Designing and building an OCXO and GPSDO
Hi folks,
I've been following the mailing list for a few weeks using Pipermail (the
web-based archive) and I figured now was a good time to jump in (so to speak).
I'm working on a GPS-disciplined oscillator, based on a Trimble SVeeSix GPS
receiver, and a homebrew OCXO. I've got a pair of 10MHz 50-degree-C oven
crystals, and have a pretty good idea how to handle the temperature regulation.
What I'm planning to do is mount the crystal on a copper plate with two
power transistors, using heatsink compound between the copper and
transistors/crystal case, and fit a temperature sensor to the top side of the
crystal case. I'm planning to use a copper bracket to hold the sensor onto the
crystal, and in turn mount the crystal to the copper base.
As far as temperature regulation goes, I'm going to use a PIC
microcontroller (one of the 8-pin chips with an A/D converter) to monitor the
temperature of the crystal, and use a PID loop to control the two power
transistors to maintain a temperature of 50C +/- 2 Celsius (the accuracy spec
of the temperature sensor). I also have other higher-accuracy sensors (Dallas
DS18S20 and DS18B20) that I can calibrate with; these are accurate to around
half a degree Celsius with a resolution of 0.5C.
The whole thing is going to be mounted in a metal box lined with 1/2in
thick polystyrene, with all external connections made via Molex KK connectors
and standard hookup wire. If there's any advantage to doing so, I might use
RG174 cable for the oscillator output, but otherwise I'll stick to the KKs and
maybe twist the OUT/GND wires together.
What I'm stuck on is the oscillator itself. The crystals are standard
parallel-resonant parts, with a load capacitance of 30 picofarads. I've got a
few varicap diodes (varactors) that I'm planning to use to allow external
trimming of the frequency, on top of what the ~20pf "coarse" preset will
allow. So on one side of the crystal I'll have a 33pf capacitor, and on the
other a 20pf load capacitor, the varicap and a low-value DC-blocking capacitor
for said varicap.
The standard oscillator circuit for TTL seems to be a pair of 74HC04
inverters and a few passives, or a transistor version that outputs a
sine-wave. Are there any particular types of oscillator that are more suitable
for high-accuracy timing?
What I'd like to do is use this oscillator to calibrate frequency counters
and check the calibration on oscilloscopes and similar. Being able to lock
function generators (a mix of custom DDS sine generators based on Analog
Devices DDS chips and FPGA-based complex-signal DDSes) against the oscillator
would be very useful as well. Should I be going for a 1V sine output and then
convert this to TTL in the generators (which are easy to retrofit with adapter
boards) or output TTL from the reference and leave it at that?
What design parameters should I be optimising for, and how?
Given that a standard crystal is good to roughly 100ppm, and most
commercial OCXOs are specified to be within 1x10^-9 or better, I'm aiming for
around 1ppm to start with. Is even this realistic for a homebrew device?
There seems to be quite a bit of difference between just building a 4MHz
oscillator to run a PIC MCU to building an accurate frequency reference source...
As far as parts are concerned, I'm planning to use either a BB153 or BB148
varicap, a Microchip TC1047AVNBTR temperature sensor, a National Semiconductor
LM4040CIM3-4.1 voltage reference for the PIC's A/D, two BD139 power
transistors and a PIC12F683 microcontroller.
Thanks,
Philip Pemberton wrote:
Hi folks,
I've been following the mailing list for a few weeks using Pipermail (the
web-based archive) and I figured now was a good time to jump in (so to speak).
I'm working on a GPS-disciplined oscillator, based on a Trimble SVeeSix GPS
receiver, and a homebrew OCXO. I've got a pair of 10MHz 50-degree-C oven
crystals, and have a pretty good idea how to handle the temperature regulation.
What I'm planning to do is mount the crystal on a copper plate with two
power transistors, using heatsink compound between the copper and
transistors/crystal case, and fit a temperature sensor to the top side of the
crystal case. I'm planning to use a copper bracket to hold the sensor onto the
crystal, and in turn mount the crystal to the copper base.
