Frequency translated filter
Embodiments of a frequency translated filter (FTF) are presented. An FTF includes a passive mixer and a baseband impedance. The baseband impedance includes a network of one or more passive components (e.g., resistors, inductors, and capacitors) that form a low-Q filter. The passive mixer is configured to translate the baseband impedance to a higher frequency. The translated baseband impedance forms a high-Q filter and is presented at the input of the FTF. The FTF can be fully integrated in CMOS IC technology (or others, e.g., Bipolar, BiCMOS, and SiGe) and applied in wireless receiver systems including GSM, Wideband Code Division Multiple Access (WCDMA), Bluetooth, and wireless LANs (e.g., IEEE 802.11).
1. A radio frequency (RF) receiver configured to receive and process an RF signal, the RF signal including a desired channel positioned within a particular frequency band, the RF receiver comprising:
a low noise amplifier (LNA) coupled to the RF signal and configured to provide an amplified RF signal; and
a frequency translated filter coupled between an input stage and a cascode stage of the LNA and configured to attenuate interferers in the RF signal outside the particular frequency band, the frequency translated filter comprising:
a passive mixer configured to mix the RF signal received at a first mixer input and a local oscillator (LO) signal received at a second mixer input; and
a baseband impedance coupled between an output of the passive mixer and ground,
wherein the first mixer input presents a high-impedance path to ground for frequency components of the RF signal within the particular frequency band and a low-impedance path to ground for frequency components of the RF signal outside the particular frequency band.
2. The RF receiver of claim 1 , wherein the LO signal has a duty cycle substantially equal to 25%.
3. The RF receiver of claim 2 , wherein the LO signal comprises an in-phase LO signal and a quadrature LO signal.
4. The RF receiver of claim 3 , wherein the passive mixer comprises:
a first single-balanced mixer configured to mix the in-phase LO signal and the RF signal; and
a second single-balanced mixer configured to mix the quadrature LO signal and the RF signal.
5. The RF receiver of claim 3 , wherein the passive mixer comprises:
a first double-balanced mixer configured to mix the in-phase LO signal and the RF signal; and
a second double-balanced mixer configured to mix the quadrature LO signal and the RF signal.
6. The RF receiver of claim 2 , wherein an impedance seen by the RF signal at the first mixer input is substantially equal to the baseband impedance translated in frequency by a frequency of the LO signal.
7. The RF receiver of claim 6 , wherein the impedance seen by the RF signal at the first mixer input is substantially given by:
Z IN ( jω RF )= R SW +2/π 2 {Z BB ( jω RF −jω LO )+ Z BB ( jω RF +jω LO )}
wherein:
Z IN is the translated baseband impedance;
Z BB is the baseband impedance;
ω RF is the frequency of the RF signal;
ω LO is the frequency of the LO signal; and
R SW is the switch resistance of a switching device used in the passive mixer.
8. The RF receiver of claim 6 , wherein the baseband impedance comprises at least one of a capacitor and an inductor.
9. The RF receiver of claim 8 , wherein the baseband impedance forms a low quality factor (low-Q) filter.
10. The RF receiver of claim 9 , wherein the translated baseband impedance seen by the RF signal at the first mixer input forms a high quality factor (high-Q) filter.
11. The RF receiver of claim 10 , wherein the high-Q filter is a notch filter.
12. The RF receiver of claim 11 , wherein the center frequency of the high-Q filter is determined by the frequency of the LO signal.
13. A radio frequency (RF) receiver configured to receive and process an RF signal, the RF signal including a desired channel positioned within a particular frequency band, the RF receiver comprising:
a passive mixer configured to mix the RF signal received at a first mixer input and a local oscillator (LO) signal received at a second mixer input; and
a baseband impedance coupled between an output of the passive mixer and ground,
wherein the first mixer input is coupled between an input stage and a cascode stage of an LNA and presents a high-impedance path to ground for frequency components of the RF signal within the particular frequency band and a low-impedance path to ground for frequency components of the RF signal outside the particular frequency band.
14. The RF receiver of claim 13 , wherein the LO signal has a duty cycle substantially equal to 25%.
15. The RF receiver of claim 14 , wherein the LO signal comprises an in-phase LO signal and a quadrature LO signal.
16. The RF receiver of claim 15 , wherein the passive mixer comprises:
a first single-balanced mixer configured to mix the in-phase LO signal and the RF signal; and
a second single-balanced mixer configured to mix the quadrature LO signal and the RF signal.
17. The RF receiver of claim 15 , wherein the passive mixer comprises:
a first double-balanced mixer configured to mix the in-phase LO signal and the RF signal; and
a second double-balanced mixer configured to mix the quadrature LO signal and the RF signal.
18. The RF receiver of claim 14 , wherein an impedance seen by the RF signal at the first mixer input is substantially equal to the baseband impedance translated in frequency by a frequency of the LO signal.
19. The RF receiver of claim 18 , wherein the impedance seen by the RF signal at the first mixer input is substantially given by:
Z
IN
(
jω
RF
)
=
R
SW
+
2
π
2
{
Z
BB
(
jω
RF
-
jω
LO
)
+
Z
BB
(
jω
RF
+
jω
LO
)
}
wherein:
Z IN is the translated baseband impedance;
Z BB is the baseband impedance;
ω RF is the frequency of the RF signal;
ω LO is the frequency of the LO signal; and
R SW is the switch resistance of a switching device used in the passive mixer.
20. The RF receiver of claim 18 , wherein the baseband impedance comprises at least one of a capacitor and an inductor.
21. The RF receiver of claim 20 , wherein the baseband impedance forms a low quality factor (low-Q) filter.
22. The RF receiver of claim 21 , wherein the translated baseband impedance seen by the RF signal at the first mixer input forms a high quality factor (high-Q) filter.
23. The RF receiver of claim 22 , wherein the high-Q filter is a notch filter.
24. The RF receiver of claim 23 , wherein the center frequency of the high-Q filter is determined by the frequency of the LO signal.
25. The RF receiver of claim 24 , wherein the LO signal has a frequency substantially equal to a center frequency of the desired channel.
26. The RF receiver of claim 24 , wherein the LO signal has a frequency substantially equal to a center frequency of the particular frequency band.
27. The RF receiver of claim 23 , wherein the high-Q filter presents a high-impedance path to ground for the desired channel and a low-impedance path to ground for the interferers outside the particular frequency band.
28. The RF receiver of claim 13 , wherein the particular frequency band is defined by a communications standard.
29. A frequency translated filter, comprising:
a passive mixer configured to mix a radio frequency (RF) signal received at a first mixer input and a local oscillator (LO) signal received at a second mixer input; and
a baseband impedance coupled between an output of the passive mixer and ground,
wherein the first mixer input presents a high-impedance path to ground for frequency components of the RF signal within a desired frequency band and a low-impedance path to ground for frequency components of the RF signal outside the desired frequency band,
wherein an impedance seen by the RF signal at the first mixer input is substantially equal to the baseband impedance translated in frequency by a frequency of the LO signal and is substantially given by:
Z
IN
(
jω
RF
)
=
R
SW
+
2
π
2
{
Z
BB
(
jω
RF
-
jω
LO
)
+
Z
BB
(
jω
RF
+
jω
LO
)
}
wherein:
Z IN is the translated baseband impedance;
Z BB is the baseband impedance;
ω RF is the frequency of the RF signal;
ω LO is the frequency of the LO signal; and
R SW is the switch resistance of a switching device used in the passive mixer.