Pumpkin UM-20 User manual

UM-20
User Manual
750 Naples Street • San Francisco, CA 94112 • [phone] • http://www.pumpkininc.com
last updated on Jun 16, 2017
All trademarks mentioned herein are properties of their respective companies
Low-cost Oven-controlled Crystal
Oscillator (OCXO) Assembly
Copyright 2013-2017 Pumpkin, Inc. All rights reserved.
Specifications subject to change without notice.

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2 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
TRADEMARKS
The following are Pumpkin trademarks. All other names are the property of their respective owners.
• Pumpkin™and the Pumpkin logo
• Salvo™ and the Salvo logo
• CubeSat Kit™ and the CubeSat Kit logo
DISCLAIMER
PUMPKIN RESERVES THE RIGHT TO MAKE ANY CHANGES WITHOUT FURTHER
NOTICE TO ANY PRODUCTS HEREIN TO CORRECT ERRORS AND IMPROVE
RELIABILITY, FUNCTION, APPEARANCE OR DESIGN. PUMPKIN DOES NOT
ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY
PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY
LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
744 Naples Street
San Francisco, CA 94112 USA
tel: [phone]
fax: [phone]
web: http://www.pumpkininc.com/
email: [email protected]
web: http://www.cubesatkit.com/
email: [email protected]

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UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
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Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly ................... 1
CHANGELOG ..........................................................................................................................5
Purpose ......................................................................................................................................6
Personnel ...................................................................................................................................6
Introduction ...............................................................................................................................6
OCXO Replacement..................................................................................................................7
OCXO Design .........................................................................................................................10
Critical Components .........................................................................................................10
OCXO131-40 .............................................................................................................11
Barrel Jack and MMCX Connector............................................................................11
High-quality Trimpot .................................................................................................11
Ultra-low Tempco Resistors.......................................................................................11
Circuit Design ...................................................................................................................12
12V LDO....................................................................................................................12
Op-amp Circuit...........................................................................................................12
OCXO.........................................................................................................................12
VCO Trim .................................................................................................................. 12
PCB Design ................................................................................................................13
Usage.......................................................................................................................................14
Frequency Survey .............................................................................................................14
Assembly ..........................................................................................................................14
Module Applications at Pumpkin ..................................................................................... 15
Use in XL Microwave Model 3xxx......................................................................................... 16
Installation ........................................................................................................................16
Frequency Tuning ............................................................................................................. 16
In-place Tuning.................................................................................................................17
Use in SRS DS345 .................................................................................................................. 18
Installation ........................................................................................................................18
Frequency Tuning ............................................................................................................. 19
In-place Tuning.................................................................................................................19
Usage.......................................................................................................................................20
DC Voltages......................................................................................................................20
Loading vs. Frequency......................................................................................................20
Frequency Stability ...........................................................................................................20
Warmup / Initial Behavior ................................................................................................21
Oscillator Outputs ............................................................................................................. 21
Summary .................................................................................................................................22
Future Work / Enhancements ..................................................................................................22
Attachments.............................................................................................................................23

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4 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Figure 1: GPS time and frequency reference receiver ..................................................................... 6
Figure 2: XL Microwave Model 3120 frequency counter ............................................................... 7
Figure 3: IsoTemp OCXO36-44, the "original" Option 115 OCXO for Model 3xxx frequency
counters ........................................................................................................................8
Figure 4: Top view of assembled module...................................................................................... 14
Figure 5: Bottom view of assembled module ................................................................................14
Figure 6: OCXO PCB assembly installed in Model 3xxx ............................................................. 16
Figure 7: OCXO PCB assembly installed in DS345...................................................................... 18
Figure 8: OCXO36-44 waveform (blue) compared to Z3805A GPS-disciplined waveform
(yellow) ...................................................................................................................... 21
Figure 9: OCXO131-40 waveform (blue) compared to Z3805A GPS-disciplined waveform
(yellow) ...................................................................................................................... 22
Table 1: MTI OCXO-based oscillator module from SN '142.......................................................... 9
Table 2: MTA100 connector pinout to mate to XL Microwave 3xxx ........................................... 10
Table 3: Frequency survey of OCXO131-40 units, installed on 705-01667A PCBs .................... 14
Table 4: Applications of Rev A OCXO Modules .......................................................................... 15
Table 5: OCXO131-40 frequency tuning range as a function of R6 & R8 ..................................... 17
Table 6: OCXO module pinout to mate to SRS DS345................................................................. 19

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CHANGELOG
Rev. Date Author Comments
A 20170512 AEK Initial version.
B 20170608 AEK
Added images of modules, included initial V_TRIM values, added more
explanatory text.

