A frequency mixer with antiparallel diodes1 provides high sensitivity and interference insusceptibility for direct-conversion receivers, the mixer has very low level of local oscillator signal at the antenna terminal (the local oscillator radiation is very low). But this mixer also has a drawback - the voltage of local oscillator signal should be adjusted to achieve the highest conversion gain. It takes that diodes should be "ON" (in the conduction state) only at peaks of local oscillator voltage Uosc (see Fig. 1.), and the duty cycle T1/T of current pulses Id must be about 0.5. If use silicon diodes with the cutoff voltage Ucutoff for the mixer, the local oscillator voltage should be about 0.6...0.75 Volts. If the voltage is less, diodes are closed almost all the time. If the voltage is higher, diodes are opened almost all the time. In both cases the conversion gain is decreasing.
The described above drawback can be eliminated by adding a network of the automatic bias control, it will change the cutoff voltage of diodes if the local oscillator voltage changes, so it will keep the duty cycle of current pulses through diodes at constant ratio. The modified circuit is shown on the Fig. 2. There are two additional antiparallel diodes, and the network of the automatic bias control R1C1 connected to arms of the bridge circuit (the bridge circuit is formed by diodes V1-V4). All this makes the mixer much more symmetrical. The R1C1 time constant should be higher then the lowest audible frequency at the output, or else the bias voltage will be "modulated" by the output signal.
C1 - 0.25 uF; R1 - 5.1k
fsignal = 2*fosc
At the positive half-cycle of the local oscillator signal the current pulse passes through diodes V1 and V4, and at the negative half-cycle the signal goes through diodes V2 and V3. In both cases these pulses develop the bios voltage across components R1, C1, the voltage is proportional to the local oscillator voltage.
The mixer described above can be modified (see Fig. 3), if connect the signal source to the tap of the coil L2, and connect the load to the middle node of the automatic bias control network. This modification decreases the couple between the local oscillator network and the signal network, because they are connected to the different arms of the balanced bridge.
V1...V4 - D9B (germanium USSR diodes);
C1 - 8..30 pF; C2 - 270 pF; C3 - 1 nF;
C4, C5 - 0.25 uF; C6, C7 - 0.05 uF;
R1 - 5.1 k; L4 - 0.1 H
fL1C2 = 2*fL3C3
The radio signal from the middle tap of coil L1 (the resonant tank L1C2 is tuned to the signal frequency) is fed to the middle tap of the coupling coil L2. The coil L3 is a coil of the local oscillator, its frequency is twice lower than the input signal frequency. If the local oscillator uses in the output a buffer stage, the coils L2 and L3 can be replaced with the toroidal core transformer. Diodes V1-V4 forms a bridge rectifier for the local oscillator signal, the bios voltage develops across the R1C4C5 network. The output audio signal develops at the common node of C4, C5, then it feeds to the low-pass filter L4C6C7 with the cutoff frequency of 3 kHz, then the audio signal goes to the audio amplifier. The amplifier can be connected directly to the output of the mixer without a DC-blocking capacitor, because capacitors C4, C5 block DC currents.
Both mixer circuits (see Fig. 2 and Fig. 3) were used in the direct-conversion receiver for the 80 meters band2. It turns out that both silicon and germanium diodes can be used for the mixer with automatic biasing, the results are similar. The next diodes can be used, from worst to best: D18, D20, D101-D105, D219-D223, D2, D9, D311, KD503, KDS523, KD514.
The measurements showed that the modified mixer has the same conversion gain (the sensitivity of the direct-conversion receiver is also the same - 1.5 μV). The sensitivity remains almost the same while the local oscillator signal amplitude reduces from 1 to 4..5 Volts (the signal was measured across the coil L2). The local oscillator signal with the frequency of 1.75 MHz was attenuated in 54 dB. The additional attenuation provides by the resonant tank L1C2 (it is tuned to the frequency of 3.5 MHz). Interfering AM signals were reduced by more than 80 dB: an AM signal with the modulation index of 0.3 and with the frequency detuning ±50 kHz provided the same voltage at the input of the receiver as a signal with the amplitude of 7 μV.
"Radio", 3, 1979