Understanding Frequency on a machine, well Kilohertz or KHZ on VLF machines is essential to know when you buying a detector, the gold machines are generally 19 KHZ and higher, where as coin machines for Fisher at least go down to 5 KHZ. So why the difference? summary answer the lower the KHZ the deeper the machine will go but the lower the KHZ the harder it is for a machine to detect low conductive targets such as a gold nugget.  So silver coins, copper coins are easy to detect deep with the CZ machines or even the 7 khz machines from Fisher, but remember to get the extra depth from the lower KHZ you need to slow down the swing speed on the detector or just buy the F75 ltd which can still go deep with the faster swings. Below is the review i wrote for my last website, it goes into a little more depth about KHZ

What is the best metal detector frequency? This is a very common question and people can get confused thinking the higher the KHZ the more they can find but at the risk of a loss in depth for smaller targets. Here is a little explanation of how it all works..
But what does the KHZ mean? What is the best KHZ for a metal detector? Higher and lower KHZ each have their own advantages. Understanding the advantages and disadvantages will help you decide what metal detector is best for you.
Metal Detector Frequency
First, what does the KHZ mean? Metal detector frequency is measured in kilohertz (kHz). A frequency of 5 kHz means that it transmits and receives 5,000 times per second. 71 kHz would be 71,000 per second. Lower frequencies also have wave lengths that are longer, and higher frequencies have wave lengths that are shorter.
Low frequency detectors are better at detecting highly conductive metals like silver and copper. The longer waves are also better at penetrating the ground so they can lead to getting greater depth. But it is not as good at finding objects that are small or have low conductivity such as gold. This kind of metal detector will typically be in the range of about 5 kHz and below.
High frequency detectors are better at detecting objects with low conductivity like gold and iron. They are also better at detecting small objects making it easier to find things like small gold nuggets or small pieces of trash. But the higher the frequency, the less depth it can get and the more sensitive the metal detector is the mineralization in the ground such as salt or shredded metal cans. This could make the detector useless in some soil conditions if the frequency is too high. This kind of metal detector will typically be above about 19 kHz such as the F19 or Goldbug machines.
A duel frequency metal detector 5 kHz and 15 kHz, the CZ3D and CZ21 can work in all conditions, it allows you to hunt the gold but when in salt it can be tuned to the salt mode and reduce the interference of the soil from the identification of the target.