ADL5375-LO input Amplitude and phase balance limits

Hi Sai Kiran S,

• For ADL5375 applications above 3 GHz, improved OIP2 and LO leakage may result from driving the LO input differentially.
• Amplitude and phase imbalance will cause the LO drive to be inefficient and less than ideal, that may impact the above mentioned parameters.
• Shown in datasheet page 30, figure 75 are three recommended baluns for the LO ports.
• If you reference the datasheets for those recommended baluns, the phase imbalance can be as much as +/- 25 degrees, which is on the higher side of what we would recommend, but will work nonetheless.
• When selecting a balun for the ADL5375, it's good to target less than 10 degrees phase imbalance and less than 1dB amplitude imbalance.

Best Regards,

David

Hi David

Thanks for the reply.

In figure 75 of the datasheet 1:1 ratio balun is used for interfacing with LO inputs of the device.

But I am having 1:2 ratio balun.Can you suggest how to interface this 1:2 balun with ADL5375 LO inputs to have impedance matching.

The following is the balun circuit

Hi Sai Kiran S,

You're welcome.

Yes, the baluns used for the LO port are 1:1 because the differential impedance of the LO ports is 50 ohms.

If you are going to use a 1:2 balun on your board, you could have secondary side at 50 ohms impedance and then the input impedance at the balun would be 25 ohms.To match this back to 50 ohms impedance there are a variety of solutions. Reactive element L-section matching will probably be the easiest while preserving power. If you need guidance on matching networks, I'd recommend to review chapter 5 in David Pozar's Microwave Engineering book (ISBN-13: 978-0470631553). He provides some good guidance on various matching techniques.

Best Regards,

David

Hi David,

If you can share any reference schematic or scheme for impedance matching circuit will be very much helpful to me because I need to freeze the design urgent and need to proceed further for PCB layout.

Surely I will refer the book mentioned in later stage.

Regards,

Sai Kiran S

Hi Sai Kiran S,

Please see below for example of lumped element placeholders with the balun. It would be good to use 0402 sized components placed close to the balun input.

Best Regards,

David

Hi David,

As I have mentioned earlier I am using balun with  impedance ratio (Secondary/Primary) is 2.

Primary side 50 ohms and secondary side 100 ohms single ended or 200 ohms differential. In the circuit which you have provided can you please explain how this secondary side 200 ohms transmission line differential impedance is matched with 50 ohms input impedance of ADL5375.

Is the differential input impedance of ADL5375 is 50 ohms or 100 ohms ? this detail not given in the datasheet, kindly confirm.

In figure 75 of datasheet 1:1 balun is used without any matching circuit.

1:1 balun which is used in figure 75 shall have 50 single ended or 100 diff impedance on the secondary side. If this is the case then differential input impedance of ADL5375 should be 100 ohms.

Regards

Sai Kiran S

Hi Sai Kiran S,

The circuit provided was assuming that the balun is 1:2, regardless of absolute primary/secondary impedances. If the secondary side is 50 ohms (between secondary terminals), then the primary side would be 25 ohms (between primary terminals).

The ADL5375 LO differential input impedance is 50 ohms. If you want to have the primary side of the balun at 50 ohms without matching components on that side, then you will need to move the matching components to the secondary side. That is, you would then need to match 100 ohms between the secondary terminals with 50 ohms of the LO input. Does this explanation help?

Generally, it may be easier to use a 1:1 balun, as is the case with the evaluation board. This would eliminate the need to perform the matching.

Best Regards,

David