Subharmonic double-balanced mixer

Details Subharmonic double-balanced mixer Subharmonic double-balanced mixer

2000-05-08

2002-02-19

Callahan, Timothy P. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific input to output function

Combining of plural signals

C327S359000

Reexamination Certificate

active

06348830

ABSTRACT:

TECHNICAL FIELD
This invention relates to subharmonic double-balanced mixers and, in particular, to subharmonic double-balanced mixers which can be implemented by integrated circuit techniques.
BACKGROUND ART
There is currently a great amount of activity in realizing low cost Si based transceivers for the wireless communications market. All transceiver architectures use a mixer for the frequency translation from the RF (radio frequency) to the IF (intermediate frequency) and vice versa. A double-balanced Gilbert cell mixer is routinely used in most transceivers today where high LO (local oscillator) to RF isolation and high LO/RF suppression at the IF port is needed.
A mixer is a nonlinear device containing either diodes or transistors, the function of which is to combine signals of two different frequencies in such a way as to produce energy at other frequencies. The mixer is typically built using Silicon, GaAs, or InP diodes, or using Silicon-based transistor technology comprising but not limited to, BJT (bipolar Junction Transistors), MOS and CMOS transistors, BiCOMS (bipolar/CMOS), or using SiGe-based transistors (HBT, etc.), or using GaAs-based transistor technology (MESFET, HEMT, HBT, etc.) or using InP-based transistors technology (MESFET, HEMT, etc.).
In a typical down converter application within a heterodyne receiver, a mixer has two inputs and one output. One of the inputs is the modulated carrier RF or microwave signal at a frequency f
RF
, the other is a well controlled signal from a local oscillator at a fixed frequency, or a voltage controlled oscillator (VCO), at a frequency f
LO
. The result of down conversion is a signal at the difference frequency f
RF
−f
LO
or f
LO
−f
RF
, which is also called the intermediate frequency f
IF
. A filter is sometimes connected to the output of the mixer to allow only the desired IF frequency signal to be passed on for further processing. For example, for an RF frequency of 840 MHz and an LO frequency of 770 MHz, the IF frequency would be 70 MHz. Another example is an RF frequency of 1960 MHz and an LO frequency of 1820 MHz, resulting in an IF frequency of 140 MHz at the output of the mixer.
FIG. 1
illustrates several other frequency selections for the RF, LO and IF signals.
The mixer circuit is also commonly used as an up converter. In this case, the low frequency (baseband or intermediate frequency) modulated signal is upconverted in the mixer using a local oscillator (fixed frequency or voltage controlled oscillator) to generate the modulated RF or microwave signal. The resulting signal of the up-conversion is at a frequency given by f
LO
+f
IF
, or f
LO
−f
IF
. A filter is sometimes used at the output of the up converter (mixer) to choose either one of these frequencies. In general, all the properties such as isolation between the IF, RF and LO ports, intermodulation, conversion loss, etc. of the up converter are closely related, if not identical, to the properties of the down converter mixer.
There are several kinds of receiver topologies which are commonly used today. The first one is commonly called the heterodyne receiver (or the superheterodyne receiver) which uses an intermediate frequency which is rather high, as is illustrated in FIG.
2
. The IF signal can be selected to be anywhere, at 10 MHz, 40 MHz, 140 MHz, 220 MHz, 400 MHz, etc. and even 1 or 2 GHz in high frequency systems (RF of 5-20 GHz). The IF signal is filtered, amplified in a high gain IF amplifier chain, and then down converted again to a low frequency (typically called the second IF) for demodulation and detection.
The other receiver topology is called a direct conversion receiver which uses virtually no IF signals, and down converts the RF signal directly into baseband, as illustrated in FIG.
3
