link.

You can see separate Rx and Tx paths (single ended)  connected to SKY65313.

Thanks,

PM

Hi,

I have looked at the SKY65313 reference design and this does not include a balun, it is the 'normal' single ended input design using LC networks for the phase shift.  The SKY65313 is a single ended input and output device.  I have used this type of circuit many times with the CC112x family parts.

The reason I am looking at the SKY66101 is that it has a balanced output, so should be able to interface to the balanced input of the CC1121 without the use of a phase shift network.  What I am looking for is some guidance on how to implement the impedance match between the 100R output of the SKY66101 and the 60+60J impedance of the CC1121. 

If I was to use the single ended solution then I would use a front end that has a single ended output (e.g the SKY65313)

Any help would be appreciated,

Regards,

Ian.

Hi,

What I was saying is connect that 50 Ohm Rx port from CC1120 to 50/100 Ohm Balun (Standard - 50 ohm single ended to 100 ohm differential- please see the following link - also posted in my previous post) then connect 100 Ohm Differential port of the Balun to SKY66101 Rx port.

If you don't want to use a Standard Balun (50 ohm single ended to 100 ohm differential - the following link) then you need to  design a Balun with those impedances by using some simulation tool ( for example ADS)

Thanks,

PM

Yes, If I was going to use the CC1121 in single ended mode then I would choose a different front end.

What I was hoping for was someone who could assist in the design of the matching network, its been many years since I did any of those calculations by hand! (And never for differential circuits)

Regards,

Ian.

Hi,

Do you have any RF Simulation tools?

Thanks,

PM

Hi

Unfortunately I don’t have any tools that can work with differential impedance. My tools are fairly basic and low cost!

Regards.

Hi,

OK, I will look into it but it will take some time to get back to you.

Thanks,

PM

Hi,

As promised, the following is the (simulated) Circuit. You may need to trim those values on your proto types for optimum performance.

Thanks,

PM

Thanks PM,

One final question, the CC1120 and CC1121 state that the RX inputs require a DC path to ground.  How do you suggest I implement this given the networks shown above do not have such a DC path to ground?

Regards, Ian

Hi,

The following is the Circuit with DC path to Ground.

Thanks,

PM

Thanks PM,

I was thinking of something similar also.  But, wont I need a 100nH on both legs to ground?  It is not clear from the Skyworks Datasheet if there is a DC path between the differential outputs.

Regards,

Ian