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6 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Purpose
This manual describes the design and performance of a small and
inexpensive oven-controlled crystal oscillator (OCXO).
Personnel
Andrew Kalman (AEK) purchased the frequency counters and
replacement oscillators for Pumpkin, collected information on a
new OCXO design, conferred on nuances of oscillator circuits, and
tested and characterized the OCXO.
David Wright (DJW) reverse-engineered the XL Microwave
OCXO module, created the schematics and PCB for the OCXO
module
Introduction
Like many hardware developers, Pumpkin utilizes various
laboratory instruments as part of hardware development and
experimentation. Frequency counters, waveform generators,
spectrum analyzers, time interval counters and other instruments
need accurate and stable oscillators.
An external frequency reference – like a 10.000000MHz signal
from a GPS-disciplined oscillator1– is a popular and relatively
inexpensive, stable and accurate frequency reference to distribute
within a laboratory, for use as an external frequency standard for
compatible instruments.
Figure 1: GPS time and frequency reference receiver
1E.g. the HP Z3805A GPS time and frequency reference receiver. This unit was
often used in cellular towers; these units were often sold on eBay once their
stated performance had expired. Upon first power-up and without a connected
GPS antenna, it was possible to identify the location of the cell tower where
the unit was located; the ones Pumpkin purchased were all from Chinese cell
towers.

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UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
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Using an external frequency standard physically ties an instrument
to that standard, and makes it less portable, etc. We wanted to have
an inexpensive, desktop frequency counter with an input range of
up to ca. 100MHz, with a relatively accurate and stable internal
oscillator. Additionally, we wanted to become more
knowledgeable on the matter of oscillators.
OCXO Replacement
The XL Microwave line of microwave frequency counters satisfied
our frequency counter requirements, and provides a high-frequency
power meter, to boot. These frequency counters have a
1/2/5/10MHz external reference oscillator input, and a 10MHz
reference output. Via eBay and other sources, Pumpkin purchased
the following models in good to excellent condition:
• Model 3030-6 Options: D S/N 9508233572
• Model 3120 Options: 115 S/N 9810253903
• Model 3080 Options: 115 S/N 1208251424
Figure 2: XL Microwave Model 3120 frequency counter
By comparing this particular Model 3080’s reference output to a
GPS-disciplined 10MHz, we discovered that its internal OCXO
was wildly out-of-range at 9,887,XXXHz.
Option 115’s OCXO (the second of four progressively better
internal oscillator options for these frequency counters) has the
following performance:
• Aging/second: E-11 root Allan variance
• Aging/day: 5E-10 after 72 hours
2Built in 1995, warrantied by XL Microwave for 2 years until 1997.
3Built in 1998, warrantied by XL Microwave for 5 years until 2003.
4Built in 2002, warrantied by XL Microwave for 5 years until 2007. Unknown why
the S/N code is not consistent with the other two …

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8 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
• Aging/year: 1.5E-7 after 7 days, 1E-7 after 30 days
• Warm-up @25C: 2E-8 in 30 minutes
• Temperature (0 to 50C): 1E-8
• ±10% MAINS change: 1E-9
S/N ‘357 has a 10MHz IsoTemp Research, Inc. Model
OCXO36-44 10.000MHz large OCXO S/N 9051-61 DATE 9552
that can be tuned down to 1E-10 using the method that compares
the drift of its output to the GPS-derived 10MHz signal. The two
screws on the can – marked Coarse and Fine – are covers for the
pots inside ... the OCXO must be unscrewed from the Model
3030’s chassis in order to get access to these screws … very nice.
Figure 3: IsoTemp OCXO36-44, the "original" Option 115
OCXO for Model 3xxx frequency counters
S/N ‘390 has the same oscillator.
Opening up S/N ‘142 revealed that this unit’s internal OCXO has
an MTI 220-0102-A 5.0000 MHz OCXO module S/N 124463 on a
small PCB, with a trimpot for frequency trimming. This MTI
OCXO puts out a 3V (roughly) sine wave; the reference output
could not be pulled to 10MHz with the trimpot when connected via
J4 to the Model 3080’s frequency input; it was, however, tunable
to 5MHz when there was no load on its output. Contacting MTI for
a replacement revealed that each OCXO cost around $500 new, but
there was a $1500 minimum order.

