Mixers with antiparallel diodes are used in direct-conversion receivers (DCR). This type of receivers also known as homodyne, synchrodyne, or zero-IF receiver. The first description of such a mixer was published in the "Radio" magazine of 1976 year . Let's recall its main advantages
The current versus voltage characteristic of two diodes, connected in antiparallel way, are symmetric relative to the center of a coordinate system. It can be described by an equation of the cubic parabola. Because of this, this kind of mixer doesn't detect signals of powerful AM radio stations, which frequencies are close to the frequency the receiver is tuned to, so it reduces susceptibility to radio interference of direct-conversion receivers.
A mixer with antiparallel diodes requires that the local oscillator frequency is twice lower than the signal frequency. It prevents the local oscillator signal from passing to the antenna, because of the selectivity of the resonant circuit at the input of the receiver.
And last but not least, the local oscillator, working at the lower frequency, is more stable. It is important for receiving CW and SSB signals without constantly using a tuning knob.
It is known that a field-effect transistor can be used as a voltage-controlled resistor. The voltage-controlled resistor is used in a different variety of circuits, such as phase and frequency demodulators, modulators, switchers, multiplexers and similar devices (see references [2-5]). Such devices generally having low non-linear distortion, high dynamic range, good decoupling between the local oscillator network and antenna network. This article is an attempt to combine advantages of antiparallel diode mixers and advantages of FET based voltage-controlled resistors. The result is a mixer with two FET transistors, used as voltage-controlled resistors, working in opposite phases.
The simplified circuit diagram of this mixer is shown in Figure 1. The signal from L1C1 tank circuit is fed to channels, connected in parallel, of transistors V1 and V2. A network C2R1 is the load of the mixer, it works generally as a low-pass filter. The local oscillator signal is fed to the tapped transformer T1, it provides opposite phase output signals. The signals is fed to gates of transistors. The frequency of the local oscillator is twice lower than the frequency of the input signal.
The Figure 2, A, shows the timing diagram of the voltage at the gate of V1, the Figure 2, B - at the gate of V2. The Ucutoff voltage is related to use of an insulated-gate field-effect (MOSFET) transistor, working in enhancement mode. If use field-effect transistors of other types, it requires a bias voltage. The channel of each of the transistor conducts the current if the voltage at the gate is higher than the cutoff voltage, i.e. at peaks of the positive half-wave of the local oscillator signal. Because gates are fed with opposite-phases signals, the summary conductivity of parallel-connected channels is increasing twice for the period, as it shown on the Figure 2, C. Therefore, parallel-connected channels work just like a switch, operating with the frequency 2*FLO, two times higher than the local oscillator frequency. If the local oscillator frequency is close to the input frequency, a signal with the frequency of Fsignal - 2*FLO will appear at the load of the mixer. It looks like the mixing occurs at the second harmonic of the local oscillator signal, but actually there is no real signal with the frequency of 2*FLO.
This mixer provides good suppression for local oscillator signal at input networks. Stray capacitance of gate and drain of transistors are connected to opposite pins of the high frequency transformer. The transformer is symmetric, it produces opposite phases signals. As a result, it works as a balanced bridge circuit. Because of this, the local oscillator voltage at the input network is reduced by 30..40 dB. The selectivity of the input resonant circuit provides additional reducing of the local oscillator voltage for 30 dB or even more, because the frequency of the input resonant circuit is twice more than the local oscillator frequency.
Note, that suppression of local oscillator voltage at the input of the receiver is very important. A stray signal will be synchronously detected in the mixer, it will create a bias voltage at the mixer output, that unbalances the mixer. The unbalanced mixer can detect radio stations near in frequency to the desired station. The other problem is that if the receiver radiates the local oscillator signal. As it happens, it can create interference not only for other receivers, but also for this receiver. In this case the local oscillator signal radiated by the receiver and the signal can be modulated by the frequency of the AC mains at bad contacts, by rectifier diodes, etc. The signal, "modulated" in such way, gets back to the antenna, after demodulating it can be heard in headphones as the AC hum of 60/50 Hz. If we disconnect the antenna from the receiver, the hum will gone. The mixer, described in this article, almost guaranteed from such effects even if use a simple short-wire antenna connected to the receiver without using a coaxial cable.