As far as temperature regulation goes, I'm going to use a PIC
microcontroller (one of the 8-pin chips with an A/D converter) to monitor the
temperature of the crystal, and use a PID loop to control the two power
transistors to maintain a temperature of 50C +/- 2 Celsius (the accuracy spec
of the temperature sensor). I also have other higher-accuracy sensors (Dallas
DS18S20 and DS18B20) that I can calibrate with; these are accurate to around
half a degree Celsius with a resolution of 0.5C.
The whole thing is going to be mounted in a metal box lined with 1/2in
thick polystyrene, with all external connections made via Molex KK connectors
and standard hookup wire. If there's any advantage to doing so, I might use
RG174 cable for the oscillator output, but otherwise I'll stick to the KKs and
maybe twist the OUT/GND wires together.
What I'm stuck on is the oscillator itself. The crystals are standard
parallel-resonant parts, with a load capacitance of 30 picofarads. I've got a
few varicap diodes (varactors) that I'm planning to use to allow external
trimming of the frequency, on top of what the ~20pf "coarse" preset will
allow. So on one side of the crystal I'll have a 33pf capacitor, and on the
other a 20pf load capacitor, the varicap and a low-value DC-blocking capacitor
for said varicap.
The standard oscillator circuit for TTL seems to be a pair of 74HC04
inverters and a few passives, or a transistor version that outputs a
sine-wave. Are there any particular types of oscillator that are more suitable
for high-accuracy timing?
What I'd like to do is use this oscillator to calibrate frequency counters
and check the calibration on oscilloscopes and similar. Being able to lock
function generators (a mix of custom DDS sine generators based on Analog
Devices DDS chips and FPGA-based complex-signal DDSes) against the oscillator
would be very useful as well. Should I be going for a 1V sine output and then
convert this to TTL in the generators (which are easy to retrofit with adapter
boards) or output TTL from the reference and leave it at that?
What design parameters should I be optimising for, and how?
Given that a standard crystal is good to roughly 100ppm, and most
commercial OCXOs are specified to be within 1x10^-9 or better, I'm aiming for
around 1ppm to start with. Is even this realistic for a homebrew device?
There seems to be quite a bit of difference between just building a 4MHz
oscillator to run a PIC MCU to building an accurate frequency reference source...
As far as parts are concerned, I'm planning to use either a BB153 or BB148
varicap, a Microchip TC1047AVNBTR temperature sensor, a National Semiconductor
LM4040CIM3-4.1 voltage reference for the PIC's A/D, two BD139 power
transistors and a PIC12F683 microcontroller.
Thanks,
Philip
If you are serious forget the fancy digital or semiconductor temperature
sensors they aren't good enough.
However with your crude oven structure using higher performance sensors
may perhaps be unwarranted.
For the best performance, unless you use a bridge oscillator circuit of
some type, you will need to control the temperature of all the
oscillator components as well.
Its best to bond the sensor into a well drilled in the oven using non
electrically conductive thermal epoxy.
An analog bridge using an RTD or an NTC thermistor can have much better
stability.
If you use an appropriate high resolution sigma delta ADC it can reverse
the bridge excitation polarity as part of the measurement sequence and
give you most of the benefits of an AC bridge with fewer devices and
lower cost.
The next step up from the gate oscillator for fundamental crystals is
perhaps Wenzel's circuit:
http://www.wenzel.com/pdffiles1/pdfs/xtalosc.pdf
This circuit needs a little optimisation to improve its performance.
A higher base collector voltage on the oscillator transistor is desirable.
This can be done using a pnp transistor to sense the oscillator
transistor dc collector current and regulate it by adjusting the dc base
current.
Replacing the Source follower buffer with a common base transistor
(allow the crystal current to flow into the emitter rather than Wenzel's
shunt C) will provide higher reverse isolation.
Cascade a few transformer coupled CB stages to provide gain and
increased isolation.
With a reverse terminated transformer coupled load in the collector of
the output CB stage any load from open to short circuit can be driven
without adverse effects.
