RF Electronics Chapter 5: Frequency Mixers Page 158
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 5.77 shows the Frater Gilbert Cell connected as a down-conversion mixer, with a
stepped LO input power to determine the conversion gain. Figure 5.78 shows the resulting
conversion gain. The LO and RF frequencies are chosen to be the same as that for the
microwave mixers of figure 5.53 and 5.56.
The Frater-Gilbert Cell mixer is different from the diode mixers in that the best
performance is obtained when the LO signal does not cause saturation in the transistors
of the Frater-Gilbert Cell. Comparing figure 5.78 with figure 5.34, shows that the slope
of conversion gain versus LO power is much larger for the Frater-Gilbert Cell mixer in
figure 5.73 than for the DBM in figure 5.26. As a result, variations in AM noise on the
LO will cause more of this AM noise to appear on the IF signal in the Frater-Gilbert Cell
mixer than in the DBM. Figure 5.78 shows that the largest signal that can be used without
the mixer saturating too much is about -6 dBm and that level of LO power has been used
for the subsequent measurements. Using a lower LO power gives more ideal mixer action
but reduces the conversion gain.
Figure 5.79. IF spectrum of the Frater-Gilbert Cell mixer of figure 5.77 as a down-converter.
Figure 5.80. Frater-Gilbert Cell up-conversion mixer for RF=-30 dBm.
Figure 5.79 shows the IF output spectrum of the mixer as a down-converter. The LO
power is -6 dBm and the RF level is -20 dBm. To enable a comparison to be made, the
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