Phil Lee W6HCC
This note is to aid anyone who is setting up a “brick” as a local oscillator for a microwave system. First will be a short general description and technical overview, followed by technical details of operation and tune-up.
OVERVIEW:The ubiquitous “brick” is a complete phase locked microwave local oscillator system. The VHF portion of the system consists of a crystal oscillator, a power amplifier, a phase detector, sweep and lock-on circuitry, and in some, an FM modulator. The microwave portion is made up of a power oscillator, (in some a power amplifier), a varactor multiplier and a filter. The system produces some tens of miliwatts at the microwave output frequency. The frequency stability is determined by the crystal oscillator. If better stability is required, the internal oscillator is phase locked to an external reference or replaced by a precision external oscillator.
CONFIGURATION: A reference signal in the 96-106 MHz range is produced by the crystal oscillator. After power amplification, it is applied to a step recovery diode. The SRD produces a spectrum in the 1.2 GHz to 2.2 GHz. range. The appropriate line of this spectrum is phase compared with the output of the power oscillator. The error signal from the phase comparator is fed through appropriate filtering to a varactor diode in the coaxial cavity of the power oscillator. This fine-tunes the cavity to the desired spectral line of the SRD. The output of the power oscillator is fed to a varactor multiplier (x6-x8 typ. ). The varactor output is filtered using a comb-line filter to produce the final microwave output. Lock-on is assisted by a sweep which is applied to the power oscillator.
STABILITY: The internal crystal oscillator is used only when stability on the order of +-5 Hz or less is required. There are two problems with the internal oscillator. First, only the crystal itself is in a temperature oven. The balance of the oscillator circuit is exposed to any temperature variations which may be encountered during operation of the system. This can lead to unpredictable drifts. Second, the crystal unit itself is of the TO-5 type. These can be obtained from some manufacturers, but are expensive. A better solution is to remove the crystal from the unit, disable the oven, and use the oscillator circuit as a buffer stage for an external oven stabilized oscillator. The frequency of the oscillator (internal or external) ranges from about 90 MHz to 107 MHz. A typical brick will use a multiplier of x96, x102, or x108 from the oscillator to the final frequency in the x-band range. Some fine 100 MHz ovened oscillators have appeared in surplus and are the basis of several SBMS rigs. (W6HCC, K6OW, WA6CDR and WB6CWN to name several using this system.) A second solution (not recommended) is phase locking the internal oscillator to an external 5 MHz or 10 MHz reference. This requires an amateur frequency crystal in the internal oscillator and a modification to use it as a VCXO. Phase lock boards are available from several sources.
CONVERSION: The first problem with using an external reference oscillator is getting the signal into the brick. Most bricks have a small coax connector which is used to monitor the internal oscillator. This may be used “as is” if you can find a matching connector. Otherwise, the connector may be replaced with a BNC or SMA. To use a BNC, the mounting hole for the original connector is tapped for the BNC. Without a tap, the SMA is recommended, since there is very little room for a nut inside the casting of the brick. Once a suitable connector is in place, use a 1000 pf. capacitor to couple to the oscillator circuit. The oven must be disabled (it wastes a lot of power if you let it run). A simple solution is to turn the temperature pot all the way to the low temperature end. The pot is located near the oven at the end of the brick away from the reference connector. A better solution is the ground the center arm of the pot. That will completely disable the oven.
POWER UP: Most bricks operate on -20 volts at 200 ma. to 400 ma. (Check carefully!! You may find one of the rare ones that uses +20 volts.) This voltage should be regulated, but from -19 to -21 volts is usually satisfactory. It is also important that the voltage be well filtered. Any ripple or noise on this line may show up as trash on your L/O! There are several monitor and control terminals on the side of a brick. The most important of these is the “phase” terminal. It will show the actual sweep and lock-up of the system. This is monitored with a DC coupled “scope” during tune-up. Some bricks have a pair of terminals which must be jumpered together to enable sweep in the phase locked loop. This junction also serves as the monitor point. If the internal crystal oscillator is used, the “xtal” terminal may be used to monitor its output. There is an “alarm” terminal on some bricks. This may be used as a loss-of-lock warning.
TUNE UP: To tune up the brick, a minimum of test equipment is needed. A cavity wavemeter, a detector, suitable attenuators and/or couplers, and a DC coupled “scope”. A power meter is nice, but not required. (If a spectrum analyzer is available, most of this is academic!) If you have any crystal that can be used in the internal oscillator, (even better if it is the one that was originally used in the brick), it is a good plan to check the brick on its original operating frequency. This may be well away from its final frequency, but it is good to know that the brick will work before starting frequency changes. Monitor the supply current to the brick on initial power up. Very high or low current is an indication of problems. If you have a power meter, check the power output. This will be a good indication of what to strive for when retuning to a new frequency.
TEST: Connect the brick output to the wavemeter (through appropriate couplers and attenuators as required). Place a detector at the output of the wavemeter and connect it to one channel of the DC coupled “scope”. Connect the other channel of the “scope” to the “phase” terminal of the brick. Monitor the power output if possible. Fire up the brick! If the brick locks up, you will see a DC level between -2 and -18 volts at the phase terminal and a DC level from the detector. On the end of the brick there is a shaft (usually about ¼ inch in diameter with a screwdriver slot ) from the cavity of the power oscillator. This is the rough tuning. Turn this shaft slightly (use care if it does not move easily, some have a lock nut.) and there will be a change in the voltage at the phase terminal. This indicates that the phase locked loop is tracking out the change in the power oscillator frequency. Continue turning the shaft and the loop will break lock. This is indicated by a sine or sawtooth sweep waveform on the phase terminal. Check the frequency. From this frequency and the frequency of the internal crystal oscillator, determine the multiplication factor. Most bricks are x102. This factor is important and will be needed to determine the reference frequency required for ham-band use. A typical brick might use a reference frequency of 100 MHz. and a factor of x102 to produce a microwave signal at 10,200 MHz. The multiplier system used in bricks by different manufacturers is different. The varactor multiplier may use a factor of x6 to x8 for x-band bricks. The typical factors (x96, x102, x108) are for an x6 varactor multiplier. This multiplier is found in most Frequency West and California Microwave bricks.
FILTER TUNING: The filter in most bricks must be returned for operation at 10,224 Mhz. This is the low side injection frequency for a 144 MHz IF. To re-tune the filter, it must be removed from the brick and reversed on the brick to expose the tuning adjustments. Most bricks have lock screws which are under the peel-off label in the top of the multiplier. Be sure to loosen these before attempting filter tuning. They use a small (0.050 in.) Allen wrench. It is a good idea to “walk” the filter to the proper frequency. To do this, disable the sweep by grounding the phase terminal. Use a power meter to monitor the output and slowly move the power oscillator toward the desired frequency. Re-tune each of the filter tuning screws for maximum power, then tune the oscillator a bit more in the desired direction and repeat the process. When the final frequency is reached, tighten the lock screws. As they are tightened, the power will vary slightly. Touch up the filter tuning for maximum power as the locking screws are tightened. When these procedures are complete, the brick will be ready to use and form a reliable and stable local oscillator for your system.