~ PROJECT PAGE ~
... Accuracy & Stability in
the Ham Shack
A GPS Clock, or GPS-Disciplined
Oscillator (GPS-DO) is a combination of a GPS receiver
and a high-quality, stable oscillator such as a quartz or rubidium
output is controlled to agree with the signals broadcast by GPS and GNSS
satellites. GPSDOs work
well as a source of timing because the satellite signals must be
accurate in order to provide positional accuracy for GPS in navigation.
These signals are accurate to nanoseconds and serves as an excellent reference for
timing applications in the ham shack ... and on the air!
The GPSDO Project is presented in four parts ...
2: OCXO --
Oven Controlled Crystal Oscillator ...
Temperature-controlled "oven" + VCXO = "OCXO"
Part 3: GPS
Receiver ... Providing the GPS-accurate
4: GPS Motherboard
+ Phase-Lock Loop Circuits ... Phase-locking the OCXO
to the stable and accurate GPS reference
(Click photo for bigger view)
YouTube video of VCXO Breadboard being tuned up
... Check out how our Breadboard of
this circuit actually works ...
(Be sure to turn up your computer's audio volume for this one ... I
had a FUBAR moment during video recording! -- n2apb)
Part 1: VCXO ...
Voltage Controlled Crystal
Some time ago, we developed a 10 MHz voltage
controlled crystal oscillator, or VCXO, for use as a fun project at the
immensely popular Atlanticon QRP Forums. This VCXO is able to be
put to use in many ways around the shack. For example, once calibrated, this
VCXO can serve as an accurate frequency standard for receiver alignment,
or as a PLL standard, or even as an LO for a transmitter.
view/download the PDF version of the schematic and all kit information
The CWTD “VCXO”
produces approximately 1-2V p-p at
when C3 is
in the top position (C3a on the pcb). This signal is quite ragged but
very suitable for driving a balanced mixer like an SA612 (e.g., in
mixing applications). When C3 is placed at the “C3b” position, a much
more sinusoid-like100-200 mV p-p signal is delivered at RF Out, which is
more suitable for use as a “standard” 10 MHz oscillator standard in the
Capacitors, crystals and semiconductors have
temperature coefficients that represent how the component values change
with changing temperature. When the component values change, the
oscillator frequency changes. This drift is not good if you want a very
stable and unchanging oscillator frequency.
So the real fun challenge for the builder,
and the excitement with our resurfacing this project here on CWTD, is to
find ways to dynamically adjust, compensate or insulate the circuit
against temperature changes. The Adjust signal on connector P1 pin 1
is delivered by a simple potentiometer that presents a nominal 3.4 volts to
D1, which is a "voltage variable capacitor" commonly called a
But this only a static level and if the temperature changes, the pot
setting would need to be changed to compensate for that change . One
could instead dynamically adjust that control signal in accordance to
the ambient temperature. Or perhaps use of NP0 “negative
coefficient” capacitors in the circuit could compensate for drift
.. which is what we've done now with the CWTD version of the circuit.
But other techniques will come along in Phase 2 and Phase 3 for this
Construction of the
VCXO is straightforward – just use the
schematic as a guide for placement of the components at the silkscreened
locations on the board.
Resistors are mounted “on end” with the top lead
bent over and going into the hole next to the bottom lead. Be sure
to use your ohmmeter to double check your guess at the color
coding on these resistors. You may be fooled with the
47-ohm and 12-ohm resistors! ;-)
NP0 ("NP Zero")
capacitors are supplied in the "outer" kit bag. NP0 caps
have zero drift with temperature (or at least to +/- 30ppm/Deg-K
... which is pretty darn close to zero at the temperature ranges
we'll be ultimately operating the oven (in Part 2).
a 2.1mm coaxial power jack is also provided for convenience.
Just plug in your standard 12V power source on the bench to
power the whole project.
Place R11, the 500-ohm, 10-turn trim pot into the used pins 3, 4
and 5, as shown in the top view photo. Then add the wire
jumpers to the bottom of the board as shown in the bottom view.
(Alternatively, you could mount the trimpot off-board, as we did
in the Breadboard photo at the top here, but it's convenient to
have it located firmly right there on the board.) BTW,
note how I placed the ground jumper, as a bare wire "outboard"
of the board, able to serve as nice clipping point for attaching
your scope and/or frequency probe!
