Background: The WA7GIE beacon was installed in 2004 atop one of the
higher mountains in the Oquirrh (pronounced
"Oh-Kerr") range, west of Salt Lake City and with the
locals just starting to build/buy narrowband 10 GHz gear, it was a
welcome addition, being useful to both test their gear to see if it
was working, but also as a common frequency reference when making
contacts to simplify coordination of QSO as in "I'll call you
25 kHz below the beacon."
The beacon originally used the DB6NT beacon transmitter, model
"MKU10 BAKE" with an internal crystal oscillator and was set for
somewhere in the area of 10368.15-10368.20 MHz range (I'm not
sure...) but over the course of the day it would drift 5-10 kHz -
depending on ambient temperature at the radio site - and it seemed
to be drifting slowly down in frequency over time. When Dave,
WA7GIE, had the opportunity to do so, he'd occasionally nudge the
frequency back up, but during 2011, he ran out of tuning capacitor
range and the beacon was beginning to encroach on the weak-signal
calling frequency of 10368.100 MHz.
This left Dave with two options:
Replace the original crystal. Clearly, something had
"aged" - probably the crystal, but we couldn't be absolutely
sure...
Use an external oscillator.
The second option - the use of an external oscillator - was chosen
as a good-quality OCXO (Oven-Controlled Crystal Oscillator)
could be used to hold the frequency to much greater precision and
stability than had been possible with the oscillator that was built
into the DB6NT unit and I took this on as a project.
Modification:
From the factory, the DB6NT unit is available with either an
"external" or "internal" reference - the WA7GIE beacon originally
using the latter. Converting it to use an external input
simply involved coupling the external reference via a 680pF
capacitor to the appropriate side of the crystal's connection (e.g.
the one "closest" to the input of the transistor in the oscillator
circuit as viewed on the schematic) To accommodate this new
connection a right-angle female SMA connector was soldered to the
outside of the case to which the RF output from the OCXO could be
connected. Because the MKU10 uses a 96-fold multiplication of
the crystal to obtain the 10 GHz frequency, a stable signal source
in the 108 MHz region was required. In this case the target
frequency was 10328.200 MHz, so the oscillator's output frequency
was to be 108.00208333 MHz due to to the multiplication by 96 in the
DB6NT beacon transmitter itself.
Using a die-cast box, I modified the DB6NT unit slightly moving the
voltage regulator - the main heat-generating component - to the same
side of the box as the RF output SMA connector and in the side of
the die-cast box a large-ish hole was also drilled to accommodate
the SMA connector and its mounting hardware. A hole was then
drilled through the DB6NT box that matched the hole in the regulator
chip's heat sink and used both as a mounting point and to transfer
heat from the regulator to the die-cast box while another hole was
drilled on that same side of the DB6NT box - as far away from the
regulator as possible - and used as another mounting point, both of
these serving to hold the DB6NT unit inside the die-cast box.
Comment:
I originally took pictures while constructing the
beacon, but unfortunately they were lost before I could
offload them from the camera, an incident which occurred after
the beacon was already installed atop the mountain.
Figure 1: An OCXO of the type used in the rebuilt WA7GIE beacon - a
Vectron "229" series oscillator. Click on the image for a slightly larger version.
Re-working the OCXO:
Having had recently worked on the K7RJ
beacon I was already familiar with what was
involved in putting together a stable frequency reference and as
with the K7RJ beacon, a surplus Vectron "229" series OCXO was chosen
- see Figure 1 for a picture of the OCXO used in
the K7RJ beacon. This unit operates from a single +24
VDC source, requiring about 400 milliamps when "cold" and taking
10-15 minutes to warm up to reasonable stability and was designed to
operate in the 90-110 MHz region (something in the 101 MHz area
was stamped on the case). Because the target frequency
was 10328.200 MHz, the oscillator's output frequency was to be
108.00208333 MHz due to to the multiplication by 96 in the DB6NT
beacon transmitter itself.
These units are soldered shut, so the tuning access screw was
removed to allow air to escape and to prevent it "spitting" hot
solder and the device was wrapped in a rag and firmly clamped
upside-down in a vise while the outside edges of the end of the case
with the soldered connection were quickly and carefully heated with
a propane torch - always keeping the flame moving - while
firmly (but gently) tugging on 6-32 nuts spun onto the four mounting
studs. Just as the solder flowed on one side, the flame was
moved about while each corner was eased up and with a bit of care,
the "bottom" portion will came off while I was being very careful
not to yank out any wires!
If you were careful, the circuity and wiring in the "bottom" of the
case will remain intact, but I've frequently had to re-work the
circuits, usually due to something becoming un-soldered by the heat
(usually a ground) or a blob of molten solder bridging across
something that it should not. Once it has cooled, it should be
possible to extract the oscillator unit in its foam cocoon from the
case and you should note the arrangement of the wires. It's
not unusual to break the bottom foam piece into 2 or 3 pieces, but
just as long as you keep it from breaking apart even more and save
all of the pieces, you'll be fine. Once it is apart, use a hot
soldering iron or soldering gun to remove "blobs" of solder that
might impeded reassembly.
