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Why a frequency reference?
When operating on the microwave amateur radio bands, narrowband modes (such as SSB or CW) are often used to maximize the link margin - that is, be able to talk when signals are weak!
The use of microwave frequencies and narrowband modes such as SSB or CW means that the one must maintain pretty good frequency stability and accuracy:
- Stability is important as a drift of even a few hundred Hz at the operating frequency (in the GHz range!) can affect intelligibility of voice - or, if CW is being used for weak-signal work, such drifting can move the received signal outside the receiver's passband filter! Having to "chase" the frequency around is not only distracting, but it complicates being able to communicate in the first place.
- Accuracy is also important because it is important that both parties be confident that their operating frequencies are reasonably accurate. If a contact is arranged beforehand, it is vital that both parties be able to find each other simply by knowing the intended frequency of communication. As long as the two parties are within several hundred Hz of each other, it is likely that they will be able to find each other. If the error was on the order of several kHz, "hunting" would be required to find the signal and if those signals are weak, it might be missed entirely!
Having one common reference can also be convenient if one is operating portable using battery power. Using an external reference means that one doesn't need to keep all of those individual pieces of gear "warmed up" all of the time to maintain stability, turning it on (and draining battery power) only when it is needed.
In addition to a Rubidium frequency standard, I have a 10 MHz "ovenized" crystal oscillator as well. While not as accurate, the crystal oscillator's stability and accuracy is more than adequate for operation at least through 24 GHz, consuming significantly less power to operate than the Rubidium standard - an important consideration when operating from battery. Nevertheless, it's nice to have something that is portable and "dead on" - and can also be used as a backup if necessary!
About this frequency reference:
The Rubidium frequency standard - an Efratom LPRO-101 - was obtained on the surplus market, having been originally pulled (probably) from obsolete cellular telephone gear. Where crystal-based oscillators use the mechanical properties of the quartz to determine the operating frequency, a Rubidium standard utilizes an atomic resonance based on the passage of light passing through a Rubidium-vapor lamp in an RF field. This mechanism of operation is fundamentally more stable than that of a quartz oscillator, allowing orders of magnitude greater long-term stability.
Unlike a quartz crystal oscillator which has no clearly-defined "wear out" period and, if well-designed, can actually improve as time goes on, a Rubidium standard has a definite lifetime associated with its lamp: As the unit operates, the Rubidium within the lamp is gradually consumed and eventually, too little vapor is available for the atomic resonance to be detected and the unit fails. For this reason, many amateurs that use Rubidium standards choose not to leave them on all of the time. For "base station" use, a GPS-based disciplined quartz oscillator is often used as the "primary" standard against which the Rubidium unit is compared. Since a GPS-based disciplined reference is, by its nature, not "portable" (that is, you can't just move it around unless you stay in one location for many hours, giving it time to re-lock) a Rubidium standard fills the niche providing high accuracy in a portable package..
At room temperature, the LPRO-101 takes 3-5 minutes to warm up and "lock", immediately providing accuracy equal or better than a good-quality "ovenized" quartz oscillator, gradually achieving something approaching its ultimate accuracy after a few 10's of minutes. The LPRO-101 has two "frequency trim" adjustments - one mechanical and the other electrical - that can be used to provide fine-tuning of its output frequency. Generally speaking, one would set the Rubidium's frequency just once using the mechanical adjustment (a potentiometer) using a "primary" reference (such as a GPS disciplined oscillator) as a basis of comparison while the electrical adjustment would be used if one were to incorporate the standard into, say, a GPS disiplined system or were to add a means of temperature compensation to further-increase accuracy.
The LPRO-101 has a number of other outputs. In addition to the "BITE" signal (see below) there are also several other signals that indicate the unit's internal health - such as the "Lamp Voltage" (which provides a relative indicator of remaining life of the Rubidium lamp) and crystal tuning voltage. While these signals are useful for diagnostic purposes, but I chose not to bring them out of the box with the idea that if the unit were to fail in the field where it might be used, I wouldn't likely be able to do any in-depth diagnosis, anyway!
Assembling the unit:
The LPRO-101 unit was originally mounted to a heat sink plate in its originally-configured chassis, but as-shipped from the surplus vendor there was no heat sinking provided for its baseplate. In order to avoid thermally stressing the unit - especially at higher ambient temperatures - it is necessary that it be operated only with a heat sink.
Shown in Figure 1 is the packaged unit, contained in a die-cast aluminum enclosure - a Hammond 1580D series. Mounted to the lid, the aluminum cover alone seems to adequately spread and radiate the heat from the unit's baseplate and when assembled, much of the lid's heat is also transferred to the body of the enclosure: In original testing of the LPRO-101 without using a heat sink it was noted that one part of its baseplate got significantly hotter than the rest, but with the aluminum lid, this "hot spot" was considerably reduced. To maximize heat transfer, it is necessary to make sure that the inside surface of the box's lid is smooth (one must grind off stamping or mold marks) and mates well with the baseplate and that heat-sink compound is also used. A total of six 4-40 machine screws were used in the holes provided to bolt the lid and baseplate together.
