Tweet
I wanted to take advantage of fast coils in the 270uH – 300uH range by maximizing fast decay and have the capability to sample as early as possible with variable sample delay. To that effect I determined to keep the pre-amp bandwidth as close to 1.5 MHz as possible while still achieving acceptable gain. I settled for a trade-off of BW= ~1MHZ @ G=726. For variable sample delay, I opted to use the ADC to control the delay for sample 1. I ended up with a sample delay variable from 4.5usec – 49.5usec. I wanted to keep sample 2 as close to sample 1 in time yet far enough down the decay curve to only get ambient noise/interference. I opted to have sample 2 occur just before TX start. This put sample 1 and sample 2 ~105 usec apart yet ~900 usec down the decay curve. I also wanted to keep the sample time as short as possible, but yet maintain high sensitivity. I also wanted to experiment with single tone audio so I opted for the single tone 555 option.
After completeing this build of the HH2, I did some testing with a 9 inch elliptical figure 8 coil of 300 uH. Power supply voltage 10.5V, Pulse repetition rate 1kHz, TX pulse width is set at 99.5 usec, sample width is set in software at 7.5usec. At timer 1 interrupt the following sequence occurs: Sample 2, TX, Sample 1, delay ADC check.
I am testing in a fairly noisy environment (inside our 5th wheel RV with TV, flouescent lights, etc). The HH2 is not yet mounted inside a shielded box which should further improve performance. With delay set at 6usec, I can detect a nickel at 21cm.
I am attaching the source code that for the uPc.
_________________________________________
After making the board modifications outlined in the ‘HH2 Board Mdifications’ thread, the following changes were made to Carl’s design to achieve the desires:
Increased R12 to 2.2 to reduce effects on damping and enhance decay;
Reduced gain of preamp to 726 (21.36 stage 1, 34 stage 2) to enhance effective BW. At G = 726, effective BW is ~1 Mhz. At original design gain (G=1122) the effective bandwidth is ~700kHz;
Changed r16 to 1 Meg and modified circuit to make offset adjust to affect stage 1 instead of stage 2 (not to correct any issues, but to support future experiments with preamp);
Omitted D7 as there is no benefit in preventing saturation of stage 1 while allowing stage 2 to saturate. Also the capacitance of the 4148 caused the effective bandwith to be reduced to ~300 kHz. Also the recovery characteristics of the MAX412 are excellent and no real benefit is realized in attempting to improve recovery by limiting saturation. The detriments outweigh any perceived benefit in using D7;
Omitted C24. Tests indicated that C24 reduced the bandwidth of stage 2 to ~10Hz.
Changed R26 & R27 to 220K. This was done to help make up for the reduction of gain in the preamp and improve the Low Pass characteristic of the integrator.
Changed IC8 from TL072 to a TLC2262. Reason… 1: I had them available. 2: The TLC2262 draws 0.25ma/stage vs 2.5ma/stage for the TL072. 3: The output of the TLC2262 is rail-to-rail.
Used the 555 single tone option for audio (R55, R49, Q10). Set the output at ~700Hz @ ~50% duty cycle by using R55=360, R39=10k, C22=0.1uF.
Changed Q11 to MPSA13 adrlington to increase audio sensitivity. Some may not like this option because it effectively is an on/off situation (but I like it as the audio acts a lot like the audio in my Fisher 1265). Anything that breaks threshold is clear tone. I also placed a diode (414
with anode connected to Q6/R36 junction and cathode connected to Gnd to limit the audio drive current and prevent overdriving Q11.
Changed Q3 to FQPF2N60C. I have had good results with this in the past. It has low Coss (~25pF) and enhances coil fast decay charachtaristics. The drain is insulated so that I can mount a heat sink without introducing up noise in a monocoil arrangement. Some may not like the Ron charactaristics, but I find it ideal if I take care to keep the coil resistance low. I try to keep my coils at <= 300uF @ < 3 ohm.
Changed Q2 to MPSA13. As I operate from a NMH 8 cell battery pack, my operating voltage is between 9 – 10.7, Q3 stops working at the lower end. With the MPSA13 I have operated the TX with voltage as low as 8V.
After completeing this build of the HH2, I did some testing with a 9 inch elliptical figure 8 coil of 300 uH. Power supply voltage 10.5V, Pulse repetition rate 1kHz, TX pulse width is set at 99.5 usec, sample width is set in software at 7.5usec. At timer 1 interrupt the following sequence occurs: Sample 2, TX, Sample 1, delay ADC check.
I am testing in a fairly noisy environment (inside our 5th wheel RV with TV, flouescent lights, etc). The HH2 is not yet mounted inside a shielded box which should further improve performance. With delay set at 6usec, I can detect a nickel at 21cm.
I am attaching the source code that for the uPc.
_________________________________________
After making the board modifications outlined in the ‘HH2 Board Mdifications’ thread, the following changes were made to Carl’s design to achieve the desires:
Increased R12 to 2.2 to reduce effects on damping and enhance decay;
Reduced gain of preamp to 726 (21.36 stage 1, 34 stage 2) to enhance effective BW. At G = 726, effective BW is ~1 Mhz. At original design gain (G=1122) the effective bandwidth is ~700kHz;
Changed r16 to 1 Meg and modified circuit to make offset adjust to affect stage 1 instead of stage 2 (not to correct any issues, but to support future experiments with preamp);
Omitted D7 as there is no benefit in preventing saturation of stage 1 while allowing stage 2 to saturate. Also the capacitance of the 4148 caused the effective bandwith to be reduced to ~300 kHz. Also the recovery characteristics of the MAX412 are excellent and no real benefit is realized in attempting to improve recovery by limiting saturation. The detriments outweigh any perceived benefit in using D7;
Omitted C24. Tests indicated that C24 reduced the bandwidth of stage 2 to ~10Hz.
Changed R26 & R27 to 220K. This was done to help make up for the reduction of gain in the preamp and improve the Low Pass characteristic of the integrator.
Changed IC8 from TL072 to a TLC2262. Reason… 1: I had them available. 2: The TLC2262 draws 0.25ma/stage vs 2.5ma/stage for the TL072. 3: The output of the TLC2262 is rail-to-rail.
Used the 555 single tone option for audio (R55, R49, Q10). Set the output at ~700Hz @ ~50% duty cycle by using R55=360, R39=10k, C22=0.1uF.
Changed Q11 to MPSA13 adrlington to increase audio sensitivity. Some may not like this option because it effectively is an on/off situation (but I like it as the audio acts a lot like the audio in my Fisher 1265). Anything that breaks threshold is clear tone. I also placed a diode (414
with anode connected to Q6/R36 junction and cathode connected to Gnd to limit the audio drive current and prevent overdriving Q11.Changed Q3 to FQPF2N60C. I have had good results with this in the past. It has low Coss (~25pF) and enhances coil fast decay charachtaristics. The drain is insulated so that I can mount a heat sink without introducing up noise in a monocoil arrangement. Some may not like the Ron charactaristics, but I find it ideal if I take care to keep the coil resistance low. I try to keep my coils at <= 300uF @ < 3 ohm.
Changed Q2 to MPSA13. As I operate from a NMH 8 cell battery pack, my operating voltage is between 9 – 10.7, Q3 stops working at the lower end. With the MPSA13 I have operated the TX with voltage as low as 8V.
Comment