The inductor shown in Wenzel's circuit wont be required with your
crystal either with or without a CB output stage.
The limiting action occurs by cutting off the oscillator transistor
during part of the cycle.
The dc collector current of the oscillator transistor sets the crystal
current.
I would build a room temperature version first for debugging.
To minimise the phase noise contributed by the varicap the EFC range
should be as small as is practical.
A very low noise power supply is also required for good performance.
A modified version (uses 2 transistors and larger capacitors) of Wenzels
active supply filter can be used to reduce the power supply noise by
30-40dB for frequencies above 1Hz or so.
http://www.wenzel.com/documents/finesse.html
I can provide circuit schematics if you are interested.
Bruce
Philip
When using a crystal in an oven you should use a crystal specified for
oven operation at a specific temperature.
The crystal frequency should be specified for the desired oven temperature.
For example for an AT cut crystal the crystal frequency can be
approximated by as a cubic function of temperature.
There are usually a couple of stationary points on the curve where the
slope of frequency versus temperature is zero.
The crystal should be cut so that one of these points coincides with the
oven temperature as this minimises the effect of small errors in the
oven temperature set point on the frequency stability.
A crystal specified for non oven operation is usually cut to minimise
the frequency variation over the specified range of operating temperaures.
The frequency of such a crystal at one of the turning points may be
several tens of ppm away from the nominal frequency.
The upper turning point may not even be suitable as it may be too high
or even within the expected ambient temperature range.
It is also possible to cut the crystal so that the stationary points
coincide at a point of inflection.
In this case the frequency change corresponding to small deviations from
this temperature are much smaller than those at the turning points of a
conventional oven crystal.
However this inflection point will lie close to room temperature for an
AT cut crystal so that the "oven" will have to be cooled when the
ambient temperature is above this point and heated when it is below this
point.
Bruce
Hello Philip,
I agree with Bruce about the digital stuff and semiconductor
temperature sensors, etc. From your commentary I think you should do
some reading before proceeding. Here are some suggestions;
The first is a series of Application notes from Agilent (old hp test
div) called AN-200. A total of 5 App notes comprise the AN-200
series. If you go to the following Web page and enter AN-200 at the
top of the page in the search box, you will get a return of all the
AN-200 booklets in PDF that can be downloaded. The BIG one is
AN-200-2, but it would be to your advantage to collect all of them.
You need to paste in the entire link below if your browser doesnt see
the whole thing when clicking on it.
[1]http://www.home.agilent.com/agilent/facet.jspx?t=80030.k.1&cc=US&lc=
eng&sm=g&pageMode=TM
Next is the AN-52 series, also at the above site. The original,
produced in the 1960's is AN-52. Later, in the 1970's, they rewrote
and split this App note into two titled AN-52-1 & AN-52-2. There is
also AN-52-4, but that does not cover your interests at the moment. I
would suggest downloading ALL of them, including the original AN-52.
An-52 does have historical perspective and a few things not included in
the later rewrites.
That should keep you busy for a while. A lot of stuff on the WEB, some
good and some not so good, just take it with a grain of salt ! NIST
(the old NBS) has several things worth reading, however, most of that
deals with the measurement process or is a rigorous mathematical
analysis of one thing or another.
In my experience, an inexpensive metal can crystal and a decent
oscillator circuit will hang in there under 10ppm in a regular room
with a stable ambient temperature. HP used such a crystal in their
60KHz receiver because it was controlled in a loop from the 60KHz, thus
approaching the accuracy and stability of the transmitted signal.
Second it does not take much to get parts in 10^-7 range. Temperature
compensated crystal oscillators easily handle that level. With care, a
crystal oscillator in a well designed circuit can reach parts in 10^-8
with a bit-bang oven control. HP did that in the late 1950's. From
that point the difficulty is logarithmic.
Bill....WB6BNQ
Philip Pemberton wrote:
Hi folks,
I've been following the mailing list for a few weeks using
Pipermail (the
web-based archive) and I figured now was a good time to jump in (so
to speak).
