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Measurement and Control of RF
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2009-09-23
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Measurement and Control of RF Power (Part II):Log Amps
The term log amp, as it is generally understood, refers to a device which calculates the
logarithm of an input signal’s envelope. In the response of an AD8307, a 500-MHz 90-dB
log amp, to a 10-MHz sine wave modulated by a 100-kHz triangular wave (see Fig. 8)
note that the input signal on the scope photo consists of many cycles of the 10-MHz
signal, compressed together using the time/div knob of the oscilloscope. We do this to
show the envelope of the signal with the much slower repetition frequency of 100 kHz.
As the envelope of the signal increases linearly, we can see the characteristic, log(x) form
in the output response. In contrast, if our measurement device was a linear envelope
detector (e.g. a diode detector) the output would still be a tri-wave.
Fig. 8:
The term log amp, as it is generally understood, refers to a device which calculates the
logarithm of an input signal’s envelope. In the response of an AD8307, a 500-MHz 90-dB
log amp, to a 10-MHz sine wave modulated by a 100-kHz triangular wave (see Fig. 8)
note that the input signal on the scope photo consists of many cycles of the 10-MHz
signal, compressed together using the time/div knob of the oscilloscope. We do this to
show the envelope of the signal with the much slower repetition frequency of 100 kHz.
As the envelope of the signal increases linearly, we can see the characteristic, log(x) form
in the output response. In contrast, if our measurement device was a linear envelope
detector (e.g. a diode detector) the output would still be a tri-wave.
Fig. 8:
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