Typically, the OCXO itself is in a small, copper case wrapped with
the oven's heating element. On one end (usually that with the
access hole for the frequency adjustment capacitor) there are blobs
of silicone adhesive holding two protruding bumps of the circuit
board in place and scraping these clean and then pushing on them
will allow the copper case to come apart.
Inside, one will be able to see a TO-5 crystal case (e.g. one that
looks like an old metal-cased transistor) and attached to it - or
floating freely within the box - will be a small, round, paper tag
on which is written a number - the operating temperature of the
original crystal in Centigrade: Keep that number as you will
need it when you specify the temperature at which the new crystal
will be cut!
At this point, you'll be ready to order a new crystal and expect to
pay about $50 plus shipping from International Crystal. The
folks there actually have the "formula" of this particular OCXO in
their records, but to save cost, you can order a crystal cut with a
lower precision: Since you'll be tweaking it with the tuning
capacitor, its "natural" frequency is less importance. When
ordering the crystal, also specify that it be cut so that it
operates at the same temperature of the original device using the
information written on the small, round paper tag that you should
have saved!
The actual frequency of the crystal is 1/2 of the operating
frequency - or, in the case of a 10 GHz beacon with a crystal
operating near 108 MHz, the crystal will be "cut" for 54 MHz or so
and the doubled in the OCXO to the final 108 MHz frequency.
When the crystal arrived, I carefully removed the old one noting the
lead length and its orientation, re-using the spacing hardware that
was been used on the original and cutting the crystal leads
only after they have been soldered to avoid damaging it from the
mechanical shock!
Using a service monitor with a spectrum analyzer I applied +24 volts
to the OCXO and observed that it was outputting a signal on the
appropriate frequency - in this case, close to 108.0020833
MHz. While observing the output power, stretch and squeezed
the two air-core inductors on the board next to the one with the
crystal until maximum power was obtained at the 108 MHz frequency
which was around +10dBm. Now that the output tuning network
had tuned, I was ready to put it back together!
Reassembling the two halves of the copper case back together, I put
new "blobs" of RTV adhesive (the "non-vinegar" type!) on the
knubs from which they were originally removed, allowed the silicone
to cure overnight, and then put it back in the foam, taking care to
orient it so that one can access the tuning capacitor through the
hole in the case. After this, I carefully put the pieces of
foam (it's nearly impossible to disassemble these things without
breaking some foam!) and firmly pressed the bottom of the OCXO
back into the case. I then powered up the OCXO again and
waited 20 minutes before determining if the mechnical adjustment
easily tuned through the desired operating frequency. Once
satisfied that it was operating properly, with solder at the ready I
used a torch to quickly re-seal it with minimum heat.
Once it had cooled, I re-tested the oscillator and found that
everything was still working so kept it powered up and set it aside
for several days until I got time to continue work on the
project. In that time, the oscillator drifted down 10-15 kHz
at the 10 GHz frequency - but this was entirely expected: This
oscillator had been powered off for years and it likely took some
time for the oven to stabilize in temperature. In addition to
this, it was likely that the parts inside were "baking out" and
losing absorbed moisture, mechanically settling in, and most of all,
the brand new "green" crystal was aging. In the several weeks
that I had the completed beacon before handing it over to WA7GIE, I
kept it powered up, testing it on my roof, and it was heard
throughout the Salt Lake valley and in this time, it drifted down a
few more kHz indicating that the effects of aging and "bake-out" had
already slowed. Comment:
If you don't want to re-crystal the OCXO yourself, you may be able
to find someone on a microwave-related internet group that may be
willing to do it for you for a reasonable cost. It's
possible that International Crystal may be willing to re-work the oscillator - also
for an additional cost!
Figure 2:
Diagrams of the power supply and oscillator
keying (top) and beacon keyer (bottom) Click on an image for a larger version.
Diagrams:
Power supply and oven section: Figure 2 shows the diagrams of the external
portions of the beacon. While the DB6NT beacon module is not
shown schematically, it is just a matter of feeding the output of
the oscillator into the position formerly occupied by the crystal,
using a blocking capacitor, as described above. The output of
the oven oscillator is at the 108 MHz frequency of the original
crystal and because of this, no further modification is needed to
the DB6NT unit. If it was originally ordered with the
"external" option, the output of the crystal oven oscillator would
be connected there. This Vectron ovenized oscillator has an external frequency
control and as shown in the diagram, this is used to provide "fine
tuning" of the crystal frequency using a multi-turn potentiometer
(R202) but as we can see, an external FSCW (Frequency Shift CW)
signal is applied via R201. The other components - C201 and
C202 help to stabilize the keying, both reducing "chirp" from keying
as well as a low-frequency warble that emanates from noise of the
Zener diode regulator internal to the oscillator and present on the
"+REF" pin.