NEW ... In order
to have your VCXO ready to be combined with the Oven Control
circuit board coming in the next episode, please mount the
crystal about 1/4" off the bottom of the board, as shown
in the photo below. If you have already mounted the
crystal on top, just unsolder the crystal and reattach it on the
bottom. Don't worry, it's easy. Just gently pull one side and
then the other while heating each pad. Then re-attach to the
same pads on the bottom side. And if you don't have 1/4" of lead
length, just solder some bare wire in the pcb holes and then
attach the crystal to these.
What do you put at the L1 position on the VCXO pcb? The
answer isn't obvious! We provide a 10 pF NP0 capacitor for
use at the "L1" position on the
board. This spot was originally intended to be for an inductor,
which would tend to lower the operating frequency of the
oscillator. But as we built up many boards in the
prototyping phase, it was found that the crystal was running a
little "too low" for the adjustment range offered by the
voltage variable cap (diode D1), so we shall instead use a capacitor to allow adjustment up to, and beyond the 10 MHz operating
point. But as Dave AD7JT points out, and nicely
illustrates below, it's not quite as simple as that!
Figures above: Finding the right capacitor value to place
at the L1 position on the VCXO pcb to provide
adjustability above/below the target 10,000,000 Hz point.
Dave AD7JT writes: "I
had to do some cut-and-try stuff to get the frequency range to
include 10 MHz. The initial frequency range with "L1" set to 10 pF
didn't come within 2 KHz of 10 MHz. Shorting L1 got it in range but
just barely (took about 4V on the Adjust line). I wanted 10 MHz to
be at about 1.5 V or midway on the DAC output range for a dsPIC. I
finally settled on a value of 68pF for "L1". The attached shows my
readings for all the values I tried. In all cases the frequency
range was about 700 - 750 Hz (not enough to even see on a scope)."
just connect up the 8-12V power source (a 9V battery will work for a
short while, with a current draw of about 130 mA), and you'll be off to the races!
remember, you'll want to use a frequency counter to serve as your
adjustment guide ... Tweak the trimpot to bring the VCXO operating
frequency as close as possible to 10 MHz, just as I show in the YouTube
video (link above.)
you don't have a frequency counter?!! Well, get one of those
nifty 8-digit, blue-LED counters shown above (and on our Breadboard) on
order from ...
You just can't beat this
$12 wonder (+about $2 shipping from China!!)
Click either photo for a bigger view.
Interested in Building the VCXO?
full kit is no longer available, but we have lots of bare
Just use the parts list to gather the handful of common parts build
it up yourself and you'll quickly be oscillating for joy.
|VCXO PC Board (bare) ...
(Order up to 10 for the same shipping fee!)
Part 2: OCXO
This is Part 2 in our series on the design of a
"GPS-Disciplined Oscillator", and this time we focus on a circuit
intended to maintain temperature of a Voltage Controlled Crystal
Oscillator, and thus help to improve the stability of its output
frequency. We'll be doing this by means of a control loop that drives a
heating element keeping the local temperature inside the enclosure
constant at about 45-degC +/- 1.
This latest project installment comes in the form of a
small kit (parts + pcb) that also includes temperature hi/lo LEDs,
voltage regulators, and a motherboard to hold the whole assembly that
slides into a nice extruded aluminum (and insulated!) enclosure.
So come follow along and learn first-hand how to keep
your oscillator's temperature under control, and thereby its frequency
==> See all the details for this installment at ...
Our Approach ....
Send to ...
Hans Summers, G0UPL ...
From Hans' website ...
The control circuit is the tricky part. In the simplest
kind of control circuit, the measured temperature is just compared with
the desired target temperature. Then the heater is switched on if the
oven is too cold, and off if the oven gets too hot. This kind of
controller is common (getting les common) in the mechanical room
thermostat, or the thermostat in your kitchen oven or fridge, or the air
conditioner in your home or office. The disadvantage is that the thermal
mass of the oven takes time to heat and cool, and this means the
temperature can vary quite considerably as the heater (or cooler) cycles
on and off.