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UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
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Table 1: MTI OCXO-based oscillator module from SN
'142
It appears that at some point (after 1998?), XL Microwave
switched from IsoTemp’s (large) OCXO36-44s to MTI’s smaller
220-0102 OCXOs; the IsoTemp units appear to be of higher long-
term quality and may be double-ovenized, but probably also cost
much more.
Various Internet searches and user forums (especially
http://leapsecond.com/time-nuts.htm) revealed a large community
of tinkerers and engineers with a shared interest in frequency
standards and measurements. For example, Gerry Sweeney built a
nice reference using an IsoTemp OCXO131:
http://gerrysweeney.com/tag/ocxo131/ ... note that he is using an
OCXO100-XXX .. that is a +5V unit. There is also an OCXO191
available (probably 12V). And this has even more detail:
http://gerrysweeney.com/tag/ocxo131-191/.

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10 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Pumpkin purchased multiple IsoTemp OCXO131-40 (12V, sine-
wave output) from an eBay seller for around $21 each and set out
to create a drop-in replacement OCXO for S/N ‘142, and for other
XL Microwave Model 3xxx units.
OCXO Design
S/N ‘142s MTI-based OCXO is mounted on a 2” x 2” 0.062” PCB
using SAE dimensions and fasteners. Four 4-40 captive threaded
standoffs on a 1.5” bolt square enable the module to be mounted to
the Model 3xxx’s chassis via 4-40” flathead screws.
The assembly has a 0.100” pitch 4-position rectangular receptacle
connector TE Connectivity / AMP P/N 3-640440-4 for a few
inches of 22AWG wiring, as follows:
1. O Purple 10MHz output
2. G Black Ground
3. + Red +12V
4. S Yellow Heater status5
Table 2: MTA100 connector pinout to mate to XL
Microwave 3xxx
Note Our 705-01667 PCB’s connector pinout is opposite from
the above … the 22AWG wires are connected to the PCB based on
the O/G/+/S labels.
Critical Components
We decided to create a replacement module with a few
enhancements compared to the original:
• Compatibility with inexpensive (used) IsoTemp OCXO131-40
10.000MHz OCXOs
• An external +12V barrel jack for desktop calibration / testing
• An MMCX connector on the output, for desktop calibration / testing
• A high-quality multi-turn trimpot
• Precision resistors in the trimpot circuit
These components are expanded upon in detail below.
5Not present / not used on the fancier OCXO36-44 and units originally equipped
with them.

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UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
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OCXO131-40
This IsoTemp OCXO module is available at low cost from various
eBay sellers. These units are probably pulled from working
frequency standards, or from other equipment. It can be assumed
that they are well aged. Measuring 36.3 x 27.2 x 19.1mm, they are
a tight fit on the compatible PCB. They are SC-cut, sinewave
output with a fast warmup, and have frequency stability
specifications that are on par with the MTI 220-series OCXO that
was originally in S/N ‘142. Hard to beat for $21 each … units
purchased from eBay often have date codes in the early 2000’s …
Barrel Jack and MMCX Connector
By placing a 2.1mm/5.5mm barrel jack on the OCXO PCB, it’s
trivial to power the OCXO separately from the Model 3xxx
frequency counter. Similarly, by connecting an MMCX jack to the
OCXO’s output, it’s trivial to connect the OCXO output to an
oscilloscope or frequency counter, via an MCX-to-BNC adapter
cable. These two features facilitate initial measurements and
trimming of the 10.000MHz OCXO while on a lab bench. The
barrel jack and MMCX upright jack are located on the PCB so that
they are accessible even when the PCB is installed into the Model
3xxx.
High-quality Trimpot
We chose the Vishay Accutrim™ 1280G 26-turn trimpot with its
15ppm/ºC temperature coefficient (tempco). At $15, it is not
inexpensive, but it is similar to what was on the MTI-based OCXO
originally in the Model 3080.
Ultra-low Tempco Resistors
In an attempt to have the finest possible trim voltage adjustability,
we chose to augment the trimpot with fixed resistors. We chose the
Vishay VCFP series ultra-high-precision resistor with a 0.2ppm/ºC
temperature coefficient. These are 0.01% resistors in a 1206-size
package. At $10 each, they are also not inexpensive, and they are
available only in a limited range of values.
It has been suggested that resistors grouped in single packages
provide optimal tracking and stability, better than (separate)
discrete resistors. One option would be a non-divider version of
Vishay’s SOT-23 MPM-series divider networks.

