RF Electronics Chapter 5: Frequency Mixers Page 136
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
The measured performance of this mixer is shown in Figure 5.41. The transformers for
this mixer were designed for operation at 1 MHz. The mixer was found to perform well
as a mixer for LO and RF frequencies between 20 kHz and 50 MHz. Above 50 MHz the
mixer performance was good, but for up-conversion the LO carrier isolation became less.
The mixer was still useful as a mixer to 100 MHz.
As a result an IF frequency of 250 kHz and an LO of 10 MHz was used for the spectrum
in figure 5.41. For this figure, the LO signal had a level of +7 dBm. The IF signal was at
a frequency of 250 kHz and a level of -10 dBm. The measured conversion loss is 6 dB
and the LO RF isolation is -60 dB. This mixer performs well for RF and LO signals in
the range of 30 kHz to 30 MHz and IF signals up to 30 MHz.
Figure 5.41. Measured performance of the mixer of figure 5.40. (Hor: Centre 10 MHz,
500 kHz/div, Vert: Max -10dBm, 10 dB/div.)
Comparing the spectrum around the LO in figure 5.40 with the corresponding spectrum
around the LO in figure 5.38, shows that in practice there will be some LO RF carrier
feed-through due to a slight mismatch of the diodes or a slight mismatch in the signal path
lengths. Such path lengths are particularly difficult to match exactly at microwave
frequencies. Figure 5.41 shows that the 5IM distortion components, which are 1.250 MHz
from the LO frequency at the centre of the plot, are bigger than the 3IM components,
which are 750 kHz from the LO frequency.
RF Electronics: Design and Simulation
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