The right photograph above, shows the
crystal frequency (received on an HF receiver and plotted in Argo
spectrum analysis software on a laptop). The cycles of the heater on/off
are very clear here. The cycle duration was around 38 seconds, of which
the oven was switched on for 17 seconds and off for 21 seconds. This
kind of temperature cycling would be completely unacceptable for QRSS or
WSPR operation such as in the U3
So we come to proportional oven
control. There are several good examples of homebrewed ovenised
oscillators on the web, but two of my favourites are: Andy
G4OEP, about half way down this page and Des
M0AYF's extensive page on his QRSS ovens.
These two are my inspiration. Both of these ovens use a proportional
controller. There's a heater, a temperature sensor, and a control
circuit consisting of an op-amp with relatively low gain (not an on/off
comparator). The limited gain of the op-amp makes it possible for the
heater to be partially on. If the thermal characteristics are right, and
the gain is correctly matched to them, then the proportional control
circuit makes it possible to control the frequency without the
characteristic "hunting" cycles of an on/off oven.
However, I wasn't quite comfortable with proportional
control circuits. An ideal control circuit would maintain the oven at a
constant temperature regardless of the ambient, environmental
temperature. But a proportional circuit does not. The oven is kept at a
reasonably constant temperature but there is still some variation, as
the environmental temperature varies. Even if everything else is
perfect, the circuit inherently produces an error term. To stop
"hunting" oscillations, the amplifier gain needs to be low, but the
lower the gain, the larger the error term becomes.
So to PID
controllers (Proportional Integral Derivative) which
are used in industrial automation. Surely the subject of many text books
and the nightmare of Electronic Engineering students everywhere. There
is plenty to read on this topic, and it seems to quickly get complicated
enough. Software programs are nowadays used for the control logic. But
here, we use a simple op-amp. The quick summary (after an awful lot of
reading and research): it turns out that the "Derivative" term isn't
necessary in a simple temperature controller. The "Proportional" term we
have covered already, such as in the circuits above. Best of all, the
"Integral" term can be implemented simply by putting a capacitor in the
feedback loop of the op-amp; and the "Integral" term removes the error
that is inherent in "Proportional" only circuits. Simple and neat, and
does everything we need!
Full information on the
"partial kit" that Hans is supplying for us is at ...
==> Interested in Building the CWTD
Oscillator Oven Controller?
Purchase the CWTD Oven Control Kit
can get a kit of the "Oven Control" parts that we discussed
in this episode (see Schematic 1 / 2 above on this
pictured below), as a kit of parts that you can start adding
to your VCXO from last episode! This is a special kit
for us from Hans Summers of QRP Labs, and contains the oven
control component (that we will use), as well as a 10 MHz
crystal oscillator components and enclosure pcb set that are
included for our eventual use downstream. In order to
save us cost, it does not contain the Si5351
synthesizer chip that his standard kit has ... Hence this is
the special "CWTD OCXO Kit" from QRP Labs ... thanks Hans!
[You can see the manual for assembling this at
http://www.qrp-labs.com/ocxokit.html ... But remember to
save the pcb enclosure and 10 MHz crystal oscillator for
[Note: To complete the GPS-DO
project, we are designing a "motherboard" to to slip inside
the enclosure, holding the VCXO, Oven Control board,
temperature display circuits, power supply and the little
NEO-xx GPS receiver boards ... Should be available by the
Also, you may purchase the CWTD
GPSDO Enclosure (pictured below)
This is a nice Aluminum enclosure
(4.5"x3"x1.75") that the end product GPS-DO motherboard will
slip into, carrying the VCXO, Oven Control circuits, and the
NEO-xx GPS receiver, as depicted in the prototype hardware
above on this page. We ordered a bunch
of these enclosures in bulk from better pricing, and are
passing that savings on to our CWTD experimenters.
[Note: To complete the GPS-DO project, we are
designing a "motherboard" to to slip inside the enclosure,
holding the VCXO, Oven Control board, temperature display
circuits, power supply and the little NEO-xx GPS receiver
boards ... Should be available by the next episode.]
conventional postal mail ...
the total price and shipping fee (plus sales tax if you're in
Maryland) by first clicking the PayPal
button(s) above (just cancel after determining the shipping fee for
your location), and then write a check or M.O. payable to "
Part 3: GPS Receiver
NEO-7M GPS Receiver:
NEO-7M ... http://www.ebay.com/itm/Ublox-NEO-7M-000-GPS-Module-MWC-APM2-6-Replace-NEO-6M-GYGPSV3-NEO7M-/400938194993?hash=item5d59c75831 ...
