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.

User Manual
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]

User Manual
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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UM-20 Low-cost Oven-controlled Crystal Oscillator (OCXO) Assembly
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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.

User Manual
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.