One potential difficulty with the Vectron oscillator - and many
similar oscillators - is that they require a +24 volt supply, an
inconvenience since lower-voltage supplies (nominally 11-15 volts)
are typically already available. To accommodate this, an
LM2577-12 switching up-converter is used to supply the +24 volts for
the oscillator, using R102 and R103 to set the voltage to 24
volts: If the "adjustable" version (e.g. LM2577-ADJ)
had been available, I would have used it, along with the
appropriate-value resistors to set the voltage. As can be seen
in the diagram, while this supply is quite simple to build, suitable
voltage up-conversion modules are easily found on Ebay and similar
sites, many of them having a voltage adjust capability. If one
uses a pre-built unit, be sure to test it and adjust the voltage -
preferably under at least 100 mA load - before
connecting it to the oven!
As can be seen in the diagram there is fairly extensive input and
output filtering on the switching supply. On the input there
is L101/C101 located at the power supply input to the enclosure
containing the DB6NT unit and the oscillator as well as
C102/L102/C103/C104 on the switching power supply board itself along
with C106/C107/L102 and C107 on the output of the switcher. This
additional filtering is absolutely necessary to keep the
switching supply's noise out of the oscillator and the DB6NT
unit!
If you happen to buy a pre-built switching converter, it
is strongly recommended that one adds additional
capacitance and inductance as shown in the diagram, particularly
when one considers the fact that most of these inexpensive,
pre-built converters tend to use "cheap" capacitors that are
probably not well-suited, by themselves, for the task of removing
the switching energy! Adding additional capacitance to the
input and output - directly on the converter - will likely go a long
way to prolonging its operational lifetime. Also, these
inexpensive converters do tend to be a bit "dirty" when it comes to
the amount of switching energy that they impart on the input and
output leads and this additional filtering is arguably more
important.
For generating the keying signal a simple keyer based on an
8-pin PIC is used. When I wrote this code I wanted it to be as
flexible as possible and have produced several different versions,
but they all have one common trait: In FSCW mode they all
output differential keying. This is a fancy way of
saying that when one of the keying pins goes high, the other goes
low in voltage and vice-versa.
Connected across the keying line is a 10k potentiometer and with
this differential keying, the net result - if the potentiometer is
set exactly in the middle is a voltage that is one-half of
the supply voltage (2.5 volts in this case) no matter what the
keying state may be. The advantage to this is
that simply by adjusting this potentiometer, one may select both the
magnitude (amount of shift for the FSCW) and
the sign (whether a "key-down" is a higher or lower
frequency) with one simple adjustment.
Our preference has been to set the magnitude (amount of shift) of
the keying to something on the order of 1.5-2.0 kHz while the sign
of the keying is such that a "key-down" condition causes a shift upwards
in frequency. The reason for doing it this way are three-fold:
A smaller amount of frequency shift (e.g. 1.5-2.0 kHz) reduces
the amount of audible "chirp" that may result.
An upwards shift in frequency during "key down" works nicely
with the fact that on the microwave bands the convention is to
use upper sideband. If the beacon is tuned
in so that the code is copied properly, the "key-up" condition
is "below zero" in the audio passband and is not audible.
With just a 2 kHz shift, it is possible to hear both
key-up and key-down if desired as both frequencies will fit into
a standard SSB bandwidth which can be useful when monitoring the
signal strength as the S-meter can be more constant.
In addition to keying, the differential lines also connect
to a 2-leaded dual-color LED and key-up/key-down is indicated by a
red or green LED as desired. Of course, one could use just a
single LED or two separate LEDs for this indication! The
advantage of the dual-color LED it is always on no matter
what the keying state so that one could easily tell if it was
powered up - plus it required only one hole to be drilled!
The PIC used also has an A/D converter and it is used for two
purposes here. The voltage from the wiper of potentiometer
R201 is read and the speed of the Morse keying is adjustable in this
way, while the voltage appearing across U203, an LM335, is read and
converted into a temperature that is included on the transmitted
Morse message as telemetry. On the WA7GIE beacon it was
decided to place the LM335 inside the enclosure and as such, it
typically reads between 110 and 125 F (43-52C) since it is located
next to the crystal oven and the DB6NT module.
The results:
The re-configure beacon has been on the air since late 2010 and
during the first year, it drifted down 12-15 kHz from its original
frequency of 10250.200, this largely occurring due to aging of the
crystal and due to some "settling in" of the oscillator
components. After about 18 month of operation, the opportunity
arose and the beacon frequency was re-set to something near its
intended and since then, it has continued to drift downwards, but at
a much slower rate.
With this rebuild, the beacon frequency is much more stable and
appears to vary less than 1.5 kHz in frequency over the course of a
day - far better than the 10-15 kHz of the original
configuration! Since the beacon is easily heard over a wide
portion of Northern Utah, it is still used as a frequency reference
when setting up QSO's, as in "10 kHz below the beacon" and as a
"sanity check" to see if our transverters are anywhere near where
they are supposed to be!
If there is interest in obtaining a PIC for your beacon
project - or, if you have other questions about this beacon,
please contact me using the email link below.
