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Offline dr.jare
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« Reply #10 on: June 19, 2011, 06:38:47 am »
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mikebg sorry i did not know that it is now so complicated registration to the forum. I have recommended you register on the Serbian forum so that I would not mediated, because of my poor knowledge of English language, little free time... WM6 can help you more than me. WM6 is the administrator of the forum HOBIDETEKTORI and speak English better than me. Contact with WM6 via private message .

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Offline mikebg
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« Reply #11 on: April 26, 2012, 05:16:24 am »
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Finally I can spend parts of my free time to make revisions to the design of Relic Hawk. I will start with revision of the transmitter because at first glance I see incorrect choice principle, incompetent selected stages and improperly calculated values of the components.

Too immodest, I to blame the professional Minelab designers for incompetence, because I’m a ham designer only. To believe me, please use some SPICE program (most popular is LTspice), to repeat my experiments. I will post my SPICE circuit diagrams drawn with an outdated program CircuitMaker 2000 because I'm used to working with it.

Each participant in the forum can experiment discussed circuits with real components because they contain fewer components only.

My hobby is design of TXs, RXs and antennas for QRP amateur radio. In amateur radio, the code QRP means “use minimal TX power”, but I will show how to increase the power and efficiency of Minelab TX.

The attached diagram I call "Relic Hawk TX", although I am convinced that it has not been applied first in Relic Hawk, but in preceding models. The name of TX is not important. For me is more important that the imperfect designed transmitter is used in several models “professional”  metal detectors. The designer don’t knows the shortcomings of his work and how to test and improve them.

I see 4 shortcomings at first glance:
1. Too large resistance for R1 in emitter of Q1. That means, the pumping pulses for LC tank circuit will have more than 50% duty cycle and TX will operate with low efficiency.
2. Too large resistance for R2 in collector of Q1. That means, the input capacitance of Q2 is discharged slowly. This creates tendence for HF ringing.
3. A cascode amplifier with Q2 and Q3 is used as end stage in TX. This lowers TX efficiency because its saturation voltage is more than 3.3V. The simple stage with a transistor has saturation voltage Vce_sat about 0.2V only, ie TX can generate almost rail to rail amplitude.
4. There is no amplitude detector. The automatic controller integrates the negative half wave to maintain constant amplitude of oscillation.

To continue with the revision and redesign, I would like someone participant of the forum to simulate with SPICE the operation of this circuit diagram and to show the result here..


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Online o.mag
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« Reply #12 on: May 06, 2012, 06:50:06 am »
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hi mikebg
could you published .ckt Circuit (I use too curcitmaker 2000)

olivier


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Offline mikebg
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« Reply #13 on: May 20, 2012, 03:01:02 pm »
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Bonjour Olivier.
 I made two versions of TX circuit for SPICE analysis attached here in compressed format. They differ in SPICE names of supply rails only.
 The first one corresponds to the names given by Bulgarian reverse engineers, but I ommited the name of the 5V (Vcc) rail because in reality its voltage varies. It starts from 7.6V (when the Pb battery is fully charged until 13.6V) and diminishes to 5.2V (when the battery is discharged to a permitted lowest value 10.2V).

 The second version (given in my previous post and attached here as *ckt file) was proposed by Max (a participant in geotech1 forum). His idea is more convenient for analysis and design, because SPICE ground appears zero axis of generated sinusoidal voltage.

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