Balanced modulator with varicap

Balanced modulators and ring modulators, based on diodes, are used to produce amplitude modulation with suppressed carrier. They are operated on relatively low frequencies, but at frequencies above 10 MHz this modulators cannot be precisely balanced, therefore a carrier can't be completely suppressed. It occurs because it is quite hard to match diodes with identical parameters, and capacitance of diodes affected much more at high frequencies.

T-shaped bridge circuit

Fig. 1. Balanced modulator

The balanced modulator described here (authorís certificate 627560, bulletin 34, 05.10.1978), is quite free of this disadvantage. It based on T-shaped bridge circuit (See Figure 1). The T-shaped bridge includes a symmetrical RF transformer T1 and two impedances Z1 and Z2. This impedances can be either active (resistance) or reactive (inductive or capacitive). The coefficient of transmission of the T-shaped bridge is zero when Z1 = Z2. This coefficient is a ratio of output voltage Uout to the voltage of the carrier signal G1. If we increase the impedance Z2, then some signal will appear at the output of the bridge. The phase of this signal is the same as the phase of the carrier G1, because the current through the horizontal arm Z1 of the bridge is higher than through the vertical arm. If we decrease the impedance Z2, then the current flowing through the left part of the transformer T1 and the vertical arm Z2 will prevail. In this case the output signal will have the opposite phase respect to the carrier signal G1. Therefore, by changing the one of impedances in one of the bridge arms synchronously with an audio signal, we can get a double sideband (DSB) signal.

The circuit diagram of the balanced modulator for 28 MHz is shown in Figure 2. The capacitive impedance of the capacitor C1 is the impedance Z1, and the capacitive impedance of the varicap V1 is the impedance Z2. The bias voltage Ubias is fed to the varicap V1 from the wiper of the trimming potentiometer R2, it used to balance the modulator. If a source of bios voltage has opposite polarity, then change the polarity of the varicap V1. The capacitance of the C1 must be four times less than the capacitance of the varicap V1 at the current bias voltage. When an audio signal is fed to the varicap, it capacitance changes and T-shaped bridge imbalances, that provides AM with suppressed carrier.

Balanced modulator with varicap for 20 MHz

Fig. 2. Balanced modulator circuit for 28 MHz
C1 - 4..15 pF; C2 - 100 pF; C3 - 8..30 pF; C4 - 24 pF; C5 - 33 nF;
L1 - 2 mH, tapped at the center; R1 - 100K; R2 - 33K;
V1 - D901G (Cnom = 22..32 pF, Kc = 2.7..3.3 at Urev = 4..45 V);
G1 - an RF source; G2 - an audio source;
T1 - see description in text.

The carrier (G1) and audio (G2) signals are fed to the modulator. This signals G1 and G2 can be connected either in series or in parallel. It will provide very high input impedance (tens of megohms) for audio signals. It allows to connect the modulator to any high impedance audio source G2, for example, an RC phase shifter. The audio signal G2 also can be fed to the upper terminal of the capacitor C5, but in this case its value must be decreased to 1..3 nF, to prevent the higher audio frequencies from being filtered out. In this case the input impedance of the modulator is equal to the resistor R1 value. The wiper of the trimming potentiometer R2 must be grounded through a capacitor of approximately 0.1...10 μF. The input impedance of the balanced modulator for RF signal G1 is much lower, the impedance is capacitive and its value is about 200 Ω.

The capacitor C2 prevents the audio signal from going to the output of the modulator. The resonant tank L1C3C4 (the low-pass PI filter) is used to match the output impedance of the modulator with the impedance of a load. With the given value of components (see figure 2), the balanced modulator can be matched with a high input impedance load (just like an amplifier stage based on a vacuum tube or a FET transistor). For a low impedance load, the value of the capacitor C4 must be higher, match this capacitance to get the maximum power of the DSB signal across the load. The low-pass PI filter efficiently filtering out overtones of the carrier frequency (2f, 3f, 4f and so on). Adjusting this low-pass PI filter, it is possible to get a good linearity of the balanced modulator.

If the balanced modulator is loaded with an active load, there is some harmonic distortion on the output. The amplitude of the output signal at the negative half-wave of the modulating voltage is just a little bit higher than at the positive half-wave. It occurs because the capacitance of the varicap is higher at the negative half-wave of the modulating voltage, and it decreases the internal capacitive impedance of the modulator. All this means that there is a second overtone in the output signal. The harmonic distortion grows with the modulation index (see the curve 1 in the Figure 3). The corresponding oscillogram of the output signal is shown in Figure 4, A.

The plot of harmonic distortion of the balanced modulator circuit

Fig. 3.

The harmonic distortion, described above, can be almost completely eliminated if the low-pass PI filter is tuned just a little bit above the carrier frequency. If tune the low-pass PI filter further up the frequency, the harmonic distortion will grow. Therefore, it is possible to get very low harmonic distortion by adjusting the trimming capacitor C3 (see the curve 2 in the Figure 3 and the oscillogram in Figure 4, B). If the low-pass PI filter L1C3C4 is tuned correctly and the modulation index m agrees with the curve 2 in Figure 3, it provides the total harmonic distortion (THD) better than 2..3%. Adjusting the low-pass PI filter doesn't affect the balance of the modulator.

Oscillograms of the output signal of the balanced modulator

Fig. 4.

Any varicap with a nominal capacitance of no less than 30 pF can be used in this circuit. The RF transformer T1 is wound on the ferrite core M100NN (μ = 100) with the dimension of 8x4x2 mm. It has 20 turns of enameled copper wire of 0.25 mm (AWG = 30), tapped at the center. Any core with μ = 30..400 can be used here. Wind the transformer using two wires, take about 40 cm of copper wire, bend it in half and wind the transformer with it. After that, cut the double end and connect the beginning of one wire to the end of other. This is the tap.

The coil L1 has 20 turns of the same wire, it wound on a former of 6 mm diameter.

The adjustment of the balanced modulator is easy. Set the voltage at the wiper of the R2 about 6V, balance the modulator with the trimming capacitor C1 to get the minimum signal at the output. This is the coarse balance adjustment. Now use the trimming potentiometer R2 to make the fine adjustment. Then, feed an audio signal to the modulator, use an oscillograph to watch the signal across the capacitor C4 (see Figure 4), and adjust the low-pass PI filter to get maximum amplitude and minimum distortion of the output DSB signal. By the way, if there is no oscillograph, a radio receiver can be useful. Tune it up on the operating frequency of the modulator, adjust C1 and R2 to suppress the carrier, and adjust C3 to get a loud signal with minimum distortion.

This balanced modulator circuit has been tested at the carrier frequency of 28 MHz. The amplitude of the carrier signal was 1 V, and the audio signal has 4 V. It gave the DSB signal with the amplitude of 0.35 V with the carrier suppressed at least 30 dB.

By the way, this balanced modulator can also produce an amplitude modulated (AM) signal. To get this, just unbalance the circuit using the trimming capacitor C1. In this case, the AM signal with 100% (and with very low distortion) modulation index can be achieved.

V. Polyakov, "Radio", September 1981

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