I'm working on a GPS-disciplined oscillator, based on a Trimble
SVeeSix GPS
receiver, and a homebrew OCXO. I've got a pair of 10MHz 50-degree-C
oven
crystals, and have a pretty good idea how to handle the temperature
regulation.
What I'm planning to do is mount the crystal on a copper plate
with two
power transistors, using heatsink compound between the copper and
transistors/crystal case, and fit a temperature sensor to the top
side of the
crystal case. I'm planning to use a copper bracket to hold the
sensor onto the
crystal, and in turn mount the crystal to the copper base.
As far as temperature regulation goes, I'm going to use a PIC
microcontroller (one of the 8-pin chips with an A/D converter) to
monitor the
temperature of the crystal, and use a PID loop to control the two
power
transistors to maintain a temperature of 50C +/- 2 Celsius (the
accuracy spec
of the temperature sensor). I also have other higher-accuracy
sensors (Dallas
DS18S20 and DS18B20) that I can calibrate with; these are accurate
to around
half a degree Celsius with a resolution of 0.5C.
The whole thing is going to be mounted in a metal box lined with
1/2in
thick polystyrene, with all external connections made via Molex KK
connectors
and standard hookup wire. If there's any advantage to doing so, I
might use
RG174 cable for the oscillator output, but otherwise I'll stick to
the KKs and
maybe twist the OUT/GND wires together.
What I'm stuck on is the oscillator itself. The crystals are
standard
parallel-resonant parts, with a load capacitance of 30 picofarads.
I've got a
few varicap diodes (varactors) that I'm planning to use to allow
external
trimming of the frequency, on top of what the ~20pf "coarse" preset
will
allow. So on one side of the crystal I'll have a 33pf capacitor, and
on the
other a 20pf load capacitor, the varicap and a low-value DC-blocking
capacitor
for said varicap.
The standard oscillator circuit for TTL seems to be a pair of
74HC04
inverters and a few passives, or a transistor version that outputs a
sine-wave. Are there any particular types of oscillator that are
more suitable
for high-accuracy timing?
What I'd like to do is use this oscillator to calibrate frequency
counters
and check the calibration on oscilloscopes and similar. Being able
to lock
function generators (a mix of custom DDS sine generators based on
Analog
Devices DDS chips and FPGA-based complex-signal DDSes) against the
oscillator
would be very useful as well. Should I be going for a 1V sine output
and then
convert this to TTL in the generators (which are easy to retrofit
with adapter
boards) or output TTL from the reference and leave it at that?
What design parameters should I be optimising for, and how?
Given that a standard crystal is good to roughly 100ppm, and most
commercial OCXOs are specified to be within 1x10^-9 or better, I'm
aiming for
around 1ppm to start with. Is even this realistic for a homebrew
device?
There seems to be quite a bit of difference between just building
a 4MHz
oscillator to run a PIC MCU to building an accurate frequency
reference source...
As far as parts are concerned, I'm planning to use either a BB153
or BB148
varicap, a Microchip TC1047AVNBTR temperature sensor, a National
Semiconductor
LM4040CIM3-4.1 voltage reference for the PIC's A/D, two BD139 power
transistors and a PIC12F683 microcontroller.
Thanks,
--
Phil.
lists@philpem.me.uk
[2]http://www.philpem.me.uk/
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References
Phil,
Good on ya. When I have the time, I'll try going back to first
principles.
I've spent 40 years working with PID controllers and their sensors and
actuators.
Rule of thumb: If the transport delay (dead time between changing the
heater and sensing the change) is 10% of the system time constant, you
have to reduce the PID gain by about half to maintain stability (of the
phase margin in a feedback loop).
In your case, do NOT mount the sensor on the crystal. Mount it halfway
between your heaters and expect the copper plate to come to a
steady-state temperature inside the insulated cavity. Do not mount it
with any form of insulating adhesive which will increase the delay.
Maybe hold it down with a copper strip, like the crystal.
