Multifrequency TX circuits for TR det.

[Anonymous]

Many manufacturers offer multifreq. det.(Fischer etc.) How do they solve the problem of transmitting few freq., is it multicoil design?

 

[Anonymous]

Andrew,
Most multi-frequency detectors transmit square waves. For instance, a 5KHz square wave has a strong harmonic at 15KHz. Some of the Fisher detectors use 5KHz square waves.
The transmit and receive coil in the Fisher is balanced but it is untuned. (Answer to your question)
After amplification the receive signal is seperated into two channels. The 5KHz channel and the 15KHz channel both go to a pair of synchronous demodulators.
The demoduators provide DC output signals for both the resistive or "R" signal component and the reactive or "X" component of the signal for both frequencies.
Different targets as well as mineral ground signals provide different ratios of "R" and "X" for each frequency. This provides a very good discrimination ability.
Some multi-frequency detectors use Several square wave generators running at different frequencies or one square wave generator with a switchable frequency to generate a complex transmit signal.
Complex multi-frequency signals can also be generated using digital methods. As an example, a read only memory (ROM) can store a complex multi-frequency waveform. A Digital to analog (D-A) converter converts the stored signal in the ROM to an analog signal. A power amplifier amplifies the signal and applies it to the transmit coil.
The receiver fiters the signal into a pair of sychronous demodulators "R" and "X" for each frequency. I hope this helps, Dave. * * *

 

[Anonymous]

Thanks DAVE! As i know in clasic TX circuit (for example Fischer 1235, 1265) TX coil is driven with sq. wave, but in paralel with coil there is a capacitor. When coil is tuned well TX current is
near sinusoidal.
As you suggest - wnen coil is untuned we have got harmonics in TX current. Let say base freq. 5kHz and strong 15kHz (3)harminic freq.. So you need two independent channels- one for 5kHz and the other for 15kHz. Each channel includes bandpass filter (for separating freq.), next we need two
phase refference circuits and two demodulators and finally 4 bandpass filters X5, X15: R5, R15. am i right DAVE or wrong plese tell me what do you think about it?

 

[Anonymous]

Andrew,
your right on target. The receiver uses two filters to seperate the 5Khz and 15KHz. Each frequency has an in phase (with the transmit signal) and a quadrature detector to provide a DC signal for both the resistive and the reactive components. Motion is not required for multi-frequency operation but motion filters are used on most detectors so as to make them easier to build and operate.
You can read about the Fisher method by going to the US Patent Office (See atached URL) and looking up US patent number 4,868,504. Fisher use 5KHz and 15KHz in an even simpler way. They don't seem to worry too much about using filters! You will need a TIF viewer to see the illustrations. All the best, Dave. * * *