Popular Electronics, Feb. 1969

[technos]

Thank you technos for your kind explanations.

Yes, circuit is from russian source. Unfortunately, it is easy only regarding the number of semiconductors, but also requires the winding of trafo, which is very annoying work.

I am interesting in such simple (Tx oscillating) version of T/R schematic, only not trafos inclusive, to do some test.

Yes you are right, but a little lazy If you want to experiment you will need to wind coils. For the russian schematic I think TP1 and TP2 can be bought and don't need to be wound. For L1/L2/L3/L4 (the article doesn't tell what L4 is - why I would like to find original article) a winding jig like this needs to be built;

http://www.youtube.com/watch?v=FIOocMoRsYQ

I am very interested in building this circuit to see how it does with ground effects and depth performance. But I plan on using a winding jig.

Both of the other circuits look like BFO circuits, which are pretty simple and common. The whole thing I would like to do is have a simple TR discriminator detector. I have not seen anything with four transistors and maybe an IC.

[mikebg]

The IBR MD will work better than conventional BFO MD

The block diagram of IBR MD in posting # 36 is valid also for conventional BFO metal detectors. They have many drawbacks because are "Specific case" of that block diagram:
1. The blocks 1 and 3 are directly connected. Sensing network of BFO uses the worst type sensor "Monocoil" instead induction balanced sensor with 3 or more coils.
At Monocoil, the signal AIR is maximal because the mutual inductance between TX coil and RX coils is degenerated in a self-inductance (coefficient of linkage K = 1). The AIR signal makes the target signal with extremely low modulation index. At induction balanced sensor we can increase modulation index making AIR signal weak (linkage K = 0), however, this is not sufficient because the single RX coil receives a strong GND signal.
The GND signal not only decreases modulation index, but it is modulated with frequencies below 6Hz as target signal.
Therefore we must have second RX coil connected as TWIN LOOP to supress the GND signal.
2. The lack of inductive balanced sensor does not allow conventional BFO MD to work without oscillation as IB MD. It is just one oscillator, a special case of "induction balanced regenerator", which is not "induction balanced", nor "regenerator".
We can construct IBR MD, which is superior to conventional options BFO MD, but this is not enough. It will have inherent weaknesses of each "regen radio" - amplifyes thermal noises generated in input and preamp. Must design low noise preamp for block 4.

[mikebg]

Hi mikebg,

Please tell us more about BFO schematic that you post and it's performance,

did we can build one good deep BFO metaldetector ?

thanks...

Goldenscull, you can build one good deep penetrating metal detector, but dont call it BFO MD. It can be IBR MD and the BFO can be block #7 of block diagram.
Disadvantages of CCO MD in posting # 39 (see also the attached below file):
1. Too large supply voltage. If using HCMOS inverters instead 4069, the circuit can be powered by 2 x AA cells (even by 2.4Volts!).
2. Negative feedback via R is too weak, so the input resistance of inverting amplifier is not zero. This decreases Q factor of tuned circuit L2-C1 and increases bandwidth, ie the preamp generates more noise.
3. The 500KHz operating frequency is too high.
4. The capacitance of the capacitor C2 is too large. You can increase the number of turns on L2 and this the sensitivity, if using smaller capacitance for C2.
5. IC1a is loaded unduly with DC current in operating point. Instead of R2 must be connected a capacitor.
6. Is not intended a control knob to adjust the sound frequency.
7. Is not intended to suppress the GND signal using a second RX coil, connected in series to R2.
8. Remains not used 3 inverters of IC1. They can replace the action of IC2.

[GOLDENSKULL]

Hi,

Did you advise CCO MD is a good deep penetrating metal detector ?!

Can a BFO MD detect a medium metal object in 2-3 meter depth ?

If not please introduce a good MD for detection in 2-3 meter depth under ground...

[Esteban]

Similar to the 3 coils MD published in Popular Electronics is the PRG Gradiometer. Seems very serious scientific MD instrument. Not all the schematic I show... See pics and patent here:

[mikebg]

Search for Vaino Ronka

Hi,

Did you advise CCO MD is a good deep penetrating metal detector ?!