As Bruce Griffiths suggests, use an analog sensor and a resistance
bridge. Then the null signal can be digitized with low accuracy
requirements, compared to digitizing the temperature to 10E-9. Mount the
analog stuff in the controlled enclosure. Use fixed resistors to balance
the bridge at 50C so there are no adjustments in the oven. Plan on
putting the oscillator in there, too.
Never mind being dead on 50C because where you want to be is on a flat
spot on the temperature curve, so maybe you don't have to hold 10E-9.
Your digital PID is looking at the digitized bridge error as its
measurement. A setpoint can vary that error to find the flat spot. Don't
forget that components and solder joints age at 50C, so plan on making
adjustments once a week until the frequency stabilizes.
The accuracy of the sensor is not important. What you want is precision
and stability. You want the highest gain you can get in the PID, which
means doing the stuff for delay as well as determining the time constant
of your oven at 50C so you can set the integral term.
Bill Hawkins
-----Original Message-----
From: Philip Pemberton
Sent: Sunday, August 10, 2008 4:45 PM
What I'm planning to do is mount the crystal on a copper plate with
two power transistors, using heatsink compound between the copper and
transistors/crystal case, and fit a temperature sensor to the top side
of the crystal case. I'm planning to use a copper bracket to hold the
sensor onto the crystal, and in turn mount the crystal to the copper
base.
As far as temperature regulation goes, I'm going to use a PIC
microcontroller (one of the 8-pin chips with an A/D converter) to
monitor the temperature of the crystal, and use a PID loop to control
the two power transistors to maintain a temperature of 50C +/- 2 Celsius
(the accuracy spec of the temperature sensor). I also have other
higher-accuracy sensors (Dallas DS18S20 and DS18B20) that I can
calibrate with; these are accurate to around half a degree Celsius with
a resolution of 0.5C.
Bruce Griffiths wrote:
If you are serious forget the fancy digital or semiconductor
temperature sensors they aren't good enough.
I was intending to use the slow Dallas chips as a calibration reference
(out-of-box they're usually quite accurate) and for testing. Is there
any particular reason the analog-output (Microchip TC1047A) sensors are
no good?
For the best performance, unless you use a bridge oscillator circuit
of some type, you will need to control the temperature of all the
oscillator components as well.
I did have a "plan B" -- a hollow metal box with a metal sheet soldered
inside at the half-way point. The crystal and oscillator circuitry would
be mounted in the bottom half, and the temperature control in the top
half. The temperature sensor is a three-pin SOT23 (about the size of a
grain of rice) and the ground is a single pin on one side. I was
thinking about mounting the sensor directly on the copper, using a small
piece of Kapton tape to stop the sensor's Vout or Vcc shorting against
the grounded copper sheet.
That would leave two hollow air-filled cavities for the control
circuitry and hold the temperature of that reasonably close to that of
that of the crystal (minus a few degrees).
An analog bridge using an RTD or an NTC thermistor can have much
better stability.
That sounds about right.. I was going to use a Pt100 or Pt1000 RTD, but
couldn't find any decent information on them other than the resistance
being 100R or 1kR at 25C -- even the manufacturer's datasheets were
somewhat thin on information.
If you use an appropriate high resolution sigma delta ADC it can
reverse the bridge excitation polarity as part of the measurement
sequence and give you most of the benefits of an AC bridge with fewer
devices and lower cost.
IIRC the A/D on the PIC is a 10-bit successive-approximation type with a
built-in sample-and-hold (though other types have 12-bit converters).
That's a measurement range of 1024 counts, which with the 4.096V
reference provides a resolution of 4mV, or 1/2.5 of a degree C per
count. 4V is actually the minimum reference voltage the A/D can accept.
Sensor output is ((degrees_c * 10) + 500) mV.
I would build a room temperature version first for debugging.
I'm planning to do that anyway. I've got a few 10MHz room-temp crystals
of a similar spec to the oven crystals that I can use, and I can
probably use the same parts in the prototype oven for testing.
To minimise the phase noise contributed by the varicap the EFC range
should be as small as is practical.
That's the part that's going to need "a bit" of experimentation I think :)
A very low noise power supply is also required for good performance.
