buffer for oscillator circuit

**pc88** · 29th August 2007 07:48 PM

I've built this oscillator for determine the resonance of LC tanks:

http://www.qsl.net/iz7ath/web/02_bre.../fig/fig08.gif

except that I didn't implement the NAND gate buffer. Instead I'm trying to use just a 2n7000 N-FET (common source configuration with a 10K drain pull-up). The problem is that I'm seeing substantial glitches (~ .5 V) on the up transitions and the rise time is now very noticable. The high-to-low transitions are fine.

I'd like to square up the output signal as much as possible without using another CMOS IC. Any suggestions? Should I use a different transistor? smaller pull-up? add a cap somewhere?

For reference, here's the associated article for the schematic:

http://www.qsl.net/iz7ath/web/02_brew/21_LCMeter01/

**audioguru** · 29th August 2007 10:55 PM

Use a 15V supply so the 4011 will be very fast.
Connect the output capacitor C6 to the output of U2b.

**pc88** · 30th August 2007 12:02 AM

Originally Posted by audioguru

Use a 15V supply so the 4011 will be very fast.
Connect the output capacitor C6 to the output of U2b.

Thanks for the pointers, but I'm not using the 4011. I've replaced it with a 2n7000, and I'm getting about .5 V overshoot on the up-transitions (but no ringing). Also, FWIW, I'm using a 5V supply. I'd like not to resort to a 14-pin IC, although any CMOS Schmitt trigger gate would work very well.

I've change it to a source follower, and I'm getting a much cleaner signal, although it's only 2.5V signal instead of a 4.5V signal. Still, I'd like to understand the issues around the original circuit. I'm pretty sure the overshoot is causing extra counts to be registered by my frequency counter.

**audioguru** · 30th August 2007 01:58 AM

The pullup resistor on the LM311 is 1k so that is about as good as it will get without having a push-pull IC like a logic inverter.

**boxer4** · 30th August 2007 02:27 AM

Use an 8-pin low voltage (i.e., not 741) op-amp with fairly high gain?

Not sure why you don't want the 4011, frequency too low or something against CMOS ICs?

**pc88** · 30th August 2007 06:15 AM

Originally Posted by audioguru

The pullup resistor on the LM311 is 1k so that is about as good as it will get without having a push-pull IC like a logic inverter.

okay - thanks... now I can stop pulling my hair out :-)

boxer4 - nothing against the 4011, expect that in this case I don't want to use that much real estate.

**Hero999** · 31st August 2007 09:09 PM

Use an SMT 4011 if you haven't got enough room for the through hole DIP.

**pc88** · 1st September 2007 06:07 AM

I also have found that there are single gate chips available now:

http://www.standardics.nxp.com/products/ahc/1g/

**Hero999** · 1st September 2007 10:24 AM

I thought of that but you need four NANDs not one.

**pc88** · 1st September 2007 12:25 PM

Do you think so? I know the schematic calls for four, but they are just chained together presumably only to make sure their inputs aren't floating.

The use of R6 is a little puzzling, and I can't think of a good reason for it.

**blueroomelectronics** · 1st September 2007 01:49 PM

I've seen that LC oscillator before, used a PIC as an LC meter. What are you connecting it to?

**Hero999** · 1st September 2007 02:03 PM

Originally Posted by pc88

Do you think so? I know the schematic calls for four, but they are just chained together presumably only to make sure their inputs aren't floating.

The use of R6 is a little puzzling, and I can't think of a good reason for it.

I agree, a single gate IC is probably an ideal solution.

**pc88** · 1st September 2007 03:54 PM

Originally Posted by blueroomelectronics

I've seen that LC oscillator before, used a PIC as an LC meter. What are you connecting it to?

On the input side I'm connecting it to various LC tanks I'm toying with, the present one being an antenna for lightning detection and direction determination. On the output side I'm just connecting it to a frequency counter.

**Roff** · 1st September 2007 04:20 PM

Originally Posted by pc88

On the input side I'm connecting it to various LC tanks I'm toying with, the present one being an antenna for lightning detection and direction determination. On the output side I'm just connecting it to a frequency counter.

If your counter is sensitive, as most are, just add an attenuator to isolate the counter loading from the LM311 output, as below.

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