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I've been reading the schematic for HHII and it seems pretty clear except I was wondering if the value selection for the recieve coil damping resistor uses the same procedure as for the xmt resistor. Should the recieve coil have the same inductance value as the xmt coil?
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Hi Mike,
First lets discuss the receive coil. Now, I have built DD coils with the receive windings ranging from 300 uh to 450 uh and all worked fine. I am inclined to use the higher value now because it does give a little more gain and with the inductance different than the transmit coil, there is less tendency for interaction. I use a 300 uh transmit winding. This allows me to use factory coils with minimal changes since they have a 300 uh transmit.
The HH schematic provides for the damping resistor to be placed on the board. However, mounting the receive coil resistor on the board leaves it in place should you decide to use a mono coil later. This can interfere with the use of factory coils since they already have a receive damping resistor inside the coil housing.
If you use a multi pin connector then you can use different pins for key connections and by duplicating a common arrangement allows for the use of other coils. This will also allow you to use either a mono coil or a DD coil much easier. Now, if you add the receive damping resistor in the coil housing like what is done on factory coils, then the connection or change in coils becomes transparent.
Many people are using the 5 pin connector similar to that used for the Minelab PI compatible coils. Doing this will allow one to use Minelab PI factory made compatible coils with ease.
Reg -
Now you've REALLY got my attention! When I first started contemplating this project, I had it in mind to build a control box that could use Coiltek
coils and probes designed for use with the Minelab SD series. I can see how mounting the rcv damping resistor in the coil housing would help facilitate switching between mono and multi coil designs but what about the main signal circuit? There's pads for a mono/multi jumper shown on the HHII schematic. I was thinking of mounting a high quality toggle switch on the panel with shielded wires (RG174u) from switch to pcb pads, but am not sure if that wouldn't add extra noise to the signal or possibly screw up something with the signal timing or quality. What do you think?
Does anyone know what brand/model/part no. of connector could recieve a Minelab SD or Coiltek coil cable, and what the pin assignments are? Or is there another standard configuration that would be more applicable?
Is the 5th pin for a common (shield) ground? I was thinking of grounding any xmt and/or rcv coil shields (on my own coils) straight to the pcb ground, instead of to the coil outputs,in order to keep the coil signals as clean as possible.
Also, just outa curiosity, has anyone tried putting the xmt damping resistor in the coil housing as well? I suspect that wouldn't be as effective as a pcb mounted damper because of the extra capacitance
added by the coil cable, but I'm a long ways from being an EE so frankly I'm just speculating (guessing).
Thanx
grungymikeComment
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Hi Mike,
Minelab uses a simple 5 pin mic connector.
I have attached a pic of the connection to the pins. Also, here is a link to a pic of the connector used.
http://www.cbradiosplus.com/shop/pc/...&idproduct=736
If you look at the pic of the wiring of the connector, you will see the shield of the coax's connect to pins 1 and 5. By connecting pins 2 and 4 will allow the transmit coil become the receive coil also. In the pic, pin 2 is the receive connection, while pin 4 is the transmit coil winding connection. So, to use a DD coil, the transmit coil of the DD will connect to pins 4 and 5, while the receive winding will connect to pins 2 and 1. Pin 2 can be connected to pin 3 also if one desires. However, pin 3 really doesn't do anything that I can tell except make it easier to connect pins 2 and 4 when wiring the connector for a mono coil.
RegComment
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Yeah, I was thinking over coffee this morning that some kind of bridged pin arrangement was called for. This is perfect. Building a box compatible with ML coils will allow me to tweak it to a known baseline (factory coil),
and then start playing around with some ideas of my own. Thank you!
By the way, how are you getting graphics into a thread? When they said "a picture is worth a thousand words", they weren't kidding.
gmComment
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Hi Mike,
First, I noticed I didn't answer your question about the damping resistor for the receive winding. I have found I can use about a 1.2K but have bumped this up to a 1.5K and as low as a 1K resistor on the receive winding. So, it is a matter of experimenting more than anything and will be dependent on the final receive inductance as well as the wire used. The less capacitance involved the less resistance required. This is true for DD's and mono's.
As an example, lets say you are building a 300 uh xmit mono coil and trying different types of wire to see what happens. If you set the damping resistor for a higher value, maybe a 1K to 1.5K and test different coils all having the same inductance but made with different wire, you will get a better idea of what I am talking about if you observe the waveforms generated. A coil with very low capacitance will dampen nicely while other coils will oscillate and require a lower value of damping resistor.
Now, this is a little trick in the fact that using the best wire will allow a higher value of damping resistor, which then will allow for a faster decay. With a faster decay, one can sample sooner. This becomes important for someone wanting to find small gold.
Now, I have tried tweaking a coil by adding some damping resistance to the xmit coil inside the winding, but I have never put all the resistance there. However, if I want to experiment with a wide range of xmit coils, then something should be done with the damping to obtain the best results. Simple experimenting will best show you what will happen.
As for graphics, I develop them with a simple photo editing program that allows drawing things and then saving what I draw as a JPEG file. I simply upload that file when I want to attach it to something I post.
If you go down past the preview/save/submit blocks, you will see the additional options box. Click on manage attachments and upload any jpeg file on your computer.
RegComment
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mating connector on box???
I've been in contact with CB radios plus trying to find out what brand/# of connector mates with their 5 pin mic connector (their sku #1376).
I think I've run into a communication problem, the person on the other end can't seem to grasp the idea that somebody somewhere at some time, put a mating connector on the back of a CB radio box so that their cable connector can attach their microphone to somebody's CB radio, they're more concerned with getting the right wires to the right pins for CB radio use.
Does anybody know who the actual manufacturer of these cable connectors is? I think I need to talk to the source...
gmComment
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Hi Mike,
Here is a link to one source of the 5 pin connectors. They carry both the male and female ones, so they have the chassis and the cord parts needed.
http://www.thomasdistributing.com/sh...rsllit8gi3gna7
Here is another source.
http://www.universal-radio.com/catal...s/micconn.html
Here is a third source. However, on this one, you have to select connectors and adapters on the left side, and then select mic to get to the listing they have.
http://www.hosfelt.com/
The key at most places is to ask for the right part. Normally, the male connector is the 5 pin that fits on the chassis and is referred to as the 5 pin chassis connector that mates to the ...... model of mic connector when you are trying to get one from a source.
I tried to look up the part you want at radios plus and only found the 4 pin chassis connector, so they may not carry what you are wanting.
RegComment
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If I comprehend your question, you can find these chassis connectors on ebay and most ham radio parts suppliers.Originally posted by grungymike View PostComment
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