Help with function signal generator and frequency counter - Page 1
Forum section: electronics / beginners

author: Xenix
author_rank: Newbie
timestamp: October 02, 2016, 03:10:42 am
content: Hi All,I am a beginner level electronics hobbyist and I recently purchased a cheap Chinese function signal generator kithttps://goo.gl/Zg2diqand a frequency counter kithttps://goo.gl/Jm5a95, I was hoping I could use the frequency counter to measure the output of the function signal generator but I seem to be having some trouble as the frequency counter reads 0.I tested the function signal generator with my oscilloscope to verify it was working correctly and found the sine wave has a dc offset.After analyzing the schematic of the frequency counterhttps://goo.gl/8zM4sqI noticed pin 3 of the micro controller is connected to R3 (a 10k pull up resistor), also I think the orientation of D1 is so when the sine wave falls below 0v (not taking into account the diode forward bias voltage) it will pull pin 3 of the micro controller low.So my theory is because of the dc offset of the function signal generator; Pin 3 of the micro controller in the frequency counter will never be pulled low thus never be able to increment a counter in software.Would someone be able to verify my theory is correct?Cheers,Sean

author: ataradov
author_rank: Super Contributor
timestamp: October 02, 2016, 04:02:25 am
content: Quote from: Xenix on October 02, 2016, 03:10:42 amI would say that you are absolutely correct.

author: danadak
author_rank: Super Contributor
timestamp: October 02, 2016, 12:55:27 pm
content: If the frequency counter spec is down to DC then it is DC coupled, andoffset could be a factor. AC couple it with a cap to remove offset effects.Also measure freq cntr input V, they may have biased it up to 1/2 itspower supply voltage, which then makes ac coupling into cntr work. Ifits not biased use a couple of equal value Rs to bias it up and ac coupleits input.Regards, Dana.

author: 2N3055
author_rank: Super Contributor
timestamp: October 02, 2016, 02:04:10 pm
content: On the right side of counter schematic there is a transistor preamp for input.. You need to move J1 to the right and it will connect it..But asataradovsaid you are correct you do have large Dc offset, so you need a capacitor on the input..Also asdanadaksaid, you could put a potentiometer and few resistors and make a variable level input..But I would start with moving J1 to position with transistor preamp, and adding 0.1 uF between Pin1 on JP1(header 3) and source... That should do it..

author: Xenix
author_rank: Newbie
timestamp: October 03, 2016, 04:31:15 am
content: I seem to have it working now,The preamp on the right hand side of the schematic is for testing crystal oscillators.I managed to get it working by building the below circuit (please excuse my crude MS paint schematic)At first I built it without a 10k pull down resistor on the base of the NPN transistor and after some testing and probing around because it wasn't working I figured the base was floating or something else was going on, maybe the transistor doesn't like negative voltages on the base??? so I put in the 10k resistor and BOOM it started workingHere is a picture of the setupThankyou for your help everyone I have learnt a lot from playing around with this!Regards,Sean