Can a BFO MD detect a medium metal object in 2-3 meter depth ?

If not please introduce a good MD for detection in 2-3 meter depth under ground...

GoldenScull,
50 years ago Vaino RONKA invented and introduced good MDs for detection in 10 meters depth under ground.
Search for this name in WEB and for his patents in USPTO.

[technos]

Similar to the 3 coils MD published in Popular Electronics is the PRG Gradiometer. Seems very serious scientific MD instrument. Not all the schematic I show... See pics and patent here:

Esteban - could you post the full schematic??

[Esteban]

Esteban - could you post the full schematic??

The full schematic haven't values of materials. Seems in other part I have the PCB, but I'm not sure...

[maikl]

Goldenscull, you can build one good deep penetrating metal detector, but dont call it BFO MD. It can be IBR MD and the BFO can be block #7 of block diagram.
Disadvantages of CCO MD in posting # 39 (see also the attached below file):
1. Too large supply voltage. If using HCMOS inverters instead 4069, the circuit can be powered by 2 x AA cells (even by 2.4Volts!).
2. Negative feedback via R is too weak, so the input resistance of inverting amplifier is not zero. This decreases Q factor of tuned circuit L2-C1 and increases bandwidth, ie the preamp generates more noise.
3. The 500KHz operating frequency is too high.
4. The capacitance of the capacitor C2 is too large. You can increase the number of turns on L2 and this the sensitivity, if using smaller capacitance for C2.
5. IC1a is loaded unduly with DC current in operating point. Instead of R2 must be connected a capacitor.
6. Is not intended a control knob to adjust the sound frequency.
7. Is not intended to suppress the GND signal using a second RX coil, connected in series to R2.
8. Remains not used 3 inverters of IC1. They can replace the action of IC2.

The question ihave: L2=? C2=? Instead of R2 must be connected a capacitor C=?

[mikebg]

The question ihave: L2=? C2=? Instead of R2 must be connected a capacitor C=?

Maikl, the CCO MD in postings # 39 and # 48 is a bad circuit with 8 drawbacks. You can calculate L and C for the lower frequency, such as 15KHz, but the operation will not improve substantially because the remaining drawbacks continue to hinder.
The task of a constructor, even if amateur, is primarily to find the best block diagram of the project according to which must operate the circuit diagram. Once you know what should be block diagram, you can copy foreign designs and circuit diagrams if they have such block diagram.
There is, however incompetently designed circuit diagrams as shown below in right.

[mikebg]

Circuit to the left serves to explain how operates a conventional narrow band inverting amplifier with normal opamp. The work of the opamp needs some intermediate voltage, such as 6V, to which are connected signal source and load resistance. In the case, our signal source is RX coil L1. We can connect L1 directly to the pin 2 without using a capacitor C1 to get wide band amplifier, but it is impossible for the circuit to the right. Will understand why later. Now let's see how opamp operates:
It has a very high gain, so the feedback through R2 is very strong. It supports the potential of pin2 same with that of pin3, though a short circuit between them. This is tantamount to connect the right-hand lead of capacitor C1 with lower lead of coil L1. Thus the Q-factor of LC tank is high because depends on coil resistance r1 only. For DC operating point, output voltage at pin 1 follows the voltage of pin 3. When we connect a load resistor between output and intermediate voltage, through it will not flow DC component becauce both leads are connected to the same potential.
How operates the circuit with CMOS transistors in right:
It produces self the intermediate voltage. If the complementary transistors have the same parameters, we will have at 2 pin output 6V DC. If you conect a DC voltmeter to be sure that it does not show the half of the supply voltage. Current, which consumes one inverting amplifier depends very much on supply voltage. Amplification is much less than that of the operational amplifier shown to the left and is influenced by supply voltage. This makes weak the feedback, so input impedance in pin 1 is not zero. Appears an input resistance "r in", which deteriorates the Q-factor of LC tank because it is connected in series to the coil resistance "r1". Most importantly, we can not connect a signal source and load resistor withot capacitor because it will amend the operating point and will proceed DC component, in the case of 3mA, which is too high load for 4069. The attachment AN88 contains information about analog applications of digital CMOC IC.

