September 8, 2015
CWTD Episode #74
... Controlling temperature in our VCXO project
for frequency stability!
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 stable!
In October's grand GPSDO finale episode, we'll add the small GPS receiver
module and the PLL circuits.
BREAKING NEWS! ... We'll also be describing a brand new (free) software
load for a popular instrument that will enable you to do some wonderful
things with the inexpensive little GPS receivers we see these days. Be among the first on the block to
get our latest gadget for the shack ... and even use it on the family's entertainment
73, George N2APB and Joe N2CX
Listen to the Podcast
Text Window happing during
the show ...
<19:57:09> "Dave - AD7JT": I guess I have been lucky, I have only
had one bad experience on ebay. I have a 6T that came in a selr-contained
white plastic disc for quite a few dollars more and it seems to work
fine. The 6T has a second TimePulse output and has some additonal
timing related features.
<19:58:49> "Mike WA8BXN":
<19:59:07> "Mike WH6YH": 6T, I'll go take a look. thank you.
<20:03:19> "Joe N2CX": GPS Disciplined Oscillator - GPSDO
<20:26:24> "Joe N2CX": Microchip temp sensor IC MCP9701
<20:46:17> "Al - N8WQ": what kind of coax did you use George from
ref signal out to bnc connector
<20:47:30> "George N2APB": I have a bunch of thin UHF coax from flea
markets, some of it is "armored" sheathing ... but it's just coax at
the end of the day A. RG-174 will work just fine.
<20:48:08> "Al - N8WQ": rog
<20:48:23> "George N2APB": The SMA/SMB connectors are expensive
though! So I try to collect as many of those from scrap circuit
<20:49:55> "Joe N2CX": What George is discussing is often called
<20:52:03> "Mark NI2O": NMEA=National Maritime Electronics
<20:52:19> "Pat - W0BM": for the next session discussion, how do you
get a GPS signal into the basement? How open does the antenna for
the GPS need to be?
<20:53:18> "Mike WA8BXN": you probably will need an outside antenna
for the basement --- many are available
<20:54:51> "Pat - W0BM": Mike, what kind of feedline? I'm probably
40 feet from the opening to the outside at 3 ft above ground.
<20:55:06> "Mike WA8BXN":
<20:55:54> "Mike WA8BXN": one antenna i have has 16 feet feedline
that looks like rg-174 size coax
<20:56:06> "Mike WA8BXN": i might try quad shield RG-6
<20:56:33> "Mark NI2O": 2ghz CATV line amp works well
<20:56:46> "Al - N8WQ": Mike, can you link your gps antenna?
<20:57:41> "Mike WH6YH": Mark, NI2O can you post a link for a white
paper on PPS steering you mentioned earlier? for inquiring minds
<20:57:58> "Mike WA8BXN": i used the following, not very carefully
<20:58:01> "Mike WA8BXN":
<20:59:28> "Mike WA8BXN": down in the basement another option to
consider is put the GPS/antenna outside and run the timing signal
out of the GPS down to the basement
<21:01:01> "Mike WH6YH": IS the screen the same size as the SNA &
<21:08:53> "George N2APB": Yes it is the same size. In fact, it's
the very same hardware platform!
<21:09:22> "George N2APB": In other words, if you had an NAT or SNA
kit already, you already have the GDT!
<21:09:51> "Mike WH6YH": I do! It will be interesting to integrate
this with the SNA
<21:10:19> "George N2APB": And if you don't yet have an SNA Kit, I
created a subset of the SNA Kit, containing just the motherboard and
display (i.e., none of the measurement and analysis stuff), and you
can get the "GDT" platform, for a lesser price.
<21:15:58> "Mike WH6YH": of 9600 modem fame?
