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176
Dual Reference Detection
The time constant is adjusted to remove the high frequency sum components at 2ω
the analog output of this first lock-in is the signal input to the second lock-in. The second
lock-in is set to detect at ω
The second lock-in time constant is adjusted to remove the 2ω
signal proportional to A
SR865A Dual Reference Mode
The SR865A Dual Reference mode makes this measurement in a single lock-in. One of
the frequencies is the external reference and the other is the internal reference. In Dual
Reference mode, the SR865A detects at |ω
doesn't matter which is the carrier and which is the modulation. It also doesn't matter
which frequency is greater or by how much. The detection frequency is the difference
between the two frequencies and will always be lower than the greater frequency.
The original experimental signal contains components at the sum and difference
frequencies
In Dual Reference mode, the SR865A multiplies by cos({ω
resulting in the output
The SR865A time constant is adjusted to remove all components at 2|ω
and 2ω
lock-in reports the measured amplitude at (ω
for rms, the displayed result will be (A
The Dual Reference mode in the SR865A does not care about the "order" of ω
The expermental signal could be a fast modulation of a slow carrier or vice versa. The
SR865A does not require that one of the frequencies be much greater than the other. The
two frequencies can be very close together. The only requirement is that the time constant
remove all output components other than dc.
The SR865A does require that one of the frequencies be the internal reference and the
other the external reference and the difference between them must be less than 4 MHz.
SR865A DSP Lock-in Amplifier
A
A
>
car
mod
2
sin(
4
mod
A
A
Z
car
mod
sin(
mod
2
A
car
mod
A
A
>
car
mod
cos(
2
A
A
>
car
mod
1
cos(
4
leaving a dc signal proportional to A
mod
^
`
Z
Z
Z
t
)
car
mod
car
. The output of this second lock-in is
A
Z
u
t
)
sin(
t
)
mod
as desired.
− ω
|, i.e. the difference frequency. It
ext
int
^
`
Z
Z
t
)
cos(
car
mod
^
`
Z
Z
2
t
)
cos(
car
mod
A
car
mod
− ω
car
mod
)/(2√2).
A
car
mod
A
A
@
Z
car
mod
sin(
mod
2
A
>
Z
car
mod
1
cos(
2
mod
4
component leaving a dc
mod
@
^
`
Z
Z
t
)
car
mod
− ω
}t) in a single step
car
mod
Z
Z
2
t
)
cos(
2
car
mod
car
as desired. It should be noted the
), which is (A
A
car
mod
Appendix E
t
)
and
car
@
t
)
@
t
)
− ω
|, 2ω
mod
car
)/2. Correcting
and ω
.
ext
int
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