Detectors 053
Detectors 053 : Particle Detectors: (4.2) Bat Detector: (2) Bat Detector Types: (2. 1) Heterodyne: Heterodyne detectors are the most commonly used, and most self-build detectors are of this type. A heterodyne function is often also built into the other types of detector. A heterodyne bat detector simply shifts all the ultrasound frequencies downwards by a fixed amount so we can hear them. A "heterodyne" is a beat frequency such as can be heard when two close musical notes are sounded together. A heterodyne bat detector combines the bat call with a constant internal frequency so that sum and difference frequencies are generated. For instance a bat call at 45 kHz and an internal frequency of 43 kHz produces output frequencies of 2 kHz and 88 kHz. The 88 kHz frequency is inaudible and is filtered out and the 2 kHz frequency is fed to a loudspeaker or headphones. The internal frequency is displayed on a dial or on a display. A better quality version of a heterodyne, or direct conversion, bat detector is the super-heterodyne detector. In this case the bat signal is mixed with a high frequency oscillator, typically around 450-600 kHz. The difference frequency is then amplified and filtered in an 'intermediate frequency' or i. f. amplifier before being converted back to audible frequencies again. This design, which is based on standard radio design, gives improved frequency discrimination and avoids problems with interference from the local oscillator. In more recent DSP-based detectors, the heterodyne conversion can be done entirely digitally. The tuning issue can be addressed by using a scanning circuit to enable the detector to scan the spectrum automatically and stop scanning when a bat call is heard. One example of such a detector is theBat Scanner. It is also possible to use a 'comb spectrum' generator as the local oscillator so that the detector is effectively tuned to lots of frequencies, 10 kHz apart, all at once. Some early bat detectors used ex Navy, low frequency radio sets, simply replacing the aerial with a microphone and pre-amplifier. It is also possible to modify a portable Long Wave radio to be a bat detector by adjusting the tuning frequencies and replacing the ferrite rod aerial with a microphone and pre-amplifier
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