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 I will post my SPICE circuit diagrams drawn with an outdated program CircuitMaker 2000 because I'm used to working with it.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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http://www.thunting.com/smf/index.php/topic,25812.msg221333.html#msg221333
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