### Converting a signal into a bit stream simplifies the correlation function calculation.

This innovation is capable of correlating two analog signals by using an analog- based signal conditioning front end to hard-limit the analog signals through adaptive thresholding into a binary bit stream, then performing the correlation using a Hamming “similarity” calculator function embedded in a one-bit digital correlator (OBDC). By converting the analog signal into a bit stream, the calculation of the correlation function is simplified, and less hardware resources are needed. This binary representation allows the hardware to move from a DSP where instructions are performed serially, into digital logic where calculations can be performed in parallel, greatly speeding up calculations.

Each of two analog signals (channels
A and B) is converted to a digital bit
stream by phase correcting it and comparing
it to an average of itself at a sampling
clock rate *f*. The hard-limited conversions
of A and B are bitwise compared
to measure the level of similarity
between the two by the OBDC. This similarity
measurement *X* is equal to the
maximum possible Hamming distance
(*N* bits in disagreement) minus the
measured number of bits in disagreement.

The OBDC functions are embedded
into a field programmable gate array
(FPGA). The OBDC is made up of two
shift registers containing the current
sample values (of length *N*) from each of the two input channels (A and B).
During each sample clock, a new sample
from each A and B input is clocked into
the input linear shift register for each
respective channel; this input shifts the
current values in the linear shift register.
The oldest (*N* + 1 sample clocks ago)
sample is clocked out of the register.
Once the inputs have been clocked in,
the correlation routine can start. This rising
edge of the sample clock also clears
the max correlation value, the best correlation
index, and the shift counter registers,
initializing the correlator.

When the two registers match exactly,
or are correlated, the *X* value will equal
*N*. Once the correlation value has been
calculated, this result is forwarded to
compare with the max correlation value
register. If the *X* value is greater than the
current max correlation value, then the
max correlation value becomes *X*, and
the shift counter register is latched and
put into the best correlation index register,
providing the index of the current
best correlation.

This index is the number of sample
clock periods difference between the
two input signals and thus, for sample
clock rate *f*, indicates the delay between
the signals A and B.

*This work was done by Norman Prokop
and Michael Krasowski of Glenn Research
Center.
*

*
Inquiries concerning rights for the commercial
use of this invention should be addressed
to NASA Glenn Research Center, Innovative
Partnerships Office, Attn: Steven Fedor, Mail
Stop 4–8, 21000 Brookpark Road,
Cleveland, Ohio 44135. LEW-18902-1*