A modified version (uses 2 transistors and larger capacitors) of
Wenzels active supply filter can be used to reduce the power supply
noise by 30-40dB for frequencies above 1Hz or so.
http://www.wenzel.com/documents/finesse.html
I can provide circuit schematics if you are interested.
That would be great, if it's not too much trouble.
Thanks,
Phil.
Bruce Griffiths wrote:
When using a crystal in an oven you should use a crystal specified for
oven operation at a specific temperature.
Which is what my crystal is -- a 10MHz oven crystal, specified for a
30pF load capacitance and operation at 50 degrees Celsius.
I'm intending to use the oscillator at an ambient temperature of up to
35 Celsius (if it's warmer than that, then I'll likely have other things
to worry about!). That's on the border of what the Agilent appnotes
suggest as reasonable (15 to 20 Celsius), but still within range. In
real terms, it's probably not going to see an ambient temperature higher
than 25C. Well, not in this country anyway.
Thanks,
Phil.
Philip Pemberton wrote:
Bruce Griffiths wrote:
If you are serious forget the fancy digital or semiconductor
temperature sensors they aren't good enough.
I was intending to use the slow Dallas chips as a calibration reference
(out-of-box they're usually quite accurate) and for testing. Is there
any particular reason the analog-output (Microchip TC1047A) sensors are
no good?
You need millidegree sensistivity and stability, no sensor of this type
has the required stability.
Only good thermistors and RTDs approach this.
For the best performance, unless you use a bridge oscillator circuit
of some type, you will need to control the temperature of all the
oscillator components as well.
I did have a "plan B" -- a hollow metal box with a metal sheet soldered
inside at the half-way point. The crystal and oscillator circuitry would
be mounted in the bottom half, and the temperature control in the top
half. The temperature sensor is a three-pin SOT23 (about the size of a
grain of rice) and the ground is a single pin on one side. I was
thinking about mounting the sensor directly on the copper, using a small
piece of Kapton tape to stop the sensor's Vout or Vcc shorting against
the grounded copper sheet.
That would leave two hollow air-filled cavities for the control
circuitry and hold the temperature of that reasonably close to that of
that of the crystal (minus a few degrees).
The tempco contribution of oscillator components is such that there
temperature should be stable to far better than a few degrees.
Thermal gradients within the oven need to be minimised.
An analog bridge using an RTD or an NTC thermistor can have much
better stability.
That sounds about right.. I was going to use a Pt100 or Pt1000 RTD, but
couldn't find any decent information on them other than the resistance
being 100R or 1kR at 25C -- even the manufacturer's datasheets were
somewhat thin on information.
These sensors like most platinum resistance thermometers elements have a
tempco of about 3900 ppm/K or so.
Copper resistors have also be used for this purpose and have a similar
tempco.
Thermistors are much more sensitive to temperature and high quality ones
are almost as stable.
The output from a bridge using RTDs is relatively low and precautions
have to be taken to eliminate thermocouple effects as well as amplifier
offsets.
An AC bridge excitation technique or periodically reversing the bridge
excitation polarity can help.
If you use an appropriate high resolution sigma delta ADC it can
reverse the bridge excitation polarity as part of the measurement
sequence and give you most of the benefits of an AC bridge with fewer
devices and lower cost.
IIRC the A/D on the PIC is a 10-bit successive-approximation type with a
built-in sample-and-hold (though other types have 12-bit converters).
That's a measurement range of 1024 counts, which with the 4.096V
reference provides a resolution of 4mV, or 1/2.5 of a degree C per
count. 4V is actually the minimum reference voltage the A/D can accept.
Insufficient resolution you need a least count below 0.001C.
If you use that ADC it will need a suitable preamp.
However a linear range of a little less than 1C is sufficient.
A high resolution sigma delta ADC is probably a better choice as a low
drift preamp isnt required.
The output DAC (of whatever type) needs to be monotonic and have high
resolution (18 bits or more) to allow the temperature to be controlled
to better than 0.001C.
There are several techniques for achieving more resolution from a low
resolution DAC.
