Editing Bat detector (section)

===Heterodyne===
[[File:FreqMixing.svg|thumb|Illustration of heterodyne mixing. An incoming down chirp is combined with a constant 50 kHz frequency signal (LO, Fig. A). Fig. B shows the resulting signal with low (difference) and high (sum) frequency components. Figs. C:/D: depicts the resp. magnitudes in the frequency domain.]] 
[[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 downward 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 played 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&nbsp;kHz and an internal frequency of 43&nbsp;kHz produces output frequencies of 2&nbsp;kHz and 88&nbsp;kHz. The 88&nbsp;kHz frequency is inaudible and is filtered out and the 2&nbsp;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&nbsp;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.

It is also possible to use a 'comb spectrum' generator as the local oscillator so that the detector is effectively tuned to many frequencies, 10&nbsp;kHz apart, simultaneously.

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.

====How it is used====
The operator guesses the likely species to be present and tunes the frequency accordingly. Many users will start listening around 45&nbsp;kHz. If a bat is seen or a bat-like call is heard, the frequency is tuned up and down until the clearest sound is heard.

Species like Pipistrelles which end their call with a "hockey stick" CF component can be recognised according to the lowest frequency which gives the clearest "plop" sound. Horseshoe bats give a peeping sound at a frequency depending on their species. FM calls all tend to sound like clicks, but the start and end frequencies and the call repetition pattern can give clues as to the species.

====Pros and cons====
The advantages of a heterodyne bat detector is that it works in real time, exaggerates the frequency changes of a bat call, is easy to use, and is the least expensive. It is easy to recognise a [[Doppler effect|doppler shift]] in CF calls of flying bats due to their speed of flight. Stereo listening and recording is possible with models such as the CSE stereo heterodyne detector, and this can help to track bats when visibility is poor.

The disadvantages of a heterodyne bat detector are that it can only convert a narrow band of frequencies, typically 5&nbsp;kHz, and has to be continually retuned, and can easily miss species out of its current tuned range.