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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
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