[time-nuts] Designing and building an OCXO and GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Aug 12 02:06:58 EDT 2008


The attached circuit schematic is for a 10MHz oscillator using a 
fundamental mode AT cut crystal.

The problem with using a common base buffer with a fundamental mode 
crystal is that the crystal ESR is rather low so that the real component 
of the CB stage input impedance is not much less than that of the 
crystal. This reduces the crystal loaded Q significantly unless the 
common base stage is run at a high collector current which increases the 
CB stage output current noise.
The crystal buffer shown here (Q103+Q104) reduces the real part of the 
input buffer stage input impedance to a value much less than the ESR of 
the crystal without using a large collector current.

The oscillator transistor is in a Colpitts configuration. The transistor 
is only on for part of the cycle. The resistor (R102) connected between 
the emitter and C101+C102 junction reduces the phase noise of Q101 when 
it is on. The crystal current is determined by the dc collector current 
of Q101 and can be varied by adjusting R103.

R103 is adjusted to set the crystal current to about 1.8mA (+13dBm 
output in a 50 ohm load.)

The output stage uses an 18V supply so that it can drive an open circuit 
load without saturating the output transistor.
This supply voltage can be reduced if a differential CB output stage is 
used OR

1) The output device is allowed to saturate when driving a high 
impedance load

2) The output transformer turns ratio is reduced

3) A lower output than +13dBm is acceptable.

Since the phase noise floor is determined by the phase noise floor of 
the buffer amplifier some care has been taken to ensure that the buffer 
amplifier phase noise floor is low.

Power supply noise will increase the phase noise of the amplifier and 
oscillator so a low noise power supply is essential.

At low offset frequencies the crystal and the oscillator stage start to 
contribute to the phase noise.
Thus  the oscillator phase noise also needs to be low for low phase 
noise close to the carrier.

It can be advantageous to use transistors with low collector base 
capacitances as their PM and AM noise can be made very low with RF feedback.
The major source of flicker PM and flicker AM  noise in an amplifier is 
modulation of the various device capacitances by low frequency noise 
currents or voltages.

Thus low noise power supplies and low noise components should be used.
Only use metal film or thin film resistors.
No thick film, carbon resistor, metal oxide resistor should be used as 
they have significant flicker noise when a dc current is flowing through 
All coupling caps should be NP0/C0G or equivalent.

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