Sensor output is ((degrees_c * 10) + 500) mV.
I would build a room temperature version first for debugging.
I'm planning to do that anyway. I've got a few 10MHz room-temp crystals
of a similar spec to the oven crystals that I can use, and I can
probably use the same parts in the prototype oven for testing.
To minimise the phase noise contributed by the varicap the EFC range
should be as small as is practical.
That's the part that's going to need "a bit" of experimentation I think :)
If you use a manual trimmer as well then a range of 1E-7 or so is
usually sufficient.
If all frequency adjustment is via EFC then you need to accommodate at
least 10 years worth of aging.
A very low noise power supply is also required for good performance.
A modified version (uses 2 transistors and larger capacitors) of
Wenzels active supply filter can be used to reduce the power supply
noise by 30-40dB for frequencies above 1Hz or so.
http://www.wenzel.com/documents/finesse.html
I can provide circuit schematics if you are interested.
That would be great, if it's not too much trouble.
Thanks,
Phil.
What are the crystal parameters:
Q?
ESR?
L?
Cshunt?
Cseries?
What crystal cut is used AT?, BT?, SC?
Is it a fundamental crystal?
Will post the modified P/S filter in a few hours together with the
modified Wenzel low noise oscillator circuit.
If you are using an overtone crystal or an SC crystal then provisions
need to be made to suppress unwanted modes.
In this case a somewhat different oscillator configuration is perhaps
advisable.
Something like one of the Driscoll type oscillators which have a tuned
tank for mode suppression can be used.
Otherwise a somewhat more complex oscillator with AGC may be necessary.
Bruce
Stupid question - why build your own OCXO when you can buy a pretty good
Oscilloquartz OCXO from eBay item number 300247357254 for almost a song?
Yes I know - it's fun!
Dave
David C. Partridge wrote:
Stupid question - why build your own OCXO when you can buy a pretty good
Oscilloquartz OCXO from eBay item number 300247357254 for almost a song?
I can think of a couple of reasons:
-
If it breaks, I can rip it open, poke and probe it, figure out what's wrong
and then hopefully fix it. I've got loads of T&M gear that sadly never sees
the use it should -- a very nice Tek TDS2024B DSO that's been used maybe a
dozen times, a HP 1651B logic analyser that's been used about as much (but
bought second-hand with severe screen burn), the list goes on. -
It's nice to know how things work - I have a severe case of
blackboxophobia. I hate little black boxes that do magic things, and have zero
parts availability. This has gotten to the point where I've built a model
logic analyser to learn how they work, and most of my complex signal
generators are homebrew too. I've got a nice DDS signal generator that I'm
working on at the moment, which should by virtue of the ADI DDS chip be able
to do AM, FM, BPSK, QPSK, (maybe) 8PSK and a few other modulation modes. I'm
building it for... testing a Radio 4 timecode receiver. Bonus points if I can
make it produce DVB-S compliant streams at 70MHz IF and upconvert to 1.x GHz
L-band to test satellite receivers. That's quite far down on the projects list
though :) -
I try and avoid ebay where possible. I've had pretty good luck with UK and
US sellers, but the few I've dealt with from other countries have been
horrendous, especially those in the vicinity of China and Hong Kong. Six
months for one parcel to arrive...
Anyone here listen to the "SolderSmoke" podcast? #72 pretty much sums it up. I
like to know what things do, and I can't stand to buy something if I can build
it from parts I've got in my junk box. In my weird world, if I've already
bought the parts, then effectively my cost is zero.. I've got the bits, I
might as well use them.
Plus I was actually looking at new OCXOs -- at the time I decided to do this,
there weren't any on eBay. The lowest quote I got was £132 +VAT (rounded up
from the quote I just dug out...) for a 12V, fairly low-tech OCXO. The shiny
new high-tech one with the TTL status output and other stuff was about £240
and some change.
In effect, it's cost me £37 for the crystals, and ~£10 for a dozen each of the
temperature sensors and voltage references. I'm still below half the cost of a
brand-new commercial OCXO.
Yes I know - it's fun!