Brick oscillators for ham radio microwave

"Most of the Phase Lock Loop (PLL) microwave sources that show up at flea markets and surplus houses are useable for Local Oscillators in one form or another. The output can be used as a LO for a transverter or as a beacon transmitter. Normally the PLL sources consist of a 1.2 - 1.7 GHz free running 0.5-2 Watt oscillator that is phase locked to a crystal reference, then coupled into a frequency multiplier consisting of a Step Recovery diode and a bandpass filter, which passes the harmonic of interest. Power output can be up to several hundred mW depending on the model."

"The units come in many frequency ranges and can be used directly or modified. Brand names are Frequency West, M/A Com and Fairchild to name a few. The phase lock reference crystal can be internal OR external. Both can be used for Amateur LOs. Don't shy away from the external units! A simple oscillator/buffer with a 70 deg C thermistor attached to the crystal is easy to use. It is not necessary to buy high accuracy (expensive) crystals to get "right on frequency". As with all LOs, consider yourself lucky if it comes out on frequency. When ordering crystals, giving the vendor the model number and an oscillator circuit drawing will help you get the proper crystal. Cheap HC-8/U xtals do work in the ovenized PLLs. Refer to Jan 1995 CB Hints & Bits."

"Basically, you need to re-crystal the unit, tune the cavity for a lock, then tune the SRD output bandpass filter. Make sure you use an alarm circuit to tell you when it's locked ! Some sources can have lock problems at temperature extremes in the field. Tuning the output filter requires a Spectrum Analyzer. Most SRD filter units will come off the power cavity with 3 screws. The filter can be rotated so the input probe is still in the cavity. A C-clamp will hold it in place while adjusting the tuning screws. Usually there is sufficient LO to be split 2 ways to feed two mixers. Refer to May 1993 CB Hints & Bits for more modification info."

"Types of units and modifications:"

"1. No mod tunable for normal LO (Fo - 144): Frequency West units in the 3-4 GHz range can be re-tuned for 3312 MHz. There are quite a few M/A 5.9-6.4 GHz units around; both internal and external referenced. These tune to 5616 MHz no problem. 3312 LO Xtal = 92.00 MHz, 5616 MHz LO Xtal = 93.600 MHz."

"2. No mod tunable for high side LO injection (Fo + 435 or Fo + 144 ): This type of unit won't quite tune down to make a low side LO. Frequency West and M/A Com 10.6 GHz PLLs can be used for a 10368 MHz transverter (+ 435). The 6 GHz units work good for 5760 MHz (+ 144). Note: Use LSB to operate on USB. These units also make good beacon transmitters. Use an isolator/PIN diode switch to CW key. 10.3683 GHz beacon Xtal = 103.6830 MHz."

"3. Cavity tuning modified for normal LO: The power cavity on some units won't quite tune down to the proper harmonic. Changing the length of the tuning inductor or the size of the cavity is required. I adjusted a unit to tune 1080 MHz (for a 2160 MHz LO) by changing the inductor length and milling out the sidewall of the cavity. Xtal was 90.000 MHz."

"4. Cavity tuning and cavity output modified for normal LO. An M/A Com 5.9 GHz PLL is the 1152 MHz LO of my 1296 transverter. The multiplier is removed completely and an SMA connector/probe is installed in the cavity and connected to an external UT-141 interdigital filter. The inductor was lengthened to tune 1152 MHz. Pout is a healthy +30 dBm, but easily attenuated and split. Xtal = 96.000 MHz."

"5. Cavity output modified for homebrew SRD multiplier driver. Again on a M/A Com 5.9 GHz PLL, the output multiplier is removed and an SMA connector attached. The cavity is tuned to 1656 MHz for example to drive a X2 SRD multiplier filter for a 3456 MHz transverter. Xtal = 92.000 MHz."

"6. Using the SRD/filter module with modifications: The SRD/filter units are excellent high efficiency multipliers that are great to connect to whatever oscillator you might have. An SMA connector can be installed in place of the input probe and the unit attached to a power boosted DEM 540 MHz LO Board. Or the filter can be removed with a bandsaw and the SRD assembly can be used with your own homebrew comb-line filter or W2CQH design interdigital filter."