[technos]

Maikl, the CCO MD in postings # 39 and # 48 is a bad circuit with 8 drawbacks.

I wouldn't call them bad circuits. It depends entirely upon the application. It must be remembered that the 4069 is not really intended for linear circuit operation, but in certain instances can be made to act that way. The reason that a 4069 might be used in this manner is for a cost-sensitive application. What would make it a bad circuit is unreliability. For example, if the circuit only worked with half of the 4069s out there, then it would be a bad circuit. If the 4069 was stressed in a manner that led to its failure, then that would be a bad circuit. But if the circuit worked with all 4069s within its specification, then that would be an acceptable (good) circuit.

[maikl]

Circuit to the left serves to explain how operates a conventional narrow band inverting amplifier with normal opamp. The work of the opamp needs some intermediate voltage, such as 6V, to which are connected signal source and load resistance. In the case, our signal source is RX coil L1. We can connect L1 directly to the pin 2 without using a capacitor C1 to get wide band amplifier, but it is impossible for the circuit to the right. Will understand why later. Now let's see how opamp operates:
It has a very high gain, so the feedback through R2 is very strong. It supports the potential of pin2 same with that of pin3, though a short circuit between them. This is tantamount to connect the right-hand lead of capacitor C1 with lower lead of coil L1. Thus the Q-factor of LC tank is high because depends on coil resistance r1 only. For DC operating point, output voltage at pin 1 follows the voltage of pin 3. When we connect a load resistor between output and intermediate voltage, through it will not flow DC component becauce both leads are connected to the same potential.
How operates the circuit with CMOS transistors in right:
It produces self the intermediate voltage. If the complementary transistors have the same parameters, we will have at 2 pin output 6V DC. If you conect a DC voltmeter to be sure that it does not show the half of the supply voltage. Current, which consumes one inverting amplifier depends very much on supply voltage. Amplification is much less than that of the operational amplifier shown to the left and is influenced by supply voltage. This makes weak the feedback, so input impedance in pin 1 is not zero. Appears an input resistance "r in", which deteriorates the Q-factor of LC tank because it is connected in series to the coil resistance "r1". Most importantly, we can not connect a signal source and load resistor withot capacitor because it will amend the operating point and will proceed DC component, in the case of 3mA, which is too high load for 4069. The attachment AN88 contains information about analog applications of digital CMOC IC.

Thank you mikebg for your kind explanations.I am very interested in building this circuit to see how it does with ground effects and depth performance.

[maikl]

I wouldn't call them bad circuits. It depends entirely upon the application. It must be remembered that the 4069 is not really intended for linear circuit operation, but in certain instances can be made to act that way. The reason that a 4069 might be used in this manner is for a cost-sensitive application. What would make it a bad circuit is unreliability. For example, if the circuit only worked with half of the 4069s out there, then it would be a bad circuit. If the 4069 was stressed in a manner that led to its failure, then that would be a bad circuit. But if the circuit worked with all 4069s within its specification, then that would be an acceptable (good) circuit.

Thank you technos for your kind explanations.

[mikebg]

I am very interested in building this circuit to see how it does with ground effects and depth performance.

Maikl, read this again:
"The task of a constructor, even if amateur, is primarily to find the best block diagram of the project according to which must operate the circuit diagram. Once you know what should be block diagram, you can copy foreign designs and circuit diagrams if they have such block diagram." [by (R)EMI group]
If you are "very interested in building this circuit to see how it does with ground effects and depth performance", you should know the correct block diagram. Now you know that CCO in postings # 39 and # 48 is a bad circuit with 8 drawbacks.
The correct block diagram uses two RX coils to suppress ground effects. This is configuration TWIN LOOP. The correct block diagram of IBR regenerates weak signals but not oscillates. The block diagram in posting #16 should show the connection of both RX coils so: