Charles Eisnor wrote:
> Hi list members, > > I've a question on what "should have been" a truly simple circuit. I'm > trouble-shooting the electronic side of a gas chromatograph, all computer > controlled. I've got power but the clock signal is not there. Well it's there but > as a 1.3 V p-p sine wave of the correct frequency (7.09 MHz). That's not going to > clock anything. I know it must be a square wave. The manufacturer is using 3/6th > of a 74LS04 with the crystal across pins 1 and 4. There is a 1K resistor across > pins 1 and 2 , a 91 ohm resistor between pins 2 and 3 and a 1K resistor across > pins 3 and 4. There is a 56 pF cap from pin 3 to ground. Pin 4 drives another > inverter, pin 5. So the output I would expect should be a square wave of about > logic voltage from pin 6. But its not. Its a sine wave of about 1.4 Vp-p. I even > breadboarded the circuit and I'm still getting a sine wave. > > I'm quite green when it comes to crystals. So what am I doing wrong? And more > importantly do you have any suggestions on how I might square up the problem. I'm not sure anything is wrong. Crystals don't pass DC so the circuit must be biased into the linear region to work. This will give you a distorted sinusoid looking signal. If it is oscillating at the correct frequency it probably is working correctly. Your breadboard supports that theory. You don't say what you are using to make your measurements. It is generally not a good idea to probe either side of the crystal because of the circuit's extreme sensitivity to parasitics (like the scope probe). If you must probe the crystal, the probe should not be less than a high quality X10 probe. Even with the X10 or you will see the type distortion you describe. My advise is to stay away from the crystal and measure the signal down stream of the oscillator. I suspect you have not correctly located the source of your problem. Fred Townsend DC to Light Insultants > > > Many thanks for your time. Its greatly appreciated. > > Chuck Eisnor > > -- > Charles R. Eisnor > Service for Laboratory Equipment & Instrumentation > http://www.scientific-electronics.com > ph: 902-681-0998 > fax: 902-681-1276 > > -- > Author: Charles Eisnor > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). -- Author: Fred Townsend INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
