Biomedical Engineering Reference
In-Depth Information
single-chip IF processor, takes care of detecting the signal and producing a logarithmic out-
put of signal strength.
In this portion of the circuit, the 45-MHz IF signal is coupled to the input of a RF mixer
internal to IC8 by way of a tuned circuit formed by C18, C19, and L3. The LO input of
this mixer is fed from a 44.5-MHz crystal-controlled oscillator. The resulting 455-kHz IF
is
filters, FLT1 and FLT2. An internal received signal strength indi-
cator (RSSI) circuit is used as a detector and linear-to-logarithmic converter. The RSSI
output, as a function of the sawtooth signal driving the tuner, is thus a logarithmic repre-
sentation of the spectrum of the signal picked up by the probe. RSSI is a current signal that
requires conversion to a voltage by the network formed by R21-R23 and D3. C24 low-
pass
fi
filtered by two ceramic
fi
er and
impedance transformer for the current-to-voltage converter. Finally, IC9A blanks the out-
put during retrace.
Figure 4.12 presents the power supply circuit for the adapter. Most of the circuitry, includ-
ing the up-converter, tuner, and sawtooth generator, is powered by
fi
filters the RSSI output to produce a smooth display, and IC2C acts as a bu
ff
12 V;
5 V powers the IF
processor. The
32 V to drive the tuner's varactors is obtained by multiplying the 12 V ac input
to
48 V, reaching the desired voltage through IC10, an LM317 adjustable linear regulator.
To operate the spectrum analyzer, the Y output of the adapter is connected to the verti-
cal input of the oscilloscope, and the TRIGGER output is connected to the trigger syn-
chronization input of the scope. The horizontal frequency of the oscilloscope is set such
that one full sweep caused by the sawtooth
fits the full graticule on the oscilloscope's
screen. Fine-tuning can be accomplished either by trimming the time base of the scope or
by adjusting the value of R18 appropriately. Alternatively, a two-channel oscilloscope can
be operated in the X-Y mode by injecting the sawtooth available at pin 7 of IC2A to the
appropriately scaled X-axis channel.
The comb generator circuit of Figure 4.13 can be used for calibrating the adapter. The
circuit is simply a TTL-compatible 40-MHz crystal-controlled oscillator module feeding a
synchronous binary counter. It is called a comb generator because the spectral pattern of
any of its outputs resembles an ordinary hair comb with its prongs pointing up. Because
these spectral components occur at harmonic multiples of the fundamental square-wave
frequency selected, it follows that the frequency di
fi
erence between consecutive “prongs”
must be the same as the value of the fundamental frequency of the square wave.
Figure 4.14 presents the pattern obtained when the 20-MHz comb output of the gener-
ator is probed by a commercial spectrum analyzer. Ac coupling was accomplished through
a series-connection 15-pF capacitor, and termination to ground of the output side of this
capacitor was performed through a 50-
ff
noninductive resistor. This is the gold standard
against which the adapter should be calibrated.
Start testing the adapter by setting the sawtooth generator to vary the voltage at the
VTUNE input of the tuner between approximately 1 and 31 V. Initially, set R6 to apply
2.5 V dc to the AGC pin of IC1. The up-converter LO frequency should be adjusted to
450 MHz by trimming R20. 9.6 V dc at the VTUNE input of IC4 will typically result in the
desired LO frequency. L4 should be trimmed to achieve stable oscillation of the 44.5-MHz
IF LO oscillator. With a 40-MHz comb applied to the input of the adapter through a 15-pF
coupling capacitor and with 50-
Ω
termination, adjust L3 to obtain an approximation of the
expected 40-MHz comb pattern on the oscilloscope. After achieving a satisfactory display
for the 40-MHz comb, calibrate the linearity of the adapter using a 20-MHz comb by
Ω
rst
trimming R3 to produce equal spacing between spectral lines throughout the lower third
of the display. Then linearize the midrange by trimming R2, and
fi
fi
finally, the high range by
trimming R1.
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