<21:16:24> "Mike WA8BXN": VE2ZAZ
<21:16:40> "Mark NI2O":
<21:17:44> "Mark NI2O":
<21:18:28> "Mike WH6YH": soldersmoke
<21:18:39> "Mark NI2O":
<21:19:59> "Mark NI2O":
<21:21:46> "Mark NI2O": https://web.archive.org/web/20110305023033/http://www.g7ltt.com/10mhz/
<21:22:39> "Mark NI2O": NTP servers
<21:23:06> "Mark NI2O": +/- some Special Relativity
<21:27:19> "Mike WA8BXN": I have information on the cables/adapters
needed I will post later
<21:32:27> "Mike WA8BXN": The basic idea of a FLL (frequency locked
loop) can be thought of this way: Assume you have a very accurate
frequency counter and want to adjust the trim pot on the VCXO to put
it on frequency. That trim pot varies the DC voltage on the varicap
diode in the VCXO board, which controls the oscillator frequency to
some limited extent.
Watching the frequency on the counter, you turn the pot one way or
the other until the counter reads exactly 10.00000 MHz. A FLL does
the same thing using a microprocessor of some sort. A frequency
counter is implemented in that processor using the timing signal
from the GPS. By using a very long measurement interval over which
you count the cycles, you can measure the frequency very accurately.
A digital to analog converter provides an output voltage from the
processor board to adjust the VCXO frequency.
<21:35:51> "George N2APB": Nicely stated Mike. One of things to be
careful of in using a microcontroller (like the Arduino or even a
PIC) is the latency in the counting and interrupt circuits. Whenever
even minor delays in included in the processing, they can ultimately
affect the counting of locking scheme controlling the VCXO.
<21:36:52> "Mike WA8BXN": Yes, making it work exactly right is not
<21:37:11> "Dave - AD7JT": The tiny connector on the GPS board is a
"MMCX" connector and adapters to other connectors are available at
DigiKey, Mouser, and eBay. I was only able to find MMCX connectors
<21:38:17> "George N2APB": Even with an FLL "cycle counting scheme"
that is well-controlled and tied to hardware only (i.e., no
software), there is a "1 count ambiguity" in the end count that will
indeed provide some phase noise and jitter.
<21:38:35> "Dave - AD7JT":
In the coming days I will be adding a significant amount of commentary,
captions and better-looking diagrams to this "photo journalistic"
approach taken with the material at show-time ... de n2apb]
Our Approach ....
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 ...
The CWTD "GPS
Announcing ... The Midnight
GPS Display Terminal ("GDT")
Handheld terminal that
dynamically interprets and displays the live NMEA serial data feed
coming from any GPS receiver module!
The "GDT" is a new software load
for the Midnight SNA hardware platform, enabling anyone with an SNA to
easily upgrade to the GPS Display Terminal.
(A separate, lesser-priced "GDT
Kit" is available for those not yet having an SNA. The GDT Kit
does not have the specific parts for SNA measurement, like the AD8307,
DDS-60, BNC connectors, etc.)
The serial port into the SNA is 5V “digital” TxD/RxD. One will need
(for now) to use an RS232-to-TxD/RxD adapter. (Inexpensive from Sparkfun,
Adafruit, Parallax, et al.)
The GDT interfaces with all GPS receiver modules to dynamically display
GPS data, including additional screens showing raw NMEA "sentences" (raw
Additionally, the GDT controls GPS receivers in order to configure
Timepulse frequencies, duty cycles, and other operational parameters.
(Initially being done with the u-Blox product line (LEA, NEO, M8F
GDT Firmware ... is ready-made to
load right onto your NAT and SNA instruments! Just
right-click and save to your local computer,
place onto the NAT/SNA SD Card and use the UPLD command to load it.
Interested in Building the CWTD
Oscillator Oven Controller?
Purchase the CWTD Oven Control Kit
See the GPS-DO Project page
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.
3) PLL reference
papers that are a good read ...
AN-885 Introduction to Single Chip Microwave PLL's (TI’s LMX1501A)
MM74HC4046 CMPS Phase Lock Loop ...
Experiment #4 - CMOS 4046 Phase-Locked Loop...
CMOS Phase-Locked-Loop Applications Using the CD54/74HC/HCT4046A and
CD4046B Phase-Locked Loop: A Versatile Building Block for Micropower
Digital and Analog Applications ...
ADF4110 PLL …
ADF4001 PLL …
And a simple Phase Detector chip + a simple VCO ...
Phase/Frequency detector ...
Supposedly "goes well" with ...
VCO with vvc control ...