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12 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Circuit Design
DJW reverse-engineered the original OCXO PCB to create a
reference schematic and design a new PCB. The new schematic
and PCB layers are attached to this manual.
12V LDO
A 12V TO-220 linear regulator accepts 12V power from the Model
3xxx and provides power to the OCXO and an op-amp. The choice
of a 12V regulator is a little odd, as its input is only 12V, and it
cannot therefore regulate its output to 12V. However, most
OCXOs run at 12V or 5V, and we’d be dissipating over half a Watt
if we used a 5V-supply oscillator, so we stuck with this topology,
for use with the 12V IsoTemp OCXO131-40.
Based on our preference for 12V linear regulators, we chose the
LM2940SX-12 low-dropout (LDO) regulator. This part has
overvoltage (up to 26V) and reverse-voltage protection, and has a
low dropout at low currents, in a surface-mount package.
Op-amp Circuit
The original circuit included an op-amp that detected when the
OCXO was no longer in warm-up mode, and drove an output to
+5V to indicate that warmup was complete. While warming up, the
op-amp detected the increased current draw and drove a small FET
to pull the –WARMING signal down to 0V.
We replicated this circuit, with a 180mA trip point, using an
LT1716 precision rail-to-rail comparator.
OCXO
The original circuit included a filter on the OCXO’s +VDC
terminal, as well as a pi filter on the output. We verified that the
trim voltage on the OCXO131-40 is effective over a range of 0V to
5V, centered at 2.5V.
We replicated the earlier circuit’s filters, added an MMCX jack on
the OCXO’s output, and planned for a VCO trim voltage centered
at 2.5V.
VCO Trim
Based on Gerry Sweeney’s OCXO board’s circuit design, we
selected the high-precision Maxim MAX6350 5V regulator to
generate a stable voltage for the trim voltage from 12V. Its tempco

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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
15kΩ, 1kΩand 15kΩ, 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.

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14 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Usage
Frequency Survey
The initial accuracies of the used OCXO131-40s are listed below.6
Initial output
(Hz) after 30
minutes
Initial
V_TRIM
(Vdc)
S/N Date
VCO not connected,
i.e., untrimmed
V_TRIM (V)
required for
10.000000MHz,
output into 50Ω
Current
draw (mA)
after 30
minutes, at
12.00Vdc
1946-88 0223 10,000,001.04 2.37 2.12 110
2140-395 0301 neither assembled nor tested
2140-618 0310 9,999,999.82 2.38 2.49 113
2140-624 0310 9,999,999.19 2.42 2.76 85
2302-1000 0406 10,000,000.04 2.41 2.43 95
4231-0013 0902 9,999,999.55 2.39 2.61 112
Table 3: Frequency survey of OCXO131-40 units,
installed on 705-01667A PCBs
Assembly
Five modules were assembled by hand, using the Pumpkin PCB
and five of the six OCXO131-40s that we acquired. The module
with its attached harness is shown in Figure 4 and Figure 5.
Figure 4: Top view of
assembled module
Figure 5: Bottom view of
assembled module
6An XL Microwave Model 3120 with a GPS DO 10MHz external reference was
used for the frequency measurements; a Fluke 289 DMM was used for the
V_TRIM measurements; an Agilent E3620A provided the V_TRIM voltage and
12Vdc power.