. In this case, for an RF signal with a center frequency of 1960 MHz and a bandwidth of 1 MHz, one would choose an LO at 1960 MHz, resulting in an IF signal output of 0-1 MHz. The direct conversion receiver results in a much easier system layout, and also saves on using expensive IF filters, and the resulting IF electronics. The bandwidth of the baseband signal is typically given by the type of modulation used, and can be 0-200 KHz, 0-1 MHz, or even 0-5 MHz. There are also other kinds of receivers, such as the low-IF heterodyne receiver, which are less commonly used.
The direct conversion topology is much less expensive to build than the standard heterodyne receiver with its IF electronics, and will miniaturize the front-end electronics even further than today's state-of-the-art. It is expected to be used in the new wireless local loop transceivers at 2.4 GHz, 3.5 GHz, 4.9 GHz, 5.8 GHz, etc., in low-cost cordless telephones at 900-1100 MHz, and in the new digital cellular telephones (1700-2100 MHz).
One must be very careful in mixer designs to ensure that no LO power and no RF power leak into the IF port. The reason is that the IF port is typically followed by a high gain amplifier and any LO leakage can saturate this IF amplifier. Also, any RF or LO leakage can introduce intermodulation products in the IF amplifier and limit the sensitivity of the receiver. This is typically done using a filter at the IF port which is selected to pass the IF signal, and to greatly attenuate the RF and LO signals. This embodiment is common in single and balanced mixer designs, but the filter occupies a lot of space on the RF integrated circuit. In reality, and for RFIC applications, single and balanced mixer designs are rarely used because of the existence of the Gilbert-Cell double-balanced mixer as described hereinbelow. However, balanced mixers built with an integral IF filter are typically used at microwave frequencies (5 GHz, 10 GHz, etc.) since they offer acceptable performance without taking a lot of space on the MMIC wafer.
The double-balanced mixer topology results in no RF and LO leakage at the IF port, and therefore no IF filter is used with this topology. The “double-balanced mixer” is the most commonly used topology in integrated circuit mixers. A double-balanced mixer is essential in RF-IC mixers since it mitigates the use of complicated and expensive on-chip (or off-chip) filters to remove the LO and RF signals at the output IF port. Double-balanced mixers are typically built using a “Gilbert-Cell” topology in RFIC transistor circuits, as is illustrated in
FIG. 4
, or using a transformer-coupled diode-ring circuit. In RFIC applications which use a Gilbert-Cell topology, the double-balanced mixer requires that the LO and the RF frequencies be relatively close to each other (RF at 1960 MHz, LO at 1820 MHz and with an IF of 140 MHz).
A subharmonic double-balanced mixer has the same function as a regular double-balanced mixer, and still offers high isolation between the RF and IF ports, LO and IF ports and RF and LO ports, but uses an LO frequency which is approximately half that of the RF signal, as illustrated in FIG.
5
. The IF signal frequency is therefore the difference frequency between RF signal frequency and 2 times the LO signal frequency (f
IF
=f
RF
−2
f
LO
, or f
IF
=2f
LO
−f
IF
). In the case of heterodyne architectures with an RF signal at 1900 MHz (or 2400 MHz), rather than use an 1830 MHz LO (or a 2330 MHz LO), one could use a 915 MHz LO (or a 1165 MHz LO) to achieve an IF of 70 MHz. The use of a lower frequency local oscillator frequency simplifies the LO design, and most importantly, since the LO is at a lower frequency, one can integrate the LO on the RFIC chip and still satisfy the system phase-noise requirements. One can immediately see the need of this mixer for the new PCS bands and wireless local loop bands at 1900 MHz and 2400 MHz.
The subharmonic double-balanced mixer is also very beneficial to dual-band cellular telephones since only one LO signal is needed to cover both the 800 MHz band and the 1900 MHz band, and an example of such a system is illustrated in FIG.
6
. This will save the designer a lot of space and cost in the RF front-end, will greatly simplify the system archit

Affiliated with

Also associated with

Rating

Subharmonic double-balanced mixer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Subharmonic double-balanced mixer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Subharmonic double-balanced mixer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2962205