Variable gain mixer circuit

Details Variable gain mixer circuit Variable gain mixer circuit

2000-10-17

2004-10-19

Trost, William (Department: 2683)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

C455S139000, C455S182100, C455S232100, C455S234100, C375S344000, C375S345000

Reexamination Certificate

active

06807406

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to electronic systems and circuits for use in wireless receivers and transmitters, and more particularly, to an electronic system having a mixer circuit with variable gain.
The field of wireless technology is currently undergoing a revolution, and is experiencing exponential growth. Cell phones, once considered a novelty and referred to as “car phones” are now ubiquitous, and cordless phones in the home are commonplace. A whole new batch of wireless personal digital assistants, and Bluetooth enabled computer peripherals are now entering the market, with wireless internet access as a driving force. A mixer having a variable gain is described which facilitates the design and lower the cost of circuits for these and other related products.
Wireless devices typically transmit and receive data through the air on high frequency electromagnetic waveforms, though some systems, such as satellite dishes and pagers simply receive, and others merely transmit. Data transmission is begun by encoding the data to be transmitted. This encoded data typically has a data rate of 100 kHz to 100 MHz and modulates a high frequency carrier signal. The carrier signal is often in the 2-10 GHz range. The modulated carrier signal is then applied to an antenna for broadcasting. The broadcast signal is referred to as a radio frequency (RF) signal. Reception involves receiving the RF signal on a different antenna, and filtering undesired spectral components. The signal is demodulated, filtered again, and decoded.
In real world situations, the amplitude of a received RF signal is constantly changing. For example, a passenger using a cell phone in a moving car may be traveling toward the antenna which is providing a signal. In that case, the amplitude of the received signal increases as the car approaches the antenna. Furthermore, buildings may intervene, or the car may enter a tunnel. These cause a received signal strength to decrease.
But receivers operate best if the amplitudes of received signals remain in a specific range. Specifically, if the signal power is too small, errors occur due to noise sources such as natural radiation, television and radio broadcasting, power transmission systems and the like. If the signal is too large, the linearity of the receive channel is lost and the signal begins to clip, again resulting in errors.
A conventional solution for this is to insert a variable gain amplifier in the receive channel. If the signal strength decreases, the amplifier gain is correspondingly increased. If received power increases, the gain may appropriately be lowered.
Conventional Receivers and Mixers
FIG. 1
is a block diagram of one such conventional receiver channel. Specifically, a direct conversion receiver is represented. It may also be referred to as a low IF (intermediate frequency), zero IF, or homodyne receiver. Included is a low noise amplifier (LNA)
110
, a modulator or mixer
120
, low pass filter (LPF)
130
, variable gain amplifier (VGA)
140
, analog to digital converter (A/D)
150
, digital signal processor (DSP)
160
, voltage controlled oscillator (VCO)
170
, phase lock loop (PLL)
170
, and digital to analog converter (D/A)
190
. The PLL
160
includes a frequency synthesizer, phase-frequency detector, and loop filter.
The RF signal is received on an antenna (not shown) coupled to line
105
. A choke filter may be used to remove unwanted spectral portions from received signal. The RF signal is amplified by LNA
110
, and provided to the mixer
120
. LNA
110
may be a composite of more than one amplifier, for example a second LNA may be on a chip with the other blocks shown, while a first LNA may be off-chip. A VCO
170
generates a local oscillator (LO) signal on line
175
, and provides it to the mixer
120
and PLL
180
. The VCO may be on-chip or off-chip; alternately it may have its transistors on-chip, with some passive components external.
The mixer
120
multiplies the RFin signal on line
115
with the LO signal on line
175
. The mixer outputs a signal on line IF
1
125
, which has spectral components at the two frequencies which are the sum and difference of the RFin and LO signals. Specifically, if the RFin and LO frequencies are both 2.4 GHz, IF
1
has components at DC (0 Hz) and 4.8 GHz.
LPF
130
filters the high frequency sum products of IF
1
while passing the low frequency difference components. VGA
140
adjusts the amplitude of the signal at IF
3
in order to optimize the use of the dynamic range of A/D
150
. The A/D
150
converts the analog signal IF
3
on line
145
into a digital waveform, and provides it on bus DOUT
155
to DSP
160
. The DSP
160
decodes the data, and provides an output on line
165
. PLL
180
generates the voltage which controls the VCO's oscillation frequency. The control voltage is Vtune, and is output from the PLL to the VCO on line
185
. The PLL divides the LO signal on line
175
and compares that to a reference frequency (REF) provided on line
199
. The LO frequency is adjusted accordingly.
DSP
160
provides a second output on bus Dgain
197
. This signal is the result of a comparison of the amplitude of A/D output signal Dout on bus
155
with the range of available outputs from the A/D. The digital signal on Dgain bus
197
is converted to an analog signal used to control the gain of VGA
140
. For example, one A/D may have 256 available output levels. If the signal at Dout
155
covers a range of only 30 levels, there is excessive quantization error. The gain of VGA
140
can be increased such that 200 levels are used. This increase in VGA gain results in a 4 times increase in the accuracy of the digitization of the waveform on IF
3
145
. But if all 256 available output levels are used, the signal may be clipping, and information may be lost. The gain of VGA
140
can be reduced, again to where 200 levels are used.
A conventional mixer circuit used in conventional receivers and/or transmitters is shown in FIG.
2
. The mixer has a first input port
245
labeled RFin, a second differential input port for the LO signal on lines
215
and
225
, and a differential output signal IF
1
on lines
265
and
275
. Voltage changes at RFin generate a current in capacitor Cl
240
. This current modulates the tail current provided by M
3
230
under the control of the bias voltage on node
235
. This RFin modulated current is then multiplied in the mixer core M
1
210
and M
2
220
, resulting in the IF
1
output at nodes
265
and
275
. The output signal IF
1
will have two frequency components, one at the sum of the frequency of the RFin and LO signals, and one at the difference.
In
FIG. 1
, several blocks are involved in mixing the signals and adjusting the output amplitude. Specifically, mixer
120
mixes the signals, VGA
140
controls the signal amplitude, and DSP
160
, along with D/A
190
control the gain of VGA
140
. Each block adds noise to the signal, and consumes both power and die area. Moreover, the variation of gain in a typical VGA is often non-linear. Therefore, it would be desirable to obtain a mixer circuit that allows for a simpler topology with reduced complexity. Additionally, it would be desirable to obtain a system wherein the variable gain is substantially linear.
SUMMARY OF THE INVENTION
In accordance with embodiments of the present invention a simplified architecture for a receiver system is provided. Additionally, embodiments of the present invention provide a variable gain mixer circuit useful in the simplified receiver system.
Accordingly, one embodiment of the present invention provides an apparatus including a receiver system, the receiver system comprising a first amplifier having an input coupled to receive an input RF signal and an output to produce an amplified RF signal, a variable gain mixer circuit having a first input, a second input to receive the amplified RF signal, a mixer output, and a control input, a low pass filter coupled to the mixer output, an analog-to-digital converter coupled to the low pass filter for producing

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