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In the new audio, Q7 have to be connected in reverse polarity (exchange of the emitter and the collector connections). Which type of the speaker you think to use? - the volume of the sound is controlled via 1K potentiometer.
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upgraded audio is not correct.Comment
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Right, the other way around Q7
I'm only going to use headphones, thanks for your attentionComment
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Hi hunter63,
Also the connecting of 78S40 IC have many, many errors. In this place, it is more suitable to use MC34063A. This IC have more simple connection and the price is 3-4 times cheaper. Read carefully the datasheet - the value of the current sense resistor have to be 150mR, not 150R in your case.Comment
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I disagree, I have been dealing with the 78s40 PWM stabilizer for years and I am satisfied, the price is about 1E, the resistor value is wrong, I usually use 2.2R, while MC34063 introduces interference, best regardsComment
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The 78S40 is in fact MC34063A with added Operational amplifier and 1A diode. Added interference is absolutely the same!Comment
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Also have possibility to disable the work of DC/DC converter during real RX sample time- from start of TX pulse to the end of RX EFE pulse with an additional start-stop trigger in the circuit of the metal detector.Comment
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Yes it's trueComment
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I am thinking about changing the clock generator from 555 to 4060 or 4046 (PLL) and increasing the frequency from 800 Hz to 1.2 kHzComment
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Increasing the frequency is a good idea for increasing of the ratio signal/noise, but serious increasing will be done by changing ADA4610 with really low noise operational amplifier type OPA1612AID in first RX stages!Comment
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I took ADA4610 from the KiCad library, ultimately it will be ADA4637-1BRZComment
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The ADA4637-1BRZ have voltage noise of 30nV/sqr Hz at 2Hz (page 13 of DS) - the same as ADA4610 (page 16 of DS). The OPA1611AID have voltage noise 4pA/sqr Hz at 2Hz.Comment
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Thank you
OPA1611AID very beautiful parameters, is it only a single one or are there also double ones, if you know the name please tell me, best regards and thank youComment
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OPA1611AID is single OpAmp and OPA1612AID is Double OpAmp. The convenient cases are SOIC-8.Comment
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Originally posted by hunter63 View Post
I have been using the ADA-4637-1BRZ since I started my design of the AGD detector years ago with the knowledge that it is a fairly noisy device and as such I have used two in parallel to reduce noise levels which works very well. Thus the ADA-4637-1BRZ can be made workable and I can detect very small gold with my AGD detector design but it is a lot different that your proposed design. As I got my design more and more refined I started working more toward the RX front end front end and since my version AGD24.2 the ADA4637-1BRZ has been replaced with lower noise devices to reduce input stage noise levels to a minimum while using the same basic PCB layout. This however causes a change in the first stage input currents that the lower noise devices have when they don't see the same impedance on its positive and negative inputs. This causes substantial DC offsets at their output which must be compensated for.
If you use a single ADA4637-1BRZ in your application or you may need to set your threshold for detection level fairly high since the ADA4637 generated noise will show up as large jumps in demodulated output voltage levels and modulate your audio tone. I would not use less than two ADA4637 devices in parallel with each one configured to do its own current sharing to the total output. You cannot just solder one on top of the other. When I was crazy about audio work I have paralleled up to eight opamps which gave amazingly low noise noise levels some twenty years ago for moving coil phono cartridges that operate at microvolt levels.
Make sure that during your simulations that you look at the noise generated at mV levels or less at the output of your first two stages with the RX input at your coil connector shorted to ground.
Be careful with the ADA4610 I think I recall that it showed some tendency to oscillate at times if no compensation is used in its feedback loop.
High speed is required of all input stage gain blocks that are before analog gates so that recovery from TX coil recoil voltage is very fast. The input stage must be able to handle anything thrown at it and behave nicely. Clamping diodes will cause loss of speed and other problems which may not be easily identified.
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