Bipolar PI design

[Detectorist#1]

Hi Eduardo,
Now I'm in intensive testing of the schematic and settings of additional module for almost all simple PI designs for sure Fe/non-Fe discrimination. The results will be posted here.

[algan]

Hello Eduardo, what variant is it, do you draw the circuit board dip or smd?

[Carl-NC]

Eduardo, here's my feedback:

1. I generally agree with Detectorist#1 on the preamp, however you often have to test these things to see whether 1 stage or 2 stage is faster. You can't go by GBW alone as overvoltage recovery plays a strong role, and that can get worse with high gain. I built a little test circuit that just has a monopolar TX and the preamp that I use for testing different opamps with different gains with both 1 stage & 2 stages.
2. The LMH6646 is serious overkill for U8b. Since this is a unity-gain amp anything with several MHz GBW will work.
3. You've combined the demod input resistors into R38 & R39. Problem is, if (for example) U9d and U9c accidentally turn on at the same time then U7b and U8b outputs are shorted and they burn up. Trust me, this can happen, especially when debugging buggy software. I like to place the resistors on the preamp side of the switches to avoid this. Also, it places the 4066 switches directly on virtual ground so they behave better versus preamp signal swing.
4. I didn't do this in the Pulse5 design, but you may want to add an offset resistor between P3 and U11b. As it is, the GB pot adjusts from purely TGT only to purely GND only. You should not need this much GB range so the offset resistor can be added to reduce the range and make it more reasonable. You'll have to figure this out later as it depends on timing.
5. D13-D14 don't really do anything but add a dead zone at zero volts. This can be useful for reducing noise but it also reduces sensitivity. You can leave them in the design but try shorting them when testing.

All for now.

[eduardo1979]

Surely it will be in SMD. For now we are going to wait for detectorist#1 to finish its work, so we can include it in the final schematic. There is no rush, good jobs come when they are done slowly. Questions always arise and I hope that people are encouraged and give their opinion.

[Carl-NC]

Any reason why FETs can't be used instead of 4066? Can they not be switched directly by the pic. Seems like much less components.

You can, see the Pulse-1 design in ITMD3. However, the 4066 has advantages:

• In a multi-channel design the switches will match reasonably well which is a bonus.
• The JFET characteristics are more dependent on preamp signal levels.
• The JFET requires clock translators in the Bipolar-PI design because the micro runs on +5V/0V.
• Without translators, 4 discrete transistors take up about the same space as a 4066 chip. With any more components, the 4066 wins.

[eduardo1979]

Thanks Carl for your comments. I will have to make the modifications to the scheme, but it has to be for this weekend since I don't have enough time for now.

[Detectorist#1]

Before using of J113 transistors, I measure Rds with ohmmeter. Usually Rds is in range 40-80 ohms. This measuring have to be made with short connection between Gate and Source. For using, I take a couple with no more than 10 ohms difference. This is 1% of the value of serially connected resistors in scheme whose popular value is 1Kohm.

[dbanner]

You can, see the Pulse-1 design in ITMD3. However, the 4066 has advantages:

• In a multi-channel design the switches will match reasonably well which is a bonus.
• The JFET characteristics are more dependent on preamp signal levels.
• The JFET requires clock translators in the Bipolar-PI design because the micro runs on +5V/0V.
• Without translators, 4 discrete transistors take up about the same space as a 4066 chip. With any more components, the 4066 wins.

"clock translators" the correct term? You mean a -5V swing is needed which the micro cannot give.because it runs on +5V/0V
4066 definitely wins with so many channels. No doubt about that
Also easier to route on pcb . 4 channels of transistor switchers and other components I imagine can be silly waste of space.

[Carl-NC]

Yes, the micro runs on +5V/0V and the JFET clocks need to be 0V/-5V.

[Detectorist#1]

Hi Eduardo,

The PCOUT output of 4046 haves +/-5V output voltage levels. J113 will be open always. Plus 5V will be limited by open junction Gate-Source to maybe +0.7V and minus 5V will be limited from connected diode to -0.7V. This voltage is not enough to stop J113 to conducting. For proper operation you have to remove diode D4 and to add resistor 10K-47K between 4046-pin4 and the Gate of J113.

[Detectorist#1]

Hi Eduardo,
Also the schematic of the audio amplifier have to be seriously corrected. The diode D17 limits the voltage on the Base of Q14 to 0.7-0.8V. With 0.6V on Base-Emitter junction of Q14 - max voltage on R71 will be 0.1-0.2V. This give collector current of 0.5 -1.0 mA. This current not gives enough voltage on R70 (1mA x 470R = 0.47V) for controlling of Q15. Also it is not serious to use pot with value of 1K for load of 50 ohms. The regulation of the level of the sound will be all or nothing.

[algan]

Hello, Eduardo, you can use the same sound scheme on the TDI, it is perfect.

[Carl-NC]

Hello, Eduardo, you can use the same sound scheme on the TDI, it is perfect.

The TDI audio uses an incredibly expensive AD654 VCO and is also a power hog. There are better ways to do this.

[Carl-NC]

Hi Eduardo,

The PCOUT output of 4046 haves +/-5V output voltage levels. J113 will be open always. Plus 5V will be limited by open junction Gate-Source to maybe +0.7V and minus 5V will be limited from connected diode to -0.7V. This voltage is not enough to stop J113 to conducting. For proper operation you have to remove diode D4 and to add resistor 10K-47K between 4046-pin4 and the Gate of J113.

Also the schematic of the audio amplifier have to be seriously corrected. The diode D17 limits the voltage on the Base of Q14 to 0.7-0.8V. With 0.6V on Base-Emitter junction of Q14 - max voltage on R71 will be 0.1-0.2V. This give collector current of 0.5 -1.0 mA. This current not gives enough voltage on R70 (1mA x 470R = 0.47V) for controlling of Q15. Also it is not serious to use pot with value of 1K for load of 50 ohms. The regulation of the level of the sound will be all or nothing.

I agree with all this. D4 is backwards from what you want, but you also need a resistor between D4 and U14 or you're likely to burn up the VCO output. Next, if D17 is intended to clamp the cap-coupled (C40) signal then it is also backwards. But C40 isn't needed because the signal on Q13-D already has the correct voltage levels (+5V/0V) so you can omit C40 and D17. I would then move the volume pot to the base side of Q14 and delete R69. Personally, I find the LM386 to both sound better and have better drive efficiency compared to most discrete solutions, and it's dirt cheap.

[eduardo1979]

You are right. D4 and D17 are reversed. Rushing is simply not good. I was in a hurry to finish the design since I had very little free time and I made those two mistakes. The C40 would leave it there. The volume potentiometer is fine where it is. I have used this audio amplifier circuit on more occasions and it works very well. The volume is regulated linearly. The LM386 is a good option and I don't rule it out at all. This Saturday I will make all the corrections to the schematic and publish it.

[pechkata]

Hi everyone on the forum...
Chip file uploaded...