Resistive (passive) FET mixer for 2.4 GHz

Material & method

The active device is a 400 um (gate width) x 0.5 um (gate length) depletion pHEMT in SOT-343 package. Generally, a narrower gate width improves conversion loss and LO isolation, but degrades intermodulation performance [1]

An IF frequency of 110 MHz is chosen in order to to use a cheap and commonly available SAW IF filter.

The circuit ties the drain and the source at the same DC potential [2]. Although this biasing arrangement slightly increases circuit complexity, it yields significantly better IMD performance.

Fig. 1:

VGS is experimentally adjusted to optimize the conversion loss. However, the optimum VGS value changes with LO power; e.g. for an LO power of 7.5 dBm, the VGS value for best conversion loss is -1.15V (fig. 2).

Fig. 2: Vgs for best conversion loss versus local oscillator power

Results

Both RF and IF ports are well matched at their target frequencies (fig. 3 & 4).

Fig. 3: RF port match at 2.44 GHz

Fig. 4: IF port match at 110 MHz

Generally, the conversion loss will improve with an increase in the LO level. However, above an LO level of 2.5 dBm, the conversion loss begins to level out, and slowly deteriorates beyond that (fig. 5). Hence, the passive FET mixer offers similar conversion loss performance to the popular diode ring mixer.

Fig. 5: Conversion loss versus local oscillator power

The intermodulation performance is measured by feeding two tones (F1 & F2) into the RF port. The IF port is observed for the down-converted fundamental tones and the third order intermodulation products, (2F1-F2) and (2F2-F1). The third-order intercept point (IP3) is referred to the IF output. IP3 will steadily increase with LO power (fig. 6). However, LO power above 7.5 dBm is not recommended because at 10 dBm and higher, the side-bands' amplitudes become unequal (fig. 7). This indicates that the LO level is too high for proper mixer operation.

Fig. 6: Intercept point versus local oscillator power

Fig. 7: If the LO power is raised to 10 dBm, the side-bands' amplitudes become unequal

References

[1] L. Picheta and E.A. Allamando, "Achieve Microwave Frequency Conversion with Cold FETs", Microwave & RF, Feb. 1997.

[2] L.E. Larson (ed.), Microwave and RF Circuit Design for Wireless Communications, Artech House, 1997. Ch. 5: Mixers for Wireless Applications, written by S.A. Maas

Created: Jul. 2019

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