Re: 74HCU04 LC Osc

This triggered a 15+ year old memory... Back then I needed a 110-120
MHz
clock generator (TTL levels); I did it using a 74AS00 (or was it
04?...), an inductor
I had routed on the PCB, a crystal - I had to locate something suitable
to work
between 115 and 120 MHz at 5-th, and a tiny trimcap... Obviously each
unit
had to be trimmed so the LC frequency would match the crystal, the
"lock"
was easily identifiable on a scope. The footprint was that of a typical
metal
crystal oscillator (DIP-14 corner pins only style), the height somehat
more.
It worked and managed to stay "locked" to the crystal over a pretty
wide
temperature range (that AS or F chip was quite an oven, especially
at about 120 MHz... :-).
Just a related memory, hopefully on something mad enough to be worth
remembering :-).

Dimiter

------------------------------------------------------
Dimiter Popoff Transgalactic Instruments

http://www.tgi-sci.com
------------------------------------------------------

Jim Thompson wrote:
On Mon, 09 Oct 2006 01:05:07 GMT, "John - KD5YI"
<groups5MUNGE@xxxxxxxxxxx> wrote:


"Andrew Holme" <andrew@xxxxxxxxxx> wrote in message
news:egb173$fkc$1$8300dec7@xxxxxxxxxxxxxxxxxxx
I built this 5 MHz oscillator using a Toko KANK4174 inductor, which is
specified as having a Q of 100:



74HCU04
|\
.------| >O-----o----
| |/ |
| .-.
| | |
| | | 3k3
| 4.8uH '-'
| ___ |
o------UUU------o
| |
--- ---
--- 390p --- 390p
| |
| |
=== ===
GND GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Peak-to-peak voltage at the output is about 3V. Peak-to-peak voltage
across either capacitor is about 0.5V

The 3k3 forms a potential divider with the impedance across the capacitor:

0.5 / 3 = Z / (3k3 + Z)
Z = 660 ohms

Dynamic impedance across the capacitively-tapped tuned circuit = Rp = 4*Z
= 2640 ohms

Q = Rp/wL = 2640 / (2*pi*5e6*4.8e-6) = 17.5

Why is my calculated Q so low?

TIA
Andrew

Hi, Andrew -

Is the inductor's Q specified at 5 MHz? Be sure to account for the different
intrinsic skin effect, if not.

I have played with your circuit in LTSpice but I cannot reproduce your
voltages. Inductor Q at 5 MHz may have something to do with it, but also
Trise, Tfall, Tdelay of the inverter has a large effect. I used the Philips
data *** to specify those items and then adjusted them to get your values.
I assumed Vcc of 3V since the inverter in LTSpice is a behavorial one and
your output was 3V. What instrument did you use to measure the voltage? What
was the probe impedance?

If you want the LTSpice netlist, let me know. Good luck.

John


In PSpice, assuming QL=100, the effective Q is ~45.

A real 74HCU04 (I have the device-level models) is quite good, and has
a barely noticeable effect at 5.2MHz.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

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