Message 29936 in TECH.WW
From: G8MNY
Date: Sun, 13 Sep 26 08:17:00 Z
Newsgroups: TECH.WW
Subject: Simple Crystal Oven
Message-ID: <4989_GB7CIP>
Path: N2MH4|VE3KPG|GB7CIP
R:260913/0820Z 37124@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260913/0820Z 831@VE3KPG.#ECON.ON.CAN.NOAM BPQ6.0.25
R:260913/0817Z @:GB7CIP.#32.GBR.EURO #:4989 [Caterham Surrey GBR] $:4989_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Nov 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here is a design revamped from an old Radcom, I have used it with some success
on a commercial MW Tx to stabilise the frequency. Other uses are for frequency
counters and UHF/SHF Transceivers & Transverters where good frequency accuracy
and stability are important.
THERMAL COMPENSATION
Before making an oven try to compensate the Xtal osc with negative temperature
coefficient capacitors instead of existing caps in the osc circuit.
TEMP Net Xtal Measure the frequency e.g. on a SSB Rx. Heat the
1 2 3 4 5 6 7 8 kHz drift
When you have the best compensation you can get, make an oven around both the
Xtal and the compensation cap.
The aim of the oven is not to cook the Xtal osc and its components, but to
maintain a temperature slightly higher than the hottest running conditions, and
by doing this eliminate frequency drift due to slow warm up & ambient
temperature changes.
OVEN CIRCUIT
This is quite simple. But mechanically quite important to get a few things in
the right place.
R1 R2 Vcc The negative temperature coefficient
MOUNTING
Over component view Side view
R2 is the main heating component and limits the current and power the heater
takes, it therefore determines the warm up time. Typical value 100R 2W for a
12V rail.
After a short time as everything has started to warm up the NPN darlington then
becomes the main source of heat as the base drive is reduced.
Resistances of R1 & TH1 determine the operate temperature, R1 turning on the
NPN darlington and TH1 turning it off. To find a suitable value for R1, heat up
resistance.
Then some simple maths for
(Vcc - Vb) 12V and a darlington gives x9
Vb a starting value for R1
Do a test without the cover to check it is all functioning and that the current
goes down (NPN collector voltage goes up) when the lot is hot. Make sure it is
not too hot (increase R1 if it is) that it could melt the foam cover material.
Then make a foam polystyrene heat cover for it all, to dramatically cut down
the power consumption and maintain a steadier temperature. Higher temperature
foam can be made in a mould from expanded foam (cavity sealing/thermal
insulation etc.).
See also my related tech buls, "Crystal Drift Compensation",
"198kHz Off Air Standard", "Off Air Lock for Ref Osc",
"Comparing Off Air Freq Standards", & "Calibrating Frequency".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
← Return to Bulletin List