December 4, 1997

Current amp provides frequency modulation

M Pereira and A del Rio, University of Vigo, Spain

You can use current-feedback op amps in many VHF-band RF applications. Thanks to their 150-MHz bandwidth, current-feedback op amps can simplify your design task by replacing discrete components, such as bipolar transistors and FETs, in high-frequency circuits. The circuit in Figure 1 is a frequency modulator that generates a 55-MHz VHF signal. The circuit is basically a VCO that displays little amplitude modulation over a wide frequency range. The maximum amplitude variation over the range of 50 to 60 MHz is less than 0.3 dB. Also, the circuit's frequency-vs-voltage transfer function has two linear zones: approximately 0.7 MHz/V in the lower range and 0.4 MHz/V in the upper range. Table 1 shows the frequency-vs-voltage relationship. The design shows low sensitivity to power-supply variations or supply noise. The transfer-function changes are negligible for supply voltages from ±8 to ±12V.

The design is compact, comprising an eight-pin IC, a few resistors and capacitors, and no inductors. The circuit comprises two blocks. The first block is the VCO, which uses two BB105 varicaps (D₁ and D₂) with IC_1A. The control voltage to these diodes can vary from 2 to 20V. You can change the frequency range by varying R₃ and R₄. The output frequency increases when those resistors decrease in value. If the value of R₁₀ in the second-block gain stage decreases, the amplitude of the FM signal also decreases. The input modulating signal has a limited bandwidth because of the capacitors connected to the varicaps. The 1-nF capacitors in Figure 1 exhibit a low impedance to the carrier and a high impedance to the input signal.

With the values shown, the circuit can accommodate input signals with a 30-kHz bandwidth. You can thus modulate two or three audio channels. The FM signal has a bandwidth that varies with the amplitude and bandwidth of the input. The second harmonic in the output signal is lower than ­30 dB; the third harmonic, approximately ­30 dB. The third harmonic is somewhat higher than the second because of clipping from the high gain. You can reduce the harmonics by connecting a lowpass filter to the output.

The second circuit block isolates the output from the VCO. A low or reactive impedance connected directly to the VCO could prevent oscillation or cause the VCO to oscillate at undesired frequencies. The IC_1B buffer provides the isolation. The buffer provides 14 dBm of gain to the FM signal, with an output stage adapted for a load of 50 ohms. You could also use this circuit in frequency synthesizers, which comprise a PLL, phase comparators, and a VCO. Such synthesizers are especially useful in digital tuners. (DI #2122)

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