Anti-parallel diode mixer for direct-conversion receiver

In a few last years direct-conversion receivers became quite popular among radio amateurs. Such circuits are simple, they can provide high selectivity and sensitivity, there are almost no other image products. A radio listener can be assured that a radio station, heard in headphones, having exactly the same frequency as it displayed on the dial. In some superheterodyne receivers with a poor image rejection, an unwanted image signal can be received at the image frequency, a direct-conversion receiver doesn't have this drawback.

But direct-conversion receivers have other drawbacks such as two-channel reception, direct demodulation of AM signals, and if there is no RF amplifier before the mixer, such receivers may radiate the local oscillator signal. The first drawback can be resolved by using a quadrature detection method. Two other drawbacks can be overcome by using a balanced mixer. But it is quite hard to balance the mixer over a wide frequency range, because of stray capacitance of diodes, a difference across the transformer windings, quality of shielding, etc.

A various research to find a solution to this problem came up with a new mixer design that uses nonlinear components with the current versus voltage characteristic, shown in the Figure 1 with a solid line. It can be described with an equation of the cubic parabola:

I = AU + BU³(1)

Where A and B are constant coefficients. For a comparison, the same figure shows the current versus voltage characteristic of a diode (a dotted line in the Figure 1).

The desirable curve can be achieved if connect two silicone diodes in an antiparallel way. If it's possible, both diodes should have similar parameters.

Let's consider a simplified mixer circuit (see the Figure 2), where a nonlinear component formed by diodes V1, V2, is under the voltage U, that is a sum of a signal U_S and local oscillator U_LO voltage:

U = U_S*cos(2πF_St) + U_LO*cos(2πF_LOt)(2)

The load R_L and the blocking capacitor C_b are connected in the circuit of the nonlinear component. If we substitute the formula (2) into the equation (1), we can see currents that pass through the nonlinear component. Those currents are the signal current F_S, the local oscillator current F_LO, and currents of new frequencies 2F_LO ± F_S. If the local oscillator voltage is much higher than the signal voltage, all other output products of the mixer are very low voltage. The blocking capacitor blocks all high-frequency currents, so there is only one current through the load:

I = 3/4*B*U_S*U²_LO*cos(2*π*t*(2F_LO - F_S))

Let's see how the mixer works. When the local oscillator signal voltage crosses the zero level, both diodes are off, there is no current flow through the network. At positive and negative half-wave current of the voltage, one of diodes conducts the current, and thus the signal source connects to the load. Therefore, the mixer operates as a switch that switches the network with the frequency twice the local oscillator frequency. If the switching frequency is close to the signal frequency, it creates the beat frequency of 2F_LO - F_S (if the signal frequency is lower than the LO frequency), or in the other case 2F_LO + F_S. This is lower side band and upper side band respectively.

Thus, from said above it follows that the mixer with nonlinear characteristics has two features - first, the frequency of the local oscillator is twice lower than the frequency of the signal, and the second - there is no DC current through the load. The last one means that interfering AM signals will not be demodulated.

Of course, that is true only if the nonlinear element have a highly symmetric current versus voltage characteristic. If it not so (for example, if diodes have different parameters), then a new squared term comes to the equation (1), and the nonlinear element will directly detect the signal. However, in practical circuits it is easy to match two diodes with similar parameters than adjust a multi-diode balanced mixer. The radiation of the local oscillator signal is reduced because of the two time difference between the local oscillator frequency and the resonant frequency of the input LC circuit. But in case if there is a second harmonic of the local oscillator signal, it wouldn't be suppressed by the input LC circuit. Actually, a simple local oscillator circuit provides quite low level of harmonic voltages. The mixer also produces the third harmonic, but its frequency is far from the frequency of the input LC circuit.

Therefore, both disadvantages of a direct-conversion receiver, mentioned in this article, are quite overcome by using this mixer.

The circuit diagram of a simple mixer is shown in the Figure 3. The input resonant tank L1C2 is tuned to the signal frequency F_S. Antenna is connected to the LC tank through the capacitor C1. The tank L3C3 of the local oscillator network is tuned to the frequency F_S/2. Coupling coils L2 and L4 provide coupling between the mixer and resonant tanks. In order to obtain the maximum sensitivity of the receiver, number of turns of these coils have to be matched. The low-pass filter L5C4C5 with the cutoff frequency of 3 kHz is the load of the mixer. The audio signal from the output of the mixer is fed to an audio amplifier.

The windings of coils are different for different bands. Values of capacitors are in the range of 500...1000 pF (for 80 meter band), to 50...100 pF (for 10 meters band). Coupling coils are wound over resonant tank coils, they have 0.1..0.3 turns of resonant tank coils.

The best diodes for this mixer are KD503A (1N4148). Diodes D104, D105 also can be used in this circuit.

The disadvantage of the simple mixer is a signal attenuation in the coupling network of the local oscillator. This disadvantage can be overcome by using a balanced mixer circuit with nonlinear components that provide a "cubic" current versus voltage characteristic. In this case, the local oscillator network and the signal network are almost completely decoupled, and the local oscillator radiation is almost suppressed. Such a mixer was used in the direct-conversion receiver for 28 MHz ("Radio", 1973, 7, p. 15). The receiver was modified just a little bit - diodes are connected in other way, and added the capacitor C2 to the local oscillator resonant tank to reduce its frequency twice (see the Figure 4). Values of coils and capacitors remained the same. The transformer T1 (now only half of the second winding in use) is used to match impedances between the mixer and the input LC tank. The coil L1, L2 tapping points should be matched to achieve the maximum volume of the audio signal in headphones. The exact tapping position of L2 is very important, because both low and high local oscillator signal levels reduce the gain of the mixer, therefore it leads to receiver sensitivity decreasing.

As a result, the local oscillator voltage at the antenna connector decreased from 1 mV to 200 μV, i.e. 14 dB. Suppression of interfered AM stations are improved, even without matching diodes for identical parameters. The receiver sensitivity is the same. The tuning of the input LC circuit doesn't change the local oscillator frequency any more. The stability of the local oscillator frequency is also improved, because the LO resonant tank is tuned to a twice lower frequency, and it uses a higher value capacitor.

Mixers with "cubic" current versus voltage characteristics can be used not only in direct-conversion receiver designs, but in other designs, just like SSB modulators, superheterodyne receivers, etc.

V Polyakov,
"Radio", 1976, 12

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