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TRF37A32EVM: IF Output Balun for Single-End Conversion?

Keito Katafuchi
Intellectual 295 points
Part Number: TRF37A32EVM

Tool/software:

The datasheet for the TRF37x downconverter mixer used in the EVM recommends pullup inductors on each of the differential output lines, and shows the outputs AC coupled to a differential amplifier.
The EVM shows the TRF37x IF outputs with the same inductor pullup / AC coupling, but to an RF transformer.

I am a bit new to RF and just want to confirm the following.

The RF transformer is a 50Ω transformer with a 4:1 impedance ration (200Ω : 50Ω?)
Is the impedance seen by the mixer "real" 200Ω, or does it include conjugate impedance? (aka is it characteristic impedance, or just like a 200Ω termination resistor between the two IF pins at DC?)

I am trying to understand if termination could just be two 100Ω resistors (IFAP to GND, and IFAN to GND) at the load (differential amplifier) for ~100MHz IF output frequency...

Thanks!  

  • 0
    TI__Intellectual 1030 points

    Hi there Keito, good question. The magnitude of the mixers IF output impedance is roughly 200 ohms across its frequency range. This means that its impedance is mostly real, as if it was imaginary, it would vary significantly over its frequency range. The transformers input impedance is also 200 ohms, again mostly real.

    Please note that since the IF output is a low frequency output, you can assume the electrical length between the mixer IF output and the next stage (the transformer in this case) is going to be small. Therefore we can ignore reflection matching, and just focus on power matching (or conjugate matching). Note that for reflection matching, ZL = ZS, and for power matching ZL = ZS* (* = conjugate). Therefore, if the impedance was mostly imaginary for the mixer output, we would have to conjugate match at the transformer input to maximize power transfer.

    The transformer is there to transform the 200 ohms differential to 50 ohms single ended at the IFAP_out connector of the EVM. A transformer is an ideal solution here, as it is the only lossless way to move 200 ohms real to 50 ohms real in a wide bandwidth. As you suggested, the same could be accomplished 100 ohm shunt to ground on each output of the mixer. But this approach would have some loss associated with it.

    Let me know if you have any more questions,
    Gideon