TX RX Freq

[baht7]

anyone know off the top of their heads the TX RX Freq for eti 1500

[ApBerg]

anyone know off the top of their heads the TX RX Freq for eti 1500

http://www.geotech1.com/pages/metdet...ti1500_300.pdf

A friend made them long ago to work on 17Khz , if I remember well......

Best regards.

Ap

[d_stanley]

nominally its 15khz but it depends on the inductance of the transmit coil you use. look up colpitts osciilator on wiki and you'll get a formula for the frequency. the transmit coil is part of the oscillator cct as the coil for this project is no longer available you have to use another there is issues here so if you have troublr specifically contact me and i may try and help you. i think the original coil was round about 5mh from memory but trust me it was designed to be 15khz and the cct defines the capacitors so using the wiki link for colpits you can reverse engineer the tranmit coil inductance

[mikebg]

anyone know off the top of their heads the TX RX Freq for eti 1500

Gary, not to worry for exact TX frequency.
More important is relative resonance frequency of RX tank network because it changes discrimination and elimination of groud signal (ground balance).

[d_stanley]

eti 1500

hi Gary I happen to be building one of these at the moment I'm up to the stage of having it working on the bench in no discrimination mode. really just as a learning exercise to understand newer designs better. i understand your reasoning about matching the resonant frequencies of the transmit and receive paths. i have even noted in these forums that the receive circuit is often detuned slightly. i assume this is to induce an extra phase shift on top of the phase shift from the target to accentuate the effect and make it easier to detect (or cancel depending on weather it is discriminating or a target).

i have a problem with this suggestion.

1. the eti1500 receive path seems to be quite wide band. not resonant at all. i modeled it with a circuit simulator and performed a frequency and phase response test. quite flat till 50 khz or more. indeed my eti 1500 performs equally well with a DD coil connected at about 18Khs or the coil from my Garrett ace 250 which when connected to the eti 1500 runs at about 8khz or so. seems to suggest the receive path is indeed wide band.

2. if the transmit frequency does not change then the receive frequency will not change so there can be no extra phase response? the frequency may of course change because of the change in transmit coil inductance due to a target but my understanding is that this will be very small indeed. if this was not the case we would all be using BFO detectors which rely on such an inductance change.

am i missing something in my understanding here?

just out of curiosity i did a bench test compared with my garrett ace 250.

so it was Garrett ace 250 detecting 10c (AUS) coin on the bench
verses
eti 1500, partially complete, in pieces on the bench, wires every where, in its 1980 design spendour, with the ace 250 coil connected.
really very similar results both detected the coin at 27 to 30cm (11 to 12 inches) in air. in all fairness though the eti 1500 was drifting all over the place but it WAS able to detect the coin a a similar distance to the garrett i'll redo the test once i have finished the eti1500 and have it in a box with all the pots connected etc.
looks to me like the magic happens in the coil.



I ought to declare i am an electrical engineer but i've been out of university 15 years now so its not too fresh.