High Order Harmonic Oscillators in Microwave and Millimeter-wave ...

2 Oscillators
located nearby the desired signal frequency. The process is shown in Fig. 2.11. A local
oscillator frequency f 0 is used to down convert a desired signal to an IF frequency. Due
to phase noise, however, an adjacent undesired signal can be down converted to the same
IF frequency due to the phase noise spectrum of the local oscillator. The phase noise that
leads to this conversion is located at an offset from the carrier equal to the IF frequency
from the undesired signal. This process is called reciprocal mixing. From this diagram,
it is easy to see that the maximum allowable phase noise is order to achieve an adjacent
channel rejection (or selectivity) of S dB (S ≥ 0) is given by
L(f m ) = C[dBm] − S[dB] − I[dBm] − 10 log(B), (2.40)
where the scale of L(f m ) is (dBc/Hz) and C is the desired signal level (in dBm), I is the
undesired (interference) signal level (in dBm), and B is the bandwidth of the IF filter (in
Hz).
Desired
LO
Unwanted
signal
Desired
signal
IF
Phase
noise
Noisy LO
0
IF
IF
IF
f0
f
Fig. 2.11: Illustrating how local oscillator phase noise can lead to the reception of undesired
signals adjacent to the desired signal.
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