The wiring - both DC and RF - is connected via a series of pins on the side. While connectors are readily available, I simply fabricated something using a cut-down IDC-type connector from a discarded computer, soldering wires to its pins and stabilizing them with thermoset adhesive to prevent them from breaking off.
Power supply:
The LPRO-101's specifications note that the allowable voltage range for the unit is from 19 to 36 volts. Because linear regulators are used within the LPRO-101, higher voltages result in more heat being dissipated, higher power consumption and higher battery drain. For this reason, I chose to operate it from the lowest-possible recommended voltage - 19 volts - which also meant that in order to run it from a 12 volt power supply (such as a battery) a power converter was required.
A cheap and easy means of up-converting the voltage is to use the National Semiconductor LM2577 "Simple Switcher" (tm). This chip can handle several amps and contains most of the circuitry necessary to perform the voltage conversion, requiring only a few external components. While "pre-built" units using this same (or similar) chip are easily found on EvilBay, I chose to build my own.
This chip works by momentarily shorting the "diode end" of the 100 uH inductor, L103, to ground, causing current to build up and a magnetic field to be established. and then "un-shorting" the inductor - a process that is repeated at a frequency of about 52 kHz. When L103 is "un-shorted" by the chip, the magnetic field collapses and the resulting current "pushes" against the input voltage and C102 on one end of the coil while the current from the other end of the coil dumps through D103 and charges the output capacitor, C104. Theoretically, the voltage produced by L103's collapsing field would be extremely high, but this is quashed by the charge transfer to the C104 and this energy transfer to the output capacitor one can effect an efficient conversion of a lower voltage to a higher one. When the voltage on the "FB" pin exceeds the threshold, the switching of the regulator's oscillator is turned off, allowing the voltage to drop: By enabling/disabling the oscillator appropriately, one can effectively regulate the output voltage.
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The switching regulator entire circuit was mounted "dead bug" on a piece of circuit board material, providing both low-profile construction (as space inside the die-cast box is rather limited) and a very solid ground plane - an absolute necessity when using a switching regulator.
For heat sinking, the tab of the LM2577 was soldered directly to the circuit board's ground plane, providing plenty of heat dissipation. To minimize radiation of magnetic fields into other circuits (which might include the Rubidium box) a toroidal inductor was used and placed as far away from the LPRO-101 and driver amplifier circuitry as possible.
For power supply bypassing, "Low-ESR" electrolytic capacitors were used: These types are absolutely necessary to provide reasonable filtering and good efficiency as the use of "ordinary" capacitors will compromise both! The various components were mechanically secured in place using silicone adhesive to prevent them from moving around during transport and breaking away and to insulate some of the connections.
The output of the power supply goes through another inductor - a 47uH toroidal unit obtained from a scrapped computer power supply - followed by another low-ESR capacitor: This second set of filtering removed residual switching energy from the LPRO-101's power supply input and if it hadn't been installed, spectral components of the switching frequency (in the 52 kHz area) might have found their way into the 10 MHz output!
Distribution amplifier and status indicator:
In order to be able to drive more than one external unit, an LM7171 high-speed, high-output op amp is used to buffer and amplify the 10 MHz signal. This op amp is mounted on its own board, located as far away from the switching converter as possible, and three outputs are provided.
Of the several signals output by the LPRO-101, one of them - the "BITE" (Built-In Test Equipment) signal is the most useful for typical operation. This signal being "high" indicates that an error condition is being detected by the LPRO-101's internal circuitry. While this indication could simply mean that the unit is still warming up or that its supply voltage is too low, it could also indicate that the unit itself has failed. Any time this signal is high, one should not trust frequency output of the unit to be accurate.
If this "BITE" signal is high, Q101 is turned on which turns Q102 off, which allows current through R115 to illuminate the "red" portion of the dual LED, D102, indicating an "error" condition. If the "BITE" signal goes low - indicating that the unit is operating properly - Q101 is turned off, allowing current through R113 to flow into the "green" portion of the dual LED and turn on Q102 which turns off the "red" LED: This "green" indication signifies that the unit is operating properly and can now be trusted to provide a reasonably accurate and stable reference. The only condition not detected by this circuitry would be the complete failure of the voltage up-converter and the disappearance of the 19 volt supply to the LPRO-101 or a failure of the output amplifier - but in either case you wouldn't have any output, either!
Power supply input filtering/protection:
Finally, the input supply is RF-bypassed using a feedthrough capacitor and C101 to prevent the ingress or egress of extraneous RF along the power lead. For power-supply short-circuit and reverse-polarity protection, R101, a 1.1 amp, self-resetting PTC fuse is used in conjunction with D101.
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This page and its contents copyright 2010 by Clint, KA7OEI. Last update: 20100729