Mixers have traditionally relied on diodes as the nonlinear mixing element. In this case, the typical configuration is shown in Figure 7.6.
The input signal is the RF, while the output signal is the IF in the case of a downconverter; vice versa in the case of an upconverter. The input network provides the optimum terminations to the LO and IN signals and filters the OUT signal generated by the nonlinearity in the diode, in order to ensure minimum conversion losses and maximum isolation between the input and output ports. It must also provide isolation between the LO and the IN ports in order to avoid interference. More dangerously, the large LO signal could saturate the output of the IN amplifier stage, when present.
Similarly, the output network provides optimum loading for the OUT signal and stops the IN and LO signals. The practical design and realisation of the filtering structures can be problematic, especially when the frequency of an unwanted large signal (typically the LO fundamental or low-harmonic frequency) lies very close to the input or output frequency that requires a good match. As we will see in the following, a balanced structure can suppress, or rather attenuate, an unwanted spectral line, easing the design of the filtering and matching networks.
In the case of the diode, the main nonlinearity is the I/V exponential charac- teristic, which presents a differential resistance ranging from nearly open circuit when
LO+ IN LO+ IN OUT
filter and match
OUT filter and match
Figure 7.6 The general structure of a diode mixer
reverse biased to a very low value when forward biased. The junction capacitance has a much smaller variation range and its contribution to mixing is much less important; it can be considered constant, and neglected for approximate analysis. A large LO signal drives the diode into forward and reverse bias for the largest part of the signal period, making the diode work very much as the ideal switch in Figure 7.4. A small forward bias current, bringing the diode at the edge of forward conduction, allows the LO signal to effectively switch it between almost short circuit (forward conduction) and almost open circuit (reverse bias) even for low amplitudes of the LO signal itself, thus enhancing the mixer performances; however, the need for a path for the bias current may complicate the layout and degrade the performances.
Active mixers make use of three-terminal devices such as MESFETs, HEMTs, HBTs or BJTs as nonlinear mixing elements, providing also some gain or at least reduced losses. Different nonlinearities are exploited depending on which terminal the large LO signal is fed to; however, the predominant nonlinear element is always the drain or collector current source, while capacitances provide a minor contribution. The outputI/V characteristics of an FET are shown in Figure 7.7 in which the load curves corresponding to different modes of operation are indicated. The parameters modulated by the LO signal are the transconductance and the output conductance, that is, the derivatives of the I/V curves with respect to gate and drain voltage respectively:
gm = ∂Id
∂Vgs
Vds=const.
gd= ∂Id
∂Vds
Vgs=const. (7.3)
The load line 1 in Figure 7.7 corresponds to a gate mixer, where the main nonlin- earity is the transconductance, modulated by the LO signal applied to the gate, with the drain voltage fairly constant. The input signal is applied to the gate as well, while the output signal is taken at the drain port, as shown in Figure 7.8.
2
4
3
1 0.04
0.03 0.02 0.01 0
−0.01 1
0
−1
−2 0 1 2 3 4 5 6
0.05
Vds
Vgs Id
Figure 7.7 Load lines on the output I/V characteristics of an FET corresponding to different operation modes
IN IN filter and match
LO
OUT
LO filter and match
OUT filter and match
Figure 7.8 The general structure of a gate mixer
The LO signal modulates the transconductance, and therefore the gain of the common-source amplifier for the IN signal, from zero below pinch off to the maxi- mum value along the load line. The behaviour is very much like that of a switch with gain. In Figure 7.7, the load line has a constantVdsvoltage path, implying a short-circuit drain termination at the LO fundamental frequency and harmonics; this is discussed in some detail below, together with the terminations at the IN and OUT frequencies.
This configuration does not provide any intrinsic isolation between LO and IN signals and has a very bad isolation between LO and OUT ports since the already large LO signal is further amplified by the FET into the OUT port. The IN signal is also amplified by the FET, but its amplitude is relatively smaller and is more easily filtered out at the OUT port. The LO and IN ports are isolated from the OUT signal because of the low reverse gain of the FET. This configuration is likely to provide a conversion gain if properly terminated; however, it is also prone to instability if the gain is exceedingly large.
The load line 2 in Figure 7.7 corresponds to a drain mixer, where the main nonlin- earities are the transconductance and the output conductance, modulated by an LO signal applied to the drain, with the gate voltage fairly constant. The input signal is applied to the gate, while the output signal is taken at the drain port, as shown in Figure 7.9.
The LO signal modulates the transconductance and the output conductance of the FET, and therefore the gain of the common-source amplifier for the IN signal, while switching between the saturated and ohmic regions of the characteristics. The behaviour is again like that of a switch with gain. In Figure 7.7, the load line has a constantVgs
voltage path, implying a short-circuit gate termination at the LO fundamental frequency and harmonics.
This configuration does not provide any intrinsic isolation between LO and OUT signals and has a bad isolation between IN and both OUT and LO ports since the IN signal is amplified by the FET. The IN port is isolated from the LO and OUT signals because of the low reverse gain of the FET. It is likely to provide a conversion gain if properly terminated; however, it is also prone to instability if the gain is large.
IN
OUT LO
IN filter and match
OUT filter and match LO filter and match
Figure 7.9 The general structure of a drain mixer
IN
LO
OUT
IN filter and match
OUT filter and match
LO filter and match
Figure 7.10 The general structure of a source mixer
The load line 3 in Figure 7.7 corresponds to a source mixer, where the main nonlinearities are the transconductance and the output conductance, modulated by an LO signal applied to the source, with the gate and drain voltages fairly constant. The input signal is applied to the gate, while the output signal is taken at the drain port, as shown in Figure 7.10.
The LO signal modulates the transconductance and the output conductance of the FET and therefore the gain of the amplifier for the IN signal. The behaviour is again like that of a switch with gain. In Figure 7.7, the load line has a constant Vgd voltage path, implying short-circuit gate and drain termination at the LO fundamental frequency and harmonics.
This configuration does not provide any intrinsic isolation between LO and OUT signals and has a bad isolation between IN and both OUT and LO ports. The IN port is
LO
OUT IN
LO filter and match
OUT filter and match IN filter and match
Figure 7.11 The general structure of a resistive (channel) mixer
isolated from both the LO and the OUT signal because of the low reverse gain of the FET. It is likely to provide a conversion gain if properly terminated.
The load line 4 in Figure 7.7 corresponds to what could be called a channel mixer since the main nonlinearity is the channel conductance, modulated by an LO signal applied to the gate, with zero-drain bias. It is known as resistive mixer because the FET has no drain bias (cold FET), and therefore has no gain. The input signal is applied to the drain, while the output signal is taken at the drain or source port, as shown in Figure 7.11.
The LO signal modulates the channel (output) conductance of the FET, making the FET behave as a time-variant resistance when seen from the drain port. In Figure 7.7, the load line has a constantVdsvoltage path, implying short-circuit drain termination at the LO fundamental frequency and harmonics.
This configuration provides a moderate isolation between LO and both IN and OUT signals: on the one hand, the FET does not have any gain, but on the other hand, the gate- channel capacitance is high at zero-drain voltage, providing non-negligible coupling. No intrinsic isolation is provided between IN and OUT ports. No gain is provided because of the cold FET; however, very linear conversion is ensured by the superior linearity of the output conductance in the ohmic region compared to the linearity of transconductance and output conductance in the regions of operations described above. Therefore, this configuration is especially valuable for low-intermodulation applications.