Hi John,
Here is a copy of a post I made a couple of years ago which deals with this subject.
RELEVANCE OF PULSE FREQUENCY in a P.I. DETECTOR.
A question that is often asked about P.I. detectors is the relevance of the pulse frequency. Some detectors run at a hundred or so pulses per second while others run at several thousand. The first archaeological detectors I worked on ran at 22 p.p.s. In later versions we upped it to 43, then 170. The important thing was to avoid the same frequency as power lines, 50Hz in the UK, 60Hz in the US. Multiples of the power frequencies also had to be avoided e.g. 100 or 120. Severe beat note interference would result if the detector and power frequencies coincided; that is if you were anywhere near power lines as is often the case in the UK.
P.I. detectors running at low frequencies usually pulsed high currents through the coil, 5 - 10 amps being a typical figure. The on / off time ratio of the pulses was high so the average current drawn from the battery would be 100 - 200 milliamps.
Many of todays detectors favour much higher pulse frequencies. This puts the detector well away from power line interference but low frequency radio transmitters can then start to cause problems. Faster pulse rates also have the advantage in that the response of the detector can be made more rapid, which aids in pinpointing an object. Fast pulse rates require that the pulse current be less, otherwise things would start to run hot and the battery life would be rated in minutes. The pulse current can be restricted by a lower transmitter voltage but more usually by a resistor in series with the coil or by making the coil more resistive by using finer wire. 0.1 - 1 amp pulse currents being typical. One major factor in determining pulse current is acceptable battery life. With rechargeables 5 - 10 hours is OK, while standard alkaline cells would be expected to give 15 - 25 hours for the cost to be acceptable. One or two recent detectors on the market appear to have energy recovery circuits which make the transmitter more efficient and significantly increase battery life.
The pulse frequency has little effect on detector performance; one running at 170 p.p.s. could have the same range as one running at 5000 p.p.s. but there are many inter-related factors that determine the final performance, not least the design of the receiver circuit.
Hope this helps. I favour higher pulse rates (1500 - 5000) in my current designs.
Eric.
