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TRF37B75: Inductor selection

Annie Liu
Genius 10295 points
Part Number: TRF37B75


Hi Team,

Customer needs to select RF amplifiers. He has some questions about the selection of RF choke inductance parameters:

1. How to determine the inductance value of choke inductance? Only 100nh is provided in the manual. Is there a specific basis for the selection of sense values? Is it related to the selected RF amplifier model?

2. Due to the high frequency of the inductors used in RF, it is generally considered to use special RF inductors such as coilcraft manufacturers. When choosing this kind of inductor, he will pay attention to the indicator of self-resonant frequency. However, it is difficult for TRF37B75 or similar devices to reach 100nh inductance self-resonant frequency in the 4000mhz frequency band to 4000m. 100nh inductors can only do less than 1G. How should the self-resonant frequency srf of the 100nh inductor be selected?

3. The choke inductor is mainly to prevent the signal from passing to the DC terminal. When paying attention to the impedance of the working frequency band of the inductor, should the impedance of the working frequency band of the inductor be as high as possible?

Thanks,

Annie

  • 0
    TI__Mastermind 29595 points

    Hello Annie,

    1. RF choke inductors suppresses high frequencies. Higher values of the RF choke coil inductor in this configuration will lead to wider range into the lower frequency. The 100nH is recommended for operating frequency above 400MHz. For best performance in a lower frequency range, the external coupling capacitors and the RF choke can be increased to increase performance in linearity, gain, and NF. 
    2. That is correct, it is important to pay attention to self-resonant frequency of the inductor. The self resonant frequency must be outside the frequency band of the system. It seems like this customer also had an issue trying to locate a RF choke inductor at 100nH (Coilcraft 0603HP-F10XJLU)  with a high enough self resonance. The thread contains a very helpful reference design that suggests other values of RF inductors along with system measurements and performance. Also, this thread provides a helpful simulation with different choke values. 
    3. Increasing the value of the RF choke inductor, increases the impedance (z = 2*pi*f*L). So actually, increasing the inductor value increases the gain at the lower frequency range.

    Thank you,

    Sima

  • +1
    Guru 140200 points

    Hi Annie,

    the performance of TRF37B75 isn't fully ruined exactly at the resonance frequency of choke, because the parasitic parallel capacitance (winding capacitance) of choke still provides a high impedance, even above the resonance frequency:

    annie_trf37b75_1.TSC

    A parallel capacitance of 200fF resulting in a resonance frequency of about 1GHz still provides an impedance of 200R at 4GHz Relaxed

    So it's no surprise that the TRF37B75 is still working properly in the GHz range:

    annie_trf37b75.TSC

    To furtherly enhance the performance, the 100nH choke can be splitted into a series circuit of two 56nH chokes. This will move up a bit the self resonance frequency because of the higher self resonance frequencies of the 56nH chokes.

    Using a series circuit of two unequal chokes can even widen the frequency response in certain applications. The choke with the higher inductance enhances the low frequency performance while the choke with the lower inductance (and the higher self resonance frequency) is helping out at the higher frequencies.

    But to be honest, to be successfull at 4GHz, everything (layout, PCB material, selection of components, etc.) has to be very well organized Relaxed

    Kai