Biomedical Engineering Reference
In-Depth Information
Figure 4.9 A simple circuit can be used to convert any triggered oscilloscope into a 100-kHz to 400-MHz spectrum analyzer suitable for
near-field EMI sning.
Varactor-controlled TV tuners receive signals on their RF input at a frequency determined
by the voltage applied to the VTUNE input. With power applied to the UHF section of
a tuner, typical control voltages between 0 and 32 V span a frequency range of approxi-
mately 450 to 850 MHz. The sensitivity of the tuner can be adjusted through the AGC
input. The output of the tuner is a standard 45-MHz IF. However, 450 to 850 MHz is not a
range that is directly applicable to the large bulk of EMI sni
ng work. For this reason, the
more appropriate range 100 kHz to 400 MHz is up-converted to the tuner's input range
through a circuit formed by IC4-IC7 (manufactured by Mini-Circuits). Here, signals from
the probe are low-pass
filtered by IC6 and injected into the IF port of a TUF-2 mixer. The
LO input of the mixer is fed with the output of IC4, a self-contained voltage-controlled
oscillator tuned to 450 MHz by potentiometer R20. The RF port of the mixer outputs sig-
nals with frequency components at the sum and di
fi
ff
erence between the IF input and the
LO frequency. This output is high-pass
fi
filtered by IC5 to ensure that only up-converted
components are fed to the tuner input.
Sweeping the tuner across its range is accomplished by a sawtooth waveform that
spans approximately 1 to 31 V. The basic sawtooth is generated by IC3 and Q1 and
bu
ered by IC2B. The span of the sawtooth is set by attenuator R10, while the center of
the sweep is adjusted by introducing an of
ff
ff
set on IC2A by means of R13. The output of
IC2A is ampli
fi
ed by transistor Q2. Q2 should be selected for a gain of 50 or less. The
fi
final span and linearity of the sweep is adjusted in three ranges by R1, R2, and R3.
The IF output of the tuner is attenuated to a level suitable for processing by R7, R8, and
R9. The actual value of the resistors for this attenuator must be selected based on the output
level of the speci
fi
c tuner that you use. Then, in the circuit of Figure 4.11, IC8, a NE/SA605
Search WWH ::




Custom Search