Constructional Notes for G8ACE MKII OCXO – January 2013

Constructional Notes for G8ACE MKII OCXO – January 2013

Constructional Notes for G8ACE MKII OCXO – January 2013
Use the notes and drawings in this document during OCXO construction. When fitting components to the PCB
its best to start with the smallest parts first. A surface mount tuning trimmer is used for tuning but a small
ceramic piston tuning trimmer will still fit the PCB pads if its preferred to not use varicap tuning. Inductors L3
and L4 will obstruct the soldering of larger piston trimmer and so it should be fitted first. Some piston
trimmers have an extended metal tab that needs careful shortening before fitting to the board. Fine frequency
tuning of the OCXO is best done with an external good resolution potentiometer (not supplied) wired between
tin plate box ground and the regulated dc output feed through. The pot wiper being connected to the varicap
tuning control feed through When using a PLL the dc feedback control is connected to the varicap feed through
and the dc output is ignored. The M3 threaded hole thru the heater plate needs a small 5mm square piece of tape
applying over it on the side placed against the PCB before epoxy is applied. This prevents the epoxy flowing
into the hole and blocking the threads used for securing the TIP31A heater transistor Q7. Ensure you use an
epoxy with a suitably high working temperature when attaching the heater plate to the PCB. Araldite precision
blue and white tubes has been found satisfactory for gluing the plate to the board. Only the thinnest smear of
epoxy is required and the plate and PCB must be secured together with sticky tape or other means until the
epoxy cures to avoid positional movement during setting. Inductors L3, L5, L7 are wound using the supplied
0.5mm wire on a suitable 2mm diameter, a 2mm former such as drill bit. Deviation from 2mm will change the
number of turns needed. Use the table of coil turns/frequency as a guide for the crystal frequency in use. The
inductor lead out connections should be bent to give a 2mm clearance from the inductor to the PCB. L5 and L7
are stretch/squeeze tuned for maximum output. L3 is left tightly wound. A minimum of +4dbm will be
obtained with correctly squeeze tuning to inductors L5, L7. Initial oscillator testing can be performed without
the heater plate or Q7. The TIP31A heater transistor with its insulator, the thermistor and the crystal should all
be smeared with heat sink paste during assembly to ensure good thermal conductivity. The plastic fixing screw
insulator used for Q7 needs shortening to fit Q7 tab thickness by carefully rubbing on a fine file. Check Q7
collector is not short circuit to the heater plate once fitted. A suggested layout arrangement for the sma socket
and connections through the tin plate box again can be found on page 4. An additional hole for piston trimmer
tuning is needed when used. Locate the feed through capacitors away from the tuning hole position. When
drilling the box ensure a wooden or similar block is used to support the box side preventing distortion. The feed
through capacitors should be fed through the small earth tags supplied to help support the feed through. The
earth tag also serves for the ground connection for the service on that particular connection point. The sma
connector may need the ptfe insulation and inner conductor shortening before fitting to the box. The finished
OCXO module should be supported firmly in polystyrene or similar thermal insulation inside the box. If the
module is used in an outside installation then an extra external layer of insulation is beneficial. Do not locate
the OCXO next to a hot heatsink in your equipment which might affect oven temperature control. The fine
frequency control potentiometer can be routed back to the shack if required using two core plus screen cable.
The screen ground connection must be made only at the OCXO end and be fully floating elsewhere. Otherwise
module OCXO supply with heater supply current flowing in the control wire braid will modulate the tuning
voltage. The OCXO can be used with the RDDS PLL and other PLL circuits. Visit my website for RDDS
details.
The OCXO will operate down to just below 11.5v dc supply voltage for trouble free use when operating
portable from 12v battery supplies. In normal operation the dc supply should be kept moderately stable. The
any noticeable degree. However larger supply changes must be avoided as the oven temperature will fluctuate.
OCXO oven will tolerate a few hundred millivolts supply voltage change without moving the frequency to
A simple external regulator such as a 1 Amp 78L12 or better a low drop out type should be used for varying or
unstable dc supplies. Startup current is around 0.5A once the heater has stabilised it will drop to around 100mA
depending how good the box insulation around the pcb is. Better insulation equals a lower current demand.
The crystal for this OCXO should be HC49/U size to fit the heater plate, 5th overtone series resonant AT cut for
OCXO use at 60C with turnover point of 60C and calibrated at 60C. Capacitance 30pf. Generally other
parameters are normally set by the crystal manufacturer for amateur priced crystals.
Oven temperature setup is by measuring for the lowest output frequency versus temperature setting, that sets
operation at the knee point in the first instance but the temperature can be modified slightly to minimize the
ageing. Alternatively set RV1 to 6.3v at TP1 which will give a starting temperature of 60C. After temperature
adjustment is complete correct the precise frequency by tuning CV1 and adjustment of the varicap voltage
control. The varicap control voltage should ideally be in the range 4 to 5 volts.
Important, C17 should be omitted for beacon use to prevent slowing of rise and decay times of F1 keying.
If the varicaps are unused then use two 4K7 resistors externally between ground and the dc output to bias them
to around 5 volts. The varicap connection must not be left dc floating or just grounded when unused.
Completed OCXO Module
The heater plate should be connected to PCB ground by adding an insulated wire (to avoid shorting the tab on
the TIP31A) from the earth tag under the Tip31A fixing screw to the hole PCB top left in the same picture.
Note, the smd tuning trimmer capacitor can be replaced with a tubular ceramic trimmer when fine mechanical
tuning is required rather than fine external electrical tuning or PLL use. C17 omitted in this picture.
Suggested positions for feed through and sma socket shown below. Modify feed through positions as
appropriate for when a tubular trimmer tuning hole adjustment access is needed.