Ceramic Resonator Questions - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: June 29, 2018, 02:04:11 pm
content: Hi,I'm building a circuit with a ZTT 20.00 MX and had some questions regarding the frequencies I was getting. The datasheet says that MX's are 13.01-50.00MHz and that the "20.00" indicates a 20.00 MHz frequency. However, in the circuits I built, the largest frequency I got from the attached schematic was 4.4MHz. The 4.4MHz was the value of the circuit where the dampening resistor was left off. I was expecting a higher value. I'm using 5V to power the CD40106BE Hex Schmitt Trigger IC.I'm also wondering if the circuit I built with the 50k dampening resistor (shown in the schematic) is correct for the output of my scope (270kHz shown in scope screenshot).Thanks,Carl

author: RobK_NL
author_rank: Frequent Contributor
timestamp: June 29, 2018, 02:22:20 pm
content: Standard CMOS logic as very slow, certainly at 5V. The delay from input to output for this part is 140ns typical, which is already almost 3 times longer than the period of a 20MHz signal.If you replace the 40106 with a 74HC(T)14, you'd get much better results.

author: eev_carl
author_rank: Regular Contributor
timestamp: June 29, 2018, 02:39:55 pm
content: Thanks! I'm looking for Propagation Delay Time on the datasheets, right? It's 38ns for the SN74HCT14NE4 and 280 ns for the CD40106BE.Is the CD40106BE for the circuit in the scheamtic correct? I have a dampening resistor of 50k which brings the frequency down to 260kHZ (period = 3.8us > 280ns).

author: RobK_NL
author_rank: Frequent Contributor
timestamp: June 29, 2018, 02:52:47 pm
content: Hi,Yes, Propagation Delay Time is indeed the term you're looking for. That 38ns you mention is probably the maximum over temperature. At 25°C they are normally about twice as fast.That dampening resistor is way too high in value for the frequency concerned. Somewhere between 250 and 500 Ohms would be much more appropriate. I even doubt if it is necessary at all for use with a resonator. For a quartz crystal yes.

author: Benta
author_rank: Super Contributor
timestamp: June 29, 2018, 04:09:23 pm
content: Two comments:1: DON'T use Schmitt triggers for crystal or resonator oscillators. It's the certain path to failure. The resonator needs a linear inverter (think sine wave).2: 40106 at 20 MHz?Even a 74HC04 is marginal at best. Go for a 74AC04.

author: eev_carl
author_rank: Regular Contributor
timestamp: June 29, 2018, 07:44:37 pm
content: A Schmitt trigger is for slow, noisy edges and not a fast application like this?

author: glarsson
author_rank: Frequent Contributor
timestamp: June 29, 2018, 08:57:59 pm
content: Quote from: eev_carl on June 29, 2018, 07:44:37 pmYes, but also that the schmitt will give you a square wave. You don't use them when you need to get a quartz crystal or ceramic resonator to oscillate. Then you want a sine wave. Compare to making a pendulum swing nicely. You push it nicely at the frequency it wants to resonate at, not smash it back and fort with a tennis racket.

author: Benta
author_rank: Super Contributor
timestamp: June 29, 2018, 09:11:51 pm
content: Quote from: glarsson on June 29, 2018, 08:57:59 pmThat's one of the greatest analogies I've read.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2018, 06:37:19 am
content: Yeah, you want a 74HCU04 in fact. HC is buffered, meaning it has more delay and very high gain. That's a good recipe for making an oscillator, but not at a desired frequency...HCU is unbuffered, available in a few special types like '04, literally just a pair of transistors per section. The linear range is wide and the gain is modest, making it very good for oscillators.Tim