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Module Applications at Pumpkin
Multiple Rev A PCBs were assembled with used OCXO131-40s;
their applications are below.7
Pumpkin
S/N
IsoTemp
S/N Application Notes
101 1946-88
XL Microwave 3030
S/N 950823357
n/a 2140-395
102 2140-618 not installed
First unit to be assembled; trimpot
and precision resistor remain
unpotted.
103 2140-624 SRS DS345
S/N 32542
104 2302-1000 SRS DS345
S/N 30029
Wired for 7-pin connector to U1
(factory OCXO).
Use cal byte 0 to tune – nominal
value is 2980.
105 4231-0013 not installed
Table 4: Applications of Rev A OCXO Modules
Two applications of the OCXO Module are expanded upon below.
7An XL Microwave Model 3120 with a GPS DO 10MHz external reference was
used for the frequency measurements; a Fluke 289 DMM was used for the
V_TRIM measurements; an Agilent E3620A provided the V_TRIM voltage and
12Vdc power.

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16 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Use in XL Microwave Model 3xxx
Installation
As a test, we decided to install the assembled OCXO module with
OCXO131-40 S/N 2140-624 into an XL Microwave Model 3030
frequency counter.
Both the upper and lower covers of a Model 3xxx must be
removed in order to access the screws that hold the oscillator
assembly in place. Four 4-40 x 3/16” flathead screws hold the
OCXO assembly in place, and J1 is connected to a single
connector labeled 10 MHz BUFFER on a nearby PCB.
Figure 6: OCXO PCB assembly installed in Model 3xxx
Frequency Tuning
It turns out that the initial values of 15kΩfor R6 and R8 restricted
the VCO trim range too much on this particular unit to be able to
reach 10.000000MHz. Replacing R6 and R8 with zero-Ohm
resistors yielded a V_TRIM (TP3) of 2.73Vdc to reach
10.000000MHz.
With zero-Ohm R6 and R8, the trim range of the VCO (stated by
IsoTemp to be up to 0.9ppm or 9Hz over the full range) required a
full ±2.5V of V_TRIM at the VCO input with R7 = 1kΩ. To reach a
multi-turn trimpot for
setting V_TRIM
22AWG 0.100” pitch
harness connector to
Model 3xxx 10MHz
oscillator stage

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V_TRIM of 2.76Vdc required R6 and R8 values of under 5kΩ; 1kΩ
resistors were eventually fitted.
R6 & R8 Values V_TRIM Range Notes
0Ω± 2.50V
Entire range yields changes in output frequency of
± 6.27Hz, or around 0.6ppm (within specs).
OCXO131-44 S/N 2140-618 output was
9,999,993.55Hz @ V_TRIM= 0.00Vdc, and
10,000,006.87Hz @ V_TRIM = 5.00Vdc.
1kΩ± 0.83V
S/Ns 1946-88, 2302-1000, 2140-624 & 4231-0013
were trimmed with these values.
2kΩ± 0.50V
5kΩ± 0.23V
10kΩ± 0.12V
15kΩ± 0.08V S/N 2140-618 was trimmed with these values.
Table 5: OCXO131-40 frequency tuning range as a
function of R6 & R8
Realistically, the choice for R6 & R8 boils down to 0Ω
(guaranteeing the greatest possible frequency tuning range) or 1kΩ
(giving a tuning sensitivity that’s 3x greater that that of the 0Ω
configuration, but with less frequency tuning). Values greater than
2kΩfor R6 and R8 are likely to prevent trimming to
10.000000MHz, unless the initial V_TRIM is within roughly
100mV of 2.5V.
Note Different values for R6 and R8 could be used on a case-by-
case basis to optimize the trim range for a given OCXO131-40.
In-place Tuning
The lower cover of the Model 3xxx must be removed in order to
trim the installed OCXO assembly to 10.000000MHz. This
requires that the Model 3xxx be upside-down when adjusting the
OCXO frequency.8
8A carefully positioned hole could be drilled in the lower cover of the Model 3xxx,
to gain access to the PCB assembly’s trimpot without removing the lower
cover.

