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.

Good Luck.

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.

Transverter transmit mixers require 1 to 10 mW of IF power. When the low-level stage of a 10-watt transceiver can not be tapped into, a suitable 50-ohm load and attenuator is required.

A flanged microstrip mounted 50-ohm load of 10-30 watts is used to dissipate the power. A small value variable capacitor couples off a small amount of power. A fixed value attenuator follows to provide the required attenuation and 50 ohm match to drive the transmit mixer. The circuit shown will provide >35 dB of attenuation at 144 MHz. Additional attenuation can be accomplished with a 100 ohm pot or a chip resistor attenuator at the mixer port.

This circuit should be built into a metal enclosure. Even so, it is still possible to work a strong station on the IF frequency when running 10 watts. Good coaxial relays or a high isolation Pin diode switch must be used. Additionally, switching off your 2-meter station GaAsFET preamp during transverter operation will also help considerably.

The modification described here was done to an MFJ Grand Master keyer, but could be used for other MFJ keyers also. A tune switch is added ahead of the memory circuits so a key down could be programmed.

The simple mod uses a 330-ohm resistor and a miniature (normally open) push button switch. These components are connected in series between Pin 5 of U4 and ground.

I soldered the resistor lead directly to the IC pin. The momentary switch was mounted on the rear panel.

Since my "Hints & Bits" article in June 1994 CHEESE BITS, several homebrewers have asked questions about ovens and thermistors. Which unit is the easiest to use? How are they wired into an existing LO? Does something as simple as a thermistor really work?

Why is temperature compensation a concern? The frequency of your transverter's LO can drift many kilohertz when you go roving or during seasonal temperature changes in your basement. Few Amateurs think about the temperature drift of their transverters or commercially made transceivers when they take the equipment out to the field. I measured a 75-KHz shift on the non-ovenized LO of a 10-GHz transverter. Reducing (if not eliminating) the frequency variable when working a weak station is very significant. The easiest, cheapest and fastest way to temperature compensate a crystal is to use a PTC thermistor. The leaded thermistor KC004P is $2.31 from Digi-Key (800-344-4539) and can be added to any crystal circuit in 10 minutes. The unit is connected directly to 12V dc. The Yaesu G9090019 model thermistor comes with its own holder that slides over an HC-25/U crystal. The number for Yaesu USA is 310-404-2700.

First, quickly unsolder one lead of the thermistor without damaging the metalization. Solder tin the side of the crystal. Solder the flat side of the thermistor to the crystal case. Solder the remaining lead to the +12V line. Solder a small gauge wire from the case of the crystal to dc ground to complete the dc path.

The thermistor is nominally 50-ohms at 25-deg C, so it draws several hundred milliamps for a short while, then settles down to an idling current less than 30 mA when it reaches its operating temperature. I found the 60-deg C unit to be hot enough, even for the temperature excursions my equipment sees in the attic. Even in the winter the crystal temperature stabilizes after five minutes, instead of drifting for hours. Adding a small styrofoam or insulated cover over the crystal will further stabilize the frequency. Don't get hung up on netting crystals to an exact frequency. Adding a heater will age the crystal in addition to moving its frequency, so allow the transverter to stay on for several days to complete the aging process. You may not be able to pull it back to the original frequency with the crystal trimmer. You can either make a note of the exact frequency and use it that way, or figure out how far the heated crystal moved and order another crystal based on that frequency delta and heater temperature.

I have added thermistor compensation to several SSB Electronic transverters, all of which adjusted back to the original frequency. The thermistors work great on reference crystal oscillators for PLL sources. Adding a $2.31 thermistor to a DEM no-tune transverter produces a rock-solid LO that won't budge even in the hot sun.

This 3/4 wavelength power divider was shown to me by K1DY, Bill Olson, back in the 70's. It is made with N-connector fittings, which makes it easy to build. Its simplicity certainly does not portrait its performance. The PD is 1/4 wavelength on 432, so it also useable on that band.

The unit measures better than 20dB Return Loss at the feedline port. The phase delta at each of the 4 ports is excellent when the connector lengths are the equal. This is accomplished by using connectors made by the same manufacturer.