AVR+DSS Frequency Mod SD2100V2

[mario]

I'm still working out the kinks.

This AD9851 operates warmer than I expected and was able to cool it down by disabling an internal 6x multiplier. Unfortunately, I won't achieve higher than 15Mhz frequencies with our 30Mhz crystal OSC, but the difference in thermals is baffling. For our purposes a 15Mhz max is more than enough since these parts will never exceed 4Mhz. I don't see anyway of accommodating a heat sink.

I would like to eventually experiment with a 180Mhz crystal OSC, but the cost is too prohibitive. These chips do shine at higher speeds though...

Anyway, I'm really happy with the lower thermals...The problem I'm now currently having involves the regulator. At a 6.2 volts input it runs cool. At 7.4 it runs warmer than I would like. I'm going to experiment with a different package and/or regulator. Once I get that solved, I will finally do some field tests.

I'll have to make a BoM soon, that way people may start ordering parts....By the way, the only parts not shown in the schematic involve power supply and low-pass filter -which is very different from the one used by the AD9851 datasheets.

The software is now mainly done. I just need some code in case people use the same board to experiment with SD2000s and SD2100s. Some parameters are saved and reused. People might unknowingly damage their SD2100s if I don't include some behind-the-scenes protective mechanism.

Anyways, I won't be able to do those field tests until I get those regulators.

I want to make sure this mod works well (these detectors are not cheap, and I want to take all the necessary precautions from damaging them).

[IBGold]

Mario please put me down for two also I will also start looking for components to suit.

Regards, Ian.

[gef12]

Hi Mario
Put me down for 2 mate... pm me when all is ready
tks for now
Geoff

[mario]

Hi guys,

The signal has to be pure. The smallest jitter will cause so much noise,that it will render the detector unusable. However, after experimenting with my mod, I was able to get a very pure sine wave using a 100Mhz crystal. The audio is crystal clear as stock (kudos to Analog Devises for creating such a remarkable chip).

I pretty much have solved most of the issues. All heating problems disappeared after replacing the cheap Chinese regulator I bought from ebay.

The only issue that remains has to do with the indirect control of the frequency.

It seems that the stock VCO outputs a 50-60Khz frequency at pin 3. The VCO feeds the two caps which rev up the frequency to 1.2Mhz.And after shorting both caps, and feeding a 1.2Mhz signal, I lose audio.

It seems the only way to rev up the frequency involves removing the stock VCO and feeding those caps with a +50Khz signal.

I have a question to you SD2000 owners: are your detectors the same? Meaning, replacing those caps increases the frequency? If so, I'm not sure this mod will work on your detectors. It seems like I can indirectly control the frequency by having my mod exert a +50Khz to pin 3 instead of feeding a pure 1.2Mhz signal.

Or does your detectors tolerate a direct 1.5-4Mhz signal? I believe Jiminy mentioned something about having 1.5-4Mhz crystals, meaning a pure 1.5-4Mhz signal but I would like to confirm.

[mario]

BoM

Parts list found here: BoM

Notice it's a work in progress. I will likely add more components, as I resolve the few remaining issues.

[IBGold]

Hi Mario,
The SD2000 will run directly with crystals from 1.25MHz to over 4.0MHz I even have a 6.0 MHz one in but I only use one channel when using it I have not try-ed lower crystals as they are too hard to get in Australia but my outboard VCO that I use runs from .985 MHz to 4.95 MHz and the detector will run all through that range no worries the problem with the VCO is it has slight jitter which degrades its performance the SD2000 has a jumper between the crystals fitted so that one or the other crystal can be selected this is where I feed the VCO signal and the six position crystal switch and where your signal would be fed into the unit.
Sounds like the SD2000 is much easier than the SD2100 to do but keep up the good work.

Regards, Ian.