$14.30, free shipping
NEO-8M ... http://www.ebay.com/itm/New-Flight-Controller-GPS-Module-for-PX4-Pixhawk-V2-4-5-APM2-56-APM-NEO-M8N-/400880452240?hash=item5d56564290 ...
$21.25, free shipping
100% brand new and
GPS Chip parameters:
72-channel u-blox M8 engine
GPS/QZSS L1 C/A,
GLONASS L10F, BeiDou B1
SBAS L1 C/A: WAAS,
Nav. update rate1
Single GNSS: up to 18 HZ
Concurrent GNSS: up to
Position accuracy2 2.0
starts: 26 s
Aided starts: 2 s
Reacquisition: 1.5 s
& Nav: –167 dBm
Cold starts: –148 dBm
Hot starts: –156 dBm
AssistNow GNSS Offline
(up to 35 days)3
(up to 6 days)
OMA SUPL & 3GPP
RTC crystal Built-In
Noise figure On-chip
LNA (NEO-M8M). Extra LNA for
lowest noise figure
Anti jamming Active CW
detection and removal. Extra
onboard SAW band pass
Memory ROM (NEO-M8M/Q)
or Flash (NEO-M8N)
Active and passive
position, velocity, and time (NEO-M8N)
Operating temp. –40° C
to 85° C
Storage temp. –40° C
to 85° C (NEO-M8N/Q)
–40° C to 105° C (NEO-M8M)
according to ISO 16750
Manufactured and fully
tested in ISO/TS 16949 certified production sites
Uses u-blox M8 chips
qualified according to AEC-Q100
Supply voltage 1.65 V
to 3.6 V (NEO-M8M)
2.7 V to 3.6 V
Power consumption4 23
mA @ 3.0 V (continuous)
5 mA @ 3.0 V Power
(1 Hz, GPS mode only)
Backup Supply 1.4 to
NEO-M8N-0 u-blox M8
Concurrent GNSS LCC Module,
TCXO, flash, SAW, LNA,
0.25 Hz to 10 MHz
Connector & Cable for the GPS Boards:
2X U.FL Mini PCI to RP-SMA Pigtail Antenna WiFi
$3.40, free shipping
==> See full background on the GPS Receiver
technology being used at our CWTD Whiteboard on this topic ...
Part 4: Grand Finale ... GPS-DO Motherboard
Holds the VCXO and OCXO boards, and has circuits
for Phase-Locked Loop, Temperature Display, Power Supplies and
Here is the overall schematic that pulls everything
together! The motherboard is off to fab now and the prototypes
will be available shortly for evaluation ... then we'll be able to
get the final kit together for all CWTD followers to complete their
project. This will be happening in our July episodes of CWTD.
[n2apb, Jun 2, 2016]
Miles, KE5FX site ... <www.ke5fx.com>
... A great assemblage of links having to do with high precision,
accuracy and stability techniques can be found on the. You can literally
spend days reading up on techniques, software, measurement results and
the history of timing and frequency control here. Just plan on spending
an afternoon to browse it.
2) “Resources for
Precision Timing, Stability, and Noise Analysis” ... <http://www.ke5fx.com/stability.htm>
... If you want in-depth knowledge this is THE place to go. Plus,
you can download plenty of extremely valuable application notes and
technical papers for future reference.
References from the site of KA7OEI ...
Rejuvenating Rubidium Lamps -
The rubidium lamp in these devices has only a finite lifetime,
but this page explains how you may be
able to get more life out of it if it quits working! Note that
this page doesn't address the LPRO-101 specifically, but the
same general technique may be
The "Time Nuts" Mailing list and archive -
Covering all sorts of nerdy topics related to frequency and time
measurement, the archives of this list contain a wealth of
information about this and other frequency references. While
anyone may peruse the archives, you must join the list in order
Stability and Noise Performance of Various
Rubidium Standards by
John Miles, KE5FX - Another excellent article comparing the
important parameters of various Rubidium devices available on
the surplus market. From this page you can readily see why the
LPRO-101 works "barefoot" as a microwave reference and an
FE-5680 does not!