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transistor (BEC), which is different from a 2N3906 (EBC)]. Then adjust Tl for
maximum signal level.
The control pulses that turn on the switches in U lA-U lD are nonoverlapping
pulses, being a quarter cycle in duration at 0-, 90-, 180-, and 270-degree relative
phases. The sampling frequency of each of these nonoverlapping pulses is the
crystal oscillator's frequency divided by 4. The sample-and-hold signals for 0 and
180 degrees appear at capacitors C4 and CS. An "almost" differential amplifier
consisting of U3B and U2B amplifies roughly the difference in voltage across C4 and
CS.
The reason for saying that U3B and U2B almost form a differential amplifier is
because normally the (+) input of U38 would include a resistive divider circuit. But
since there is none, there is an error of about l/gain. Since the feedback resistors
in U6A (and U6B) result in a gain of about lOO, the error is about 1/100, or 1
percent. Similarily, the 90- and 270-degree sampled signals are amplified almost
differentially by a factor of 100 via amplifier U3A and U2A.
Taking the difference between the two signals that are sampled 180 degrees apart
in Figure 12-11B amounts to essentially the same as the circuit shown in Figure
12-8A, consisting of differential output amplifier (Ua733) U2 coupled to the two
input terminals of the single-pole double-throw switch U3A. The output signals in
Figure 12-11B of U3B and U3A then provide I and Q signals to the computer's
sound card.
It should be noted that because the gain bandwidth product of the op amps in U6
is 10 MHz, it is operating near the limit to provide a 100-kHz bandwidth for a
closed-loop gain of 100. The NE5532 is configured to provide a gain of lOO, so the
frequency response is 100 kHz, which means that there is very little negative
feedback at 100 kHz. Normally, this can be a problem because the gain and
frequency response of amplifiers are not tightly specified. That is, only a minimum
specification of the gain bandwidth product of the op amp is guaranteed. So, if U6,
a dual op amp, were replaced by two single op amps of similar specifications,
chances are that the I and Q channels would suffer a mismatch in frequency
response and ability to maintain a 90-degree phase difference over the range of
frequencies (e.g., 100 kHz). But because in a dual op amp the characteristics in the
die are matched (e.g., equally bad or equally good on both op amps in the
package), it is often possible to still have good matching on the I and Q channels.
Of course, higher-frequency dual op amps can be used in place of the NE5532.
In reference to Figure 12-11A, the crystal oscillator at about 28.6 MHz runs at four
times the mixing frequency but gets frequency divided by 2 via U1A to ensure a 50