"Tune up:"

"To decide whether your surplus unit is good for an LO, you need to find the crystal frequency, see if the cavity will oscillate and lock at the proper harmonic in the 1.2 - 1.7 GHz range and whether the output filter will tune. First put the unit on a spectrum analyzer and verify that it works at the design frequency. Externally referenced units (running without lock) will sweep +/- 40 MHz around the design frequency."

"There is one easy way to find out what the crystal frequency should be and whether the unit will work. Use a GOOD Synthesized Microwave Sweeper as a reference in the 90-105 MHz range. Set the power level for ~+5dBm. While looking at the output on the analyzer, start the sweeper at the design reference frequency. As you move the tuning screw slightly, you will see the output sweeping 20-40 MHz before it locks. It should lock when the exact reference frequency is set. Then tune the cavity while moving the sweeper frequency. See if you can lock it at the proper harmonic frequency. If not, decide if you need to modify the cavity."

"After you get a lock, see if the output filter will tune to your LO frequency. Remember, getting the unit to lock on your LO frequency is the hard part, you can always amplify the level if the filter does not tune. Some PLL units will not lock when turned on from a cold start. You should always turn the PLL on in advance so it can heat up and stabilize at the lock temperature. Use my simple LED lock circuit to be sure it is locked. An LED and a 1.3K resistor is all you need."

In the past few years I have come across a few PLL LOs that at first glance didn't look too promising. I am always looking for PLLs to modify for 1296, 2304 or 3456 MHz transverters. PLLs that work directly on 5760 and 10-GHz were easier to find than units that tune the lower microwave bands, now these too are becoming scarce. Several of the M/A Com 14.0-14.5 GHz sources have shown up. Also, some of the California Microwave 14 GHz units are floating around."

"It turns out that the M/A Com 14.0-14.5 GHz unit is a great LO for a 2304 MHz transverter. This model uses an external reference signal, which makes it easy to figure out the locking range and crystal frequency required."

"On units that don't have an output in the proper frequency range for Amateur LOs, I remove the multiplier and attach a connector to the oscillator cavity. The connector has a small disc soldered to the center pin to capacitively couple the RF from the cavity. The frequency of most of them is 1.2 to 1.7-GHz or so, in the +20 to +33dBm range. Since the cavity is running at the operating frequency, the output is very clean and no additional filtering is needed. Trimming the capacitive coupling disc for the minimum power required will also reduce the harmonics, usually down >60dB."

"I removed the SRD multiplier unit and connected an SMA connector/probe to the oscillator cavity and fired it up. The frequency was higher than normal and output was +26dBm. I was able to tune the cavity up to almost 2-GHz, and down below 1.6-GHz. This frequency range gave me an idea about its useful LO frequency range. 1872 + 432 = 2304 MHz. I connected an HP signal generator to the reference input and found that I could get a lock at 1872 with a 93.6 MHz reference signal (x20). Reference drive levels are usually 0 to +10dBm. The lock was quite broad on the tuning screw and it locked immediately when the Vcc was cycled off and on. Moving the reference signal up in frequency (while re-tuning the cavity) I found the PLL would lock at many different points, as it should, up to about 1.98-GHz."

"A stable reference signal is easy to build. It can be the oscillator/ buffer section of the DEM model LOK unit, for example. To make the PLL source stable, use a 60-degC PTC thermistor on the crystal, as I have explained in other articles."

"Since these PLLs are the same in many respects, I believe the Cal Mic units can be used as discussed also. Don't shy away from PLL sources that arenít in the ham bands. There are ways to modify them all for use as LOs or beacon transmitters. And there is nothing wrong with mixing 2 oddball PLL sources together to get a third signal to be used as a local oscillator. A way to make use of some of these PLLs is to add your own multiplier/filter to the cavity output. I machined an SRD holder and interdigital filter combination that attached to a 6-GHz PLL cavity to create a 3312 MHz LO. The filter had 2 outputs to drive 2 mixers."

"Another way to make use of some of the oddball PLLs is to try an IF other than 144 MHz. 432 MHz makes a good IF; the LO and image frequencies are easily filtered. A simple 10mW transverter from 144 to 432 MHz allows you to use your 2- meter all-mode (or transverter). Coverage will be from 432 to 436 MHz. 432 is a good IF to be used with the 10-GHz PLL LOs that are above 10.368 GHz. High side injection is no problem. Most of the time you are on CW anyway. When on SSB, just switch to the opposite sideband."

Go back