Is this a correct front end?

[brucester]

Hi,

After playing around with a few circuits and reading chapters 11 and 12 over and over I want to get started on my micro PI.

I want the simplest front end possible for tx/rx, just enough to provide my micro with a nice curve to play with, then i can finally get stuck into code.

Is this a correct circuit for tx and preamp? It seems to work in the sim: http://www.falstad.com/circuit/#%24+...625E-4+3+-1%0A





Are the any mistakes or issues to be aware of? Or is this a decent tx preamp circuit?

Sorry to bore you with such basic electronics, once i get past analog i'll be fine :P

Thanks for any input

[Qiaozhi]

Hi,

After playing around with a few circuits and reading chapters 11 and 12 over and over I want to get started on my micro PI.

I want the simplest front end possible for tx/rx, just enough to provide my micro with a nice curve to play with, then i can finally get stuck into code.

Is this a correct circuit for tx and preamp? It seems to work in the sim: http://www.falstad.com/circuit/#%24+...625E-4+3+-1%0A



[ATTACH=CONFIG]23044[/ATTACH]

Are the any mistakes or issues to be aware of? Or is this a decent tx preamp circuit?

Sorry to bore you with such basic electronics, once i get past analog i'll be fine :P

Thanks for any input

If you want a simple preamp circuit, then try looking at either Hammerhead or Surf PI. Either that, or copy the PI-1 circuit in the book.

Regarding the design you posted:
1. The coil is shown as 500mH. This should typically be somewhere between 300uH and 500uH. i.e. it's at least 1000 times too large.
2. The feedback resistor is shown as 3m, which means 3 milli-ohms. SPICE is not case-sensitive, so you must use either MEG or meg for mega-ohms.
3. Assuming the feedback resistor is supposed to be 3MEG, then the gain (8333x) of your preamp is way too high. You should not go beyond 1000x, as you are restricted by the gain-bandwidth of the opamp.

[brucester]

Thanks Q.

1) The PI 1 circuit uses a PMOS, would rather use N like most people. I also avoided directly copying the surf and others out of fear of patents and copyright etc, not sure how all that stuff works.

2) I should have mentioned to ignore the coils inductance. The sim only shows curves for high inductance coils. If i make it 400uh, the sims oscilloscopes show only a tiny spike and not the curve.

3) As for the 3M resistor, falstad circuit sim displays 3M if you type 3000000 into the resistors value. I copied it from the russian pirate preamp. I changed it to 360k and get a better result.

Thanks again.

[Pete the Builder]

G'day Brucester!

I'm in pretty much the same predicament as you are. I wanted a nice, well-behaved, fairly easy to understand (ha!) front-end that I could stick on to a fast MCU with plenty of memory. And after reading a couple of dozen threads here, I knew I did NOT want to reinvent a wheel! There's just too much to screw up without realising it. At least, for me, anyway! YMMV! But it seems that in this field (no pun intended) a little knowledge can put you well and truly on the wrong track!

I opted for a variation on BugWhisperer's old UPIM front end, it was designed from scratch for a uP, it's very simple (just 2 op-amps) and gives the MCU a very nice slope to look at and sample. OTOH, Tinkerer's design is well-discussed and designed, but it may not exactly suit your needs!

I will be interested to hear what other advice you get, and what you end up selecting... There are literally dozens of designs here, but they are all terribly hard to find, at least the latest updates are, and many have been superseded in the past 2 years or so. That doesn't make them bad, and it does allow you to try a couple of ideas out that you know will work, and choose the best one. You can then either just use it, or develop it further. There are some incredible talents here, so it's a great place to ask questions.

I hope this wasn't too long, too unhelpful, or too confusing. But I (and I suspect quite a few other) newbs would be very interested to see how you go. No pressure.

Good luck with your design. Keep us updated, please?

Cheers,
Pete

[brucester]

Thanks Pete,

I will check out the designs you suggested. Cant really do much till my oscilloscope arrives because i am getting some really weird readings from my analog reads on my uC. Once i have my scope and my front end i can do what i do best, code

Cheers