User Manual
UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
13
is 1ppm/ºC. This voltage passes through a trimpot that is flanked
by two fixed resistors, to narrow the range of the trim voltage for
greater precision when trimming the VCO.
The tempco of the trimpot
R7
is considerably higher than that of
the fixed resistors
R6
and
R8
. By choosing resistor values for
R6
and
R8
that are much (>10x) larger than that of the trimpot
R7
, the
poor tempco contribution of the trimpot is minimized.
We improved the trimpot of the original design via improved
tempco components. Our initial choice for
R6
,
R7
and
R8
was
15k
Ω
, 1k
Ω
and 15k
Ω
, respectively.
PCB Design
DJW designed a new PCB that was drop-in compatible with the
original PCB in S/N ‘140, and compatible with all Model 3xxx
internal oscillators. We followed Connor Winfield’s AN2093
OCXO Layout Guidelines
and other web resources.
The PCB has two layers, with soldered-in 4-40 threaded standoffs
in the corners. Because the OCXO131-40 is quite large relative to
the available PCB real estate, ¼” long standoffs were used, to
make room for some of the taller components in the design (
L2
,
U1
&
C1
) to be placed on the underside of the PCB.
The trimpot
R7
was placed in close proximity to OCXO
U4
’s metal
can, and resistors
R6
and
R8
were placed close to the trimpot.
These four components were placed in close proximity so that they
can be thermally coupled via a thermal encapsulating epoxy.
Trimpot
R7
was placed in a position that makes it easy to adjust
the OCXO when installed in a Model 3xxx. MMCX jack
J2
is also
relatively accessible. Barrel jack
J3
is also on the edge of the PCB,
and can be used to power the circuit.
Note
It is not recommended to power the circuit when it’s also
connected to the Model 3xxx via
J1
. It can be used when the PCB
is installed into the Model 3xxx, it’s not connected via
J1
, and test
equipment is measuring the circuit’s output via
J2
.
A few test points are provided to measure the VCO trim voltage,
the post-regulator 12V, etc.
Содержание UM-20
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