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Detector/ AF pre-amp
Figure 130. Tayloe quadrature detector and I/Q audio preamplifier schematic
The detector follows the T/R switch above. The input is a doubled tuned bandpass filter used to
improve the out of band signal rejection.
The detector makes use of a very high speed four to one analog multiplexer. The input to the detector is
biased at one half supply voltage (1.5v) in order to place the incoming RF signal in the center of its input
3v range. The output of the detector is the across the four 0.82 uf detection caps. Assuming a 50 ohm
input impedance, the output impedance of each of these four outputs is 200 ohms. The four outputs are
phase shifted with respect to each other because each of the outputs was taken from four separate quarter
cycle samples of the input signal. Thus the outputs are phase shifted at 0, 90, 180, and 270 degrees.
These four different phased are combined by the post detector, audio pre-amplifier section into two
signals called I (in phase) and Q (quadrature). The 0 and 180 degree outputs are differentially summed
using one op-amp, while 90 and 270 are summed using the second op-amp. The differential
amplification helps reduce noise that might be common to all four outputs.
Some folks have described the detector as a sample and hold device. While this is true in that the
detected signal on the detector capacitor is held from one ¼ cycle sample to the next, the actual
Norcal NC2030 v5
12-18-05
Page 110 of 128
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