of the reference signal. Either of these two inputs is selected by two jumpers. The polarity of the detected output voltage depends on these jumper selectors. As the expected output is of positive output voltage, we can change the jumper position, if we find negative output.
Fig.8. The schematic of the lock-in amplifier
The balanced modulator (AD630) actually multiplies the two input signals and the multiplied result is produced at the output. Only the two signals of exact frequency and phase will produce a DC output, others will cancel out. That is why lock-in detection is a good technique to detect only our desired signal, though very weak in nature.
As the output is a pulsating DC, a passive RC filter consisting of R7 and C9 is used to obtain stable DC, it also filters out any small and fast variation of the desired DC output. Though in the schematic only one value of C9 (10 uF) is given, later it was found that it is better to use several capacitors of different values (1 uF, 10 uF, and 100 uF), one of which is to be selected by a rotary switch.
When the distance of the object goes longer than 100 meters, signal gets weaker and higher value for C9 gives better result. However, large value for C9, makes the response time slower. As the scattered light produces very little signal, the resulting demodulated DC output is also very low, a DC amplifier consisting of IC3A and IC3B lifts this to usable level up to several hundred millivolts to few volts.
Of course, the DC output depends on the distance of the object as well. The further the object the less is the output DC level. This output DC can be seen on a DC voltmeter or on an oscilloscope. The oscilloscope-waveforms of the signal input, reference input, demodulated output and amplified DC output is shown in Fig. 10.
Fig.9. The implemented PCB of the lock-in amplifier and the associated input-output terminals and control knobs.
Fig.10. Signal waveforms at various test points of the lock-in amplifier
To detect the presence and absence of the distant object, it is better to have some sharp indication (ON or OFF) as well along with the DC level measurements. Or in many occasions the sharp detection will suffice. For this purpose a comparator (IC4A) is used at the output of the DC amplifier stage. We can set a DC reference level by a pot in the front panel. Upon receiving a small scattered light by the photo-transistor and going through the all the processes, a DC voltage proportional to the received light is applied at the non-inverting terminal of the comparator.
If the front-panel pot (R11) is now adjusted to little lower than this voltage, the comparator output will be HIGH. If the distant object is removed, the DC signal drops and the comparator output will go LOW. The comparator output can also be used to turn ON an alarm by employing an electro-mechanical or solid state relay.
The photograph of the entire distant