This mixer has very limited capability to detect interfering AM radio stations, because channels of transistors are linear resistors. Theoretically this mixer should not detect AM signals at all. But in reality transistor channels have some nonlinearity, this factor limits the susceptibility to radio interference.
This mixer have very low noise levels, because FETs are low noise devices and the RF current, passes through the FET channel is very low. In practice, the noise of field-effect transistor is just a little bit higher than the noise of a resistor with the resistance equal to the average resistance of both FET channels.
The circuit diagram of a the direct-conversion receiver with FETs is shown on the Figure 3. The input signal of 28 MHz is fed to the mixer through the L-shaped filter L1C1, L2C2. The tapped coil L2 provides impedance matching for the antenna. A bias voltage from the wiper of the trimming potentiometer R1 is fed to the gates of V1, V2, the voltage should be adjusted to the point where transistors are opened only at peaks of the local oscillator voltage. The low-pass filter C3L3C4 is connected to the output of the mixer. The characteristic impedance of the filter is 4.5 kohms. The op amp based audio preamplifier provides a gain of 1000, its output should be connected to any main amplifier (for example, the main amplifier can be based on LM386 IC) with the gain of 30...100.
V1, V2 - KP303I (Ugs cutoff=0.5...2 V, IZero-Gate-Voltage Drain Current=1.5...5 mA)
D1 - 140UD1A - μA702;
It is better use an op amp with internal frequency-compensation circuit, just like 741;
R1 - 47k trimming potentiometer; R2 - 51k; R3 - 180 Ω; R4 470k;
C1 - 6...25 pF trimmer; C2 - 51 pF; C3, C4 - 10 nF; C5, C6 - 0.1 μF, C7 - 1μF; C8 - 3.3 nF;
T1 - a center-tapped HF phase splitting transformer; http://zpostbox.ru
L1 - 19 turns of 0.8 mm wire, wound on 6 mm former;
L2 - 10 turns of 0.8 mm wire, tapped at 2..3 turn, wound on 6 mm former with a ferrite slug;
L3 - 300 mH, 520 turns of 0.07...0.1 mm wire,
wound on a ferrite toroidal core 16x8x4 mm, μ/μ0=2000
Coils L1 and L2 of the input resonant circuit are wound on a former of 6 mm diameter with a copper wire with the diameter of 0.8 mm (AWG 20). L1 has 19 turns, L2 - 10 turns, it has a ferrite slug. The coil L2 is tapped at 2nd or 3rd turn (match it to get the maximum sensitivity of the receiver) from the ground lead side. The coil L3 has 520 turns of silk enameled copper wire of 0.07...0.1 mm in diameter (AWG 38...40 ), it wound on a ferrite toroidal core with dimensions of 16x8x4 mm, the relative permeability of the core μ/μ0=2000. The transformer T1 is wound on a ferrite toroidal core with dimensions of 7x4x2 mm, μ/μ0=100. It has 3x12 turns of 0.15 mm (AWG 34...35) silk enameled copper wire. Take 3 wires, wind them together, then connect the one end of the first wire to the opposite end of the second wire.
It was found that the optimal bias voltage is 2.5 Volts, the local oscillator voltage at gates of V1 and V2 is about 1.5 Volts. With this values, the receiver has maximum sensitivity of about 0.3 μV at the signal to noise ratio of 10 dB. The signals of interfering AM radio stations and the local oscillator signal at the input of the receiver are suppressed for more than 70 dB. This parameters can be improved by using MOSFETs.
This type of mixers can be used in superheterodyne circuits, if replace the R1C2 network (see Figure 1) with a tank circuit, tuned to an intermediate frequency.
V. Polyakov, B. Stepanov
"Radio", 1983, 4