[sido]

Hi Mario, the frequency mod with crystal's is documented by Ismael Jones for your reference. It will give you pin location to be able to tap into. There is also full circuit diagram available there after sign up. Do a search here also for (SD2000 circuit), it is available.

http://www.minelabmods.com/joomla/in...id=2&Itemid=40

If i get some time in couple of days, will look into it....Hope that helps

Sido

[moodz]

DDS 50 bucks on ebay ... sometimes buying beats building :-)


See ebay .... just search on "DDS"

eg .... item 280700041512

these are new items !!

1. Signal Output function

• Output waveforms:Sine wave, Square wave and Triangle wave
• Output amplitude:≥10Vp-p(signal output, no load)
• Output impedance:50Ω±10%(signal output)
• DC offset:±2.5V(no load)
• Display:LCD160
• Frequency range:0.01Hz ~ 5MHz(SG1005)
• Resolution:0.01Hz
• Frequency Stability:±1×10-6
• Frequency accuracy:±5×10-6
• Sine wave distortion:≤0.8% (reference frequency is 1kHz)
• Trinagle linearity:≥98% (0.01Hz~10kHz)
• Rise and fall time of square wave:≤100ns
• Square Wave Duty range:1%~99%

2.TTL Output function

• Frequency range:0.01Hz ~ 5MHz(SG1005)
• Amplitude:>3Vp-p
• Fan Out:>20 TTL loads

3.COUNTER function

• Counter Range:0-4294967295
• Frequency Meter Range:1Hz~60MHz
• Input Voltage Range:0.5Vp-p~20Vp-p

4.Others function

• Save and Load Parameter:0-9(0: default load)

Package included:

• 1 x 5MHz DDS Function Signal Generator Source Module Wave
• 1 x 5V Power Supply

[mario]

Thanks Ian,

This mod should work fine with the SD2000s. i did not notice any jitter from 1-4Mhz,so there should not be any audio issues. Still, I would like confirmation.

However, with the SD2100s, the situation becomes tricky. Because I'm inderectly controlling the frequency, those two caps limit my range . if im not carefull jitter starts distorting the audio.

My next step involves finding the right cap combination before finishing up. Otherwise, I'm going to slightly mod the audio.

[mario]

Moodz,

Before building my solution, I explored several options. After my failed attempt at trying a programmable VCO (like Ian), I found the above board on ebay.

It is sometimes difficult to trust the claims of those ebay sellers. The above board does not use AD9851 for DSS, but did claim similar specs.

Sometimes it pays off to reinvent the wheel.

[moodz]

Hi Mario,
my comment was a bit cheeky....I was just indicating a possible shortcut for some who dont like surface mount soldering etc. Of course there is always opportunity to make a new an improved wheel like your good work.

regards,

moodz.

[IBGold]

What woul be nice would be to be able to set the frequency range say 1.0MHz to 4.5 MHz and then sweep that range and when a signal is picked up it locks for say 10 seconds at the frequency in question now would that not be nice may elininate detector holes but it is out of my league.

Regards, Ian.

[mario]

Hi Ian,

When I hunt, I rev up the frequency just before the ground becomes noisy. Doing so will tighten the timings enough to ignore the mineralized ground conditions while still triggering a response.

I hunt in channel one until finding a target. if I find something interesting , I confirm the target by switching to channel two. If the target triggers channel two, I start digging.

From my perspective, if we can do a frequency sweep several uS below the ground becomes noisy, we will get better sensitivity. However, if the target is too small to trigger a channel two response, then we might miss the target entirely.

So, in my honest opinion, thightening the channel two timings will probably provide us with an opportunity to make our detectors even more sensitive -assuming the user hunts in the above mentioned manner.

Unfortunately, the channel two timings and channel one timings are mutally exclusive of each other. In other words, by changng the timings through the frequency you change the timings of both channels, not one or the other.

And the only way to modify the two channel timings independently is if we go ahead and change the contents of the eprom...

[mario]

[mario]

Still working on it, putting in hours here and there...

Anyways, sorry about the parts list being inaccessible. I forgot to change the privacy settings. I might add a few more caps, but the bulk of the list will not change.

Schematic is also accurate and complete.

BoM found here:
https://spreadsheets.google.com/spre...hISGc&hl=en_US