By Chuck Houghton,* WB61GP

MICROWAVt::A bove and Beyond. 1296 MHz and Up

The Qualcomm "Omnitrack" DRO Synthesizer

The Qualcomm DRO (dielectric resonant oscillator) syn-thesizer normally operates on a frequency of 2620 MHzand has an output power of +10 dBm. Frequency stabil-ity is under control of a dielectric resonant controlled oscilla-tor (DRO). This ceramic puck stabilizes the oscillator, and syn-thesizer phase-locked-loop (PLL) circuitry must be modified tochange the output to another frequency more desirable for ama-teur microwa ve use.

Unfortunately , the DRO synthesizer is con troll ed by itsdivide-by reference of 1.25 MHz (random-acces s [RAJ counterset to divide reference 10 MHz by 8). A solution was to changethe RA counter to divide the stock frequency of 10 MHz by 64for a new reference frequen cy of 156.25 kHz. There are manypossibilities, but for this application the irregular frequen cysteps limited those frequencies that could be reached by the PLLchip. Mod ification of the DRO took the form of adding sol-der-bits of copper or short length s of copper wire soldered tothe top of the DRO, stretchin g the DRO resonance. This couldbe a small coupled pF capacitor, or an inductance used to tweakthe DRO to a new frequency (free-running frequency neardesired operation point).

The above is where this project lay for some year s. Thenwhile cleaning out the shed and trying to reor ganize materialstored there, I came upon a large box ofDRO synthesizers. Atthe same time, I revisited the file of material and modificati onnotes that John Stevens, WB 2BYP, and I had worked on pre-viously . I had done the original modification work using thestock refere nce frequencies of 1.25 MHz and 156.25 kHz, butJohn saw a much better use for these DRO synthesizers, work -ing his plan around a new reference frequ ency of 1 MH z. Thisallo wed many other possible frequency combinati ons thatwere cap able of being reached by reprogramming the PLLdividers and constructing a new divider (divide by 10) to pro-duce a I -MHz referen ce from the lO-MH z TCXO (tempera-ture-compensated crystal oscillator).

John put together a great modifi cation procedure and gaveme his permission to cover it here . I am sure it will be met withgreat interest, especially because the synthesizer PC boards arestill available from the author (WB6IGP) for modification toother useful microwave frequencies.

Modification for 2592 MHzThe sequence of images and comm ents builds upon my arti-

cle entitled "Above and Beyond," which appeared in the June1994 issue of 73 Magazine. I had known about these synthe-sizers for a few years, and only recently started thinking about

*Member San Diego Microwave Group, 6345 Badger Lake Avenue,San Diego, CA 92119e-mail:

62 . CO VHF • Winter 2005

Photo A. Bottom of the synthesizer PC board with pins of thesynthesizer chip isolated by dremel-cutting around each pin to

isolate them f or experiments on other fr equencies.

them as an LO (local oscillator) or marker generator formicrowave experimentation. I had been looking for a signalsource that I could lock to a lO-MH z TCXO for field use oras a GPS-disciplined oscillator. I wanted to know with someaccuracy where the band edge s are at 10368 and 24 192 MHz.Jud , K2CBA, uses one of these as the basis for an LO in hishomebrew lO-GHz transverter. The frequencies at which thesynthesizers lock with the Omnitrac 10-MH z TCXO are lim-ited in utility, but if you are open -minded about what to useas an IF and have a rig that will do it, you can use the boardswith minor mods.

In my 73 Magazine article I discussed some of the frequen-cies that are available with a minimum number of modifica-tions, and I provid ed some good schema tic inform ation. Notethat you may have to do some drawing out of the circuit to sat-isfy your curiosi ty regarding some of the board ' s finer detai l,such as the reference filter and YCO (voltage-controlled oscil-lator). One of the boards I modified had some workmanshiperrors that contributed to a failure in the supply voltage to theYCO buffer amp. It was an easy fix, however, and the unmod-ified board came up with +10 dBm at 2620 MHz . Jud and I dis-cussed the utility of the boards during some Monday evening2-meter chat s, and a coup le of ideas came to mind . A fewevenings spent at the test bench led to some interesting find-ings. I wanted the lock-to frequencies to come out in more con-venient values for common IFs with low side injection.

