Frequency multipliers typically work with square waves. However, the circuit in Figure 1 performs frequency multiplication on triangle waveforms and maintains the input's amplitude and uniformity.

Figure 1. |
This circuit performs frequency multiplication on triangle waveforms. |

The general idea is to apply the triangle waveform to any full-wave rectifier. The output is then a triangle wave with twice the input frequency plus some dc bias. You then can remove the dc bias using a simple highpass filter or by shifting the bias level with another op amp. You can continually repeat this trick to obtain a frequency series of 2×F_{IN}, 4×F_{IN}, 8×F_{IN}, and so on.

The actual circuit in Figure 1 uses a single-supply, dual op amp to help perform the rectification. When the input is the negative half-wave signal, IC_{1A} works as a regular inverting amplifier which R_{1} and R_{3} set to a gain of 2. The circuit then produces the algebraic sum, with a coefficient of 0.5, of the positive output signal of IC_{1A} and the negative input signal at summing point A. After IC_{1B} amplifies the signal by 4, according to the values of R_{5} and R_{6}, the output signal has the same peak-to-peak amplitude as the input, but with a positive sign.

When the input goes positive, the only path to summing point A is through R_{2}. In this case, IC_{1A} saturates, and its output sits close to ground potential. Good rail-to-rail op amps can swing within a few millivolts of ground. Thus, the signal at A remains positive, but divided by 2 due to the voltage divider of R_{2} and R_{4}. IC_{1B} restores the original amplitude of the signal. Thus, the output is a triangle wave equal in peak-to-peak amplitude to but twice the frequency of the input.

D_{1} and R_{7} are optional; they improve the dynamic performance of the circuit by preventing the input stage overload and the impact of IC_{1A}’s input capacitance when the input signal goes positive. If the input frequency doesn't exceed 1 kHz, you can omit D_{1} and R_{7}. For higher frequencies, keep these components. Moreover, you can proportionally reduce the values of all resistors. Also, you can reduce the value of R_{7} to 10 or 20% of R_{1}, but this change may cause dc bias of the output and adversely affect the circuit's accuracy.

The component values and op amp in Figure 1 make this circuit applicable to low-frequency-range applications, such as a frequency (octave) synthesizer for electro-musical instruments. For applications having frequencies higher than 20 kHz, you must choose a faster op amp and, possibly, a different rectifier topology.

In Figure 1’s full-wave rectifier topology, the input impedance differs for positive and negative waveforms. To reduce this difference, you can connect a constant resistor between the input pin and ground. When coupling numerous multiplying stages, the simple RC, highpass filter also helps minimize the impedance asymmetry. For this filter, choose as small a resistor as necessary to reduce the impedance asymmetry between positive- and negative-going signals to a reasonable level and then choose the capacitor for the filter frequency you choose.