Tweet
Health to everyone! I read and experimented a lot, but in practice I did not notice much difference in the operation of the metal detector if the frequency of the receiver and transmitter differ from 200 Hz to 1.5 kHz. with a smaller difference in frequencies, the sensitivity of the device is greater, but does not particularly affect the stability of operation. Can anyone explain the point with facts? Thank you.
-
-
Rx coil has a steep phase gradient at resonance, so the phase response would jump allover the place if you tune it on Tx frequency. It would give you better sensitivity, but discrimination would be very bad. Because most of the Rx circuitry expects less phase shift than you get at resonance, most probably your Rx would have big issues ground balancing as well.
So in short, in classic VLF frequency spacing is a compromise between sensitivity and stability. -
What do you mean by adjusting the RX frequency? (I'm asking because I don't know)Originally posted by Davor View Post
Whatever the frequency coming from TX is the frequency that RX will detect.
For example;
You have a system that you will run at 18kHz, how do you calculate the 28cm dd rx and tx coil (wire thickness, number of turns, resonance capacity)
Note: We don't put resonance capacity in RX anyway, what kind of adjustment will it be?Comment
-
Yes, a detector that transmits 18kHz will also receive 18kHz. But if you resonate the RX coil, you should not resonate it at 18kHz as the phase stability will be poor. Most detectors resonate the RX coil somewhat above or below the TX frequency for good phase linearity. If you don't resonate the RX coil, then this is not a concern.Comment
-
I said "Rx coil ... if you tune it on Tx frequency."Originally posted by Daves View Post
So it is tuning a Rx tank (coil+capacitor) with regards to Tx that needs that offset for stability.
There are always some historical reasons why something was done in a particular way. The reason behind this near-resonant solution was to improve S/N in times when OpAmps were very noisy.Comment
-
Hi, when you say "near-resonance" how much offset are we talking about? some 1% or 10% off, more or less?
Thanks for the infoComment
-
The offset is not defined in relative % (1-3-5-10-15..). And in accordance with the md scheme, phase-frequency and phase-amplitude characteristics of contours, etc. sorry for the google translationOriginally posted by Pep_T View Post
Comment
-
+/- 2kHz will do.
Comment
-
This topic interests me, too, and I would like to learn more.
The frequency difference obviously depends on the ACTUAL TX frequency, so any sensible reply to "How much different?" needs to be expressed as a Percentage ( like +/- 4% ) , or as a ratio ( like 0.95 / 1.05 ) . Any replies like "1kHz different" are meaningless - unless actual operating freq is also stated.
And it must also depend on the damping of the RX circuit ... is it highly tuned or not so ? A good coil could have a high Q-factor ( like 20 ) , but that could easily be lowered by adding a damping resistor across it, on the pre-amp input. A lower Q resonant circuit will have a flatter resonance curve, and more stable phase behaviour. As an example, a 13 kHz Fisher F75 coil ( 7mH, 40 Ohms ) has XL = 570 Ohms, so Q = 14. But the RX input has ( I think ) a 4K99 resistor across the coil; this drops Q to about 5.5 , this must be intentionally chosen by the designers.
Comment
-
Mr Artyom has told us nothing about his detector, so we can make few useful replies. If we know the operating freq, and the RX coil specification, and the pre-amp / input circuit, then we may have a chance. Maybe he saw little performance difference when he changed frequency spacing because his detector has a low Q-factor?Comment
Comment