What followed is the Qualcomm synthesizer with modifica-tions to place it on 2592.000 MHz (10368 .000 divided by 4). Iam using this for a marker generator. The reference is runnin g

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Photo B. Top ofthe synthesizer showing added perf-boardtran-sistor amp and lO-MHz TCXO used to provide clock input to

the syn thesizer via a new , added 7490-divide-by-lO chip.

at I MHz, and the R divider is set to 8. That gives an internalreference frequency of 0.125 MHz. The advantage of this is thelock-up points now occur at more convenient frequencies in the2.5-GHz region. The existing scheme divides the IO-MHz ref-erence oscillator by 8 to yield 1.25-MHz reference steps, yetthe lock-up frequencies are less useful. By dividing the refer-ence oscillator by 10 using a 7490 decade counter, and givingthe chip I MHz instead of 10 MHz, the 0.125-MHz referencestep is achieved without changing to a different external TCXO.You have to by-pass the on-board reference frequency filter tointroduce the I-MHz signal from the decade divider. The greatthing about this is that the LO or marker genera tor can be runfrom an inexpensive IO-MHz TXCO for better stability thanopen-air or heated-crystal oscillators.

For 2592 MHz the N counter has to be set to 80, and the Acounter has to be set to 16. I dremel-tooled the board to isolateall the A and N counter pins on the MC145 152 (see photo A forthe bottom of the PC board). This was more work than need-ed, but it allowed me to play with programming different fre-quencies. I put a 1/4-inch lead of #22 wire on the DRO to addsome parasitic inductance to achieve lock. Note that there is a"slice-to-tune" element on the existing PC board that can beplayed with to yield a more convenient open-loop oscillationfrequency. Output is about +10 dBm . I did not bring in the loopfilter to limit the phase noise, as I was looking for an ultimateaccuracy signal source, more so than an LO. If it were used asan LO, you would want to add capacitance to the loop filter toreduce receive reciprocal mixing noise from nearby stationsdue to oscillator phase noise. The existing amount of capaci-tance will have to be increased by a factor of about 10.

I added the lock indicator to help diagnose lock conditions.I think this really is a big help, and it is right there and easy todo. It just takes an LED hanging off one of the pins on the whitepin header. Note that it is dark when open loop, dim whenunlocked, and bright when locked. This is an inexpensive, use-ful feedback. The modified synthesizer will hit all common LOfrequencies for amateur bands, as well as the in-band signal.For example , it can be made to hit 10224/4 as well as 10368/4.It also will hit the band and LO frequency squarely at 24192 .It will hit an 18th sub-multiple of 47088 MHz, as well as the

www.cq-vhf.com Winter 2005 • CQ VHF • 63

Photo C. The modified synthesize r operating at 2688 MHz onJohn, WB2B YP's workbench with a spre adsheet in the

background for programming the synthesizer.

Photo D. Modification of DRO synthesizer by adding a dropofsolder to the DRO ceramic element to change resonance to

a fre quency other than the stock 2620 MHz.

frequency 144 MHz below that. Ifyou are using a OB6NT mixerthat runs La at half or quarter frequency, you can find these fre-quencies, too.

Note that the frequency to hit the 18th sub-multiple of 47088is easily within the range of the unmodified ORO. Keeping theORO is advantageous, as it is fairly high Q. To hit the other fre-quencies you will have to substitute anL/C combination with atiny Johanson trimmer or equivalent and resonate close to the in-tended frequency. I know this works between 2 and 2.5 GHz,although I have not pushed it up to 3 GHz yet. More to follow.

What is needed to simplify the process is a simple map thatsays, "Ground these pins to get these frequencies." (The pro-gram.pdf document is provided with the synthesizer.) It ' s from

an Excel sheet that John developed to calculate the combina-tions of programming the pin-for-pin synthesizer chip vs. fre-quency obtained. The synthesizer board is on the right, the 7490is pasted onto the perf board, and the TCXO is on the lower left(see photo B for details of the top of the modified synthesizerwith added transistor amp and TCXO 1O-MHz oscillator).Thereis a 2N5l 79 buffer amp to build up voltage to the TTL K', Itwould be better if there were a CMOS IC as the decade counter.The TTL 7490 was what I had at first. This is running off theGPS-d isciplined oscillator in the photo. The ultimate accuracyat a reasonable cost can be had by using a GPS-di sciplined 10-MHz osci llator for one part in 10 to the minus 12 accuracy.Translated to 10368 MHz, the accuracy is pretty darn good incomparison to homebrew, OEM, or OB6NT Las. The stabili-ty is as good as you feed it.

CHIP CAP STANDING ON END

Modification for 2688 MHzHere is the second unit, modified for 2688 MHz. This fre-

quency is 24192/9 and is useful as a signal source. This is doneby setting N to 83 and A to 16, and driving the reference dividerwith I MHz. In this case I am sourcing the I MHz from the othersynthesizer, running from the lO-MHz TCXO