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18 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Use in SRS DS345
Installation
We installed the assembled OCXO module with OCXO131-40 S/N
2302-1000 into a SRS DS345 synthesized function generator. This
unit was configured at the factory without Option 02 (Internal
OCXO).
Since the Pumpkin OCXO module was not designed for use in the
DS345, we had to find a robust way to mount it. We found that in
the DS345’s rear corner where the various back-panel BNCs are
located, there is enough room to locate one of our modules by
mounting it to one of the sides of the aluminum sheet metal
chassis. We machined two countersunk 82º holes sized for 4-40
flathead screws 1.500” apart, 0.325” from the top edge of the
chassis.
Figure 7: OCXO PCB assembly installed in DS345
The DS345 maintains a chassis ground that is separate from
(digital) ground; since the OCXO module Rev A PCBs connected
the four 4-40-size standoffs to (electrical) ground, we had to isolate
these modules when used in the DS345. The solution was to use
nylon screws, along with nylon washers in the top two mounting
holes.9
9The OCXO module’s lower two captive standoffs simply had the nylon screws
screwed into them until them bottomed out; the net effect is that they form a
captive spacer that insulates the lower standoffs from the DS345’s chassis.
Nylon 4-40 screws and
washers for mounting
OCXO Module Rev A to
DS345 chassis
5-wire 22AWG twisted
harness to 7-pin 0.100”
pitch connector on
DS345 top board
(at right)

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19
Prior to installation, we replaced the four-wire harness that is for a
Model 3xxx with a 5-wire, 7-pin harness suitable for use in the
DS345. The pinout for the 7-pin connector is as follows:
1. - NC +15V (not used)
2. - NC -15V (not used)
3. - White VCO trim from DS34510
4. G Black Ground
5. O Purple 10MHz output to DS345
6. S Yellow Monitor11
7. + Red +15V
Table 6: OCXO module pinout to mate to SRS DS345
N.B. The DS345 supplies +15V (and –15V) to the factory-
installed OCXO. Regulator U1 on the OCXO module is compatible
with +15V input. In contrast to the usage in the Model 3xxx, when
used in the DS345, the OCXO module’s 12V regulator U1 is
actually providing a regulated +12Vdc output to the OCXO131-40.
Frequency Tuning
The DS345 provides an OCXO tuning voltage called OPT_VCO to
an installed OCXO on its connector U203 (the factory OCXO). By
connecting this signal to the VCO input on the Pumpkin OCXO
module, DS345 users can fine-tune the OCXO’s frequency using
the DS345’s internal calibration byte #0. Since OPT_VCO is driven
by an op-amp, the effect of the OCXO module’s trimpot on its
VCO trim voltage V_TRIM are overridden. A cal byte value of 0
represents an OPT_VCO value of -5.5Vdc, and a value of 4095
represents an OPT_VCO value of +5.5Vdc. The factory default
value is 2980, corresponding to 2.5Vdc.
In-place Tuning
Both the first and second position of the 4-position DIP switch
SW300 inside the DS345 must be set to ON; The first position
selects the OCXO as the module’s internal 10MHz source; the
second position enables front-panel changes to calibration
constants. See the DS345 manual for more information.
An cal word #0 initial value of 2977 (OPT_VCO = 2.476Vdc) tuned
the OCXO to 10MHz.
10 In Rev A modules, the wire was soldered to TP3. In the Rev B design, this signal
is part of the J1 header / connector.
11 Not used by the DS345.

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20 UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
Usage
DC Voltages
TP1 (12V LDO output) typically measures 11.87Vdc for a +12Vdc
input to the module. Note that this means that the LDO is not
regulating its output.
TP2 (stable 5V for trimpot) will measure close to 4.9999Vdc.
TP3 (OCXO’s VCO) varies for 10.00000000MHz. The trim
voltage seems to be around 4-6mV/0.1Hz.
Note It was observed that the trim voltage for 10MHz when
installed typically differs from the trim voltage when the PCB
assembly is on a bench and not installed in an XL Microwave
Model 3xxx.
-WARMING rises to 4.87V once the OCXO is out of its warm-up
phase.
The total current (@12V) when warming is 250-300mA,
post-warming is 85mA. The 180mA trip threshold of U2 is
therefore confirmed.
Loading vs. Frequency
The output of the OCXO changes slightly with loading,
particularly capacitive loading. The output does not have an active
buffer. The frequency difference between the OCXO assembly
running on a bench, into a high-impedance oscilloscope, and the
10MHz output frequency from the Model 3xxx, is in the region of
0.5Hz, or 50ppb.
Frequency Stability
Once installed in the Model 3xxx, the frequency of the OCXO
PCB assembly is measured at the Model 3xxx’s output BNC on its
back panel. These tests were performed without any thermally
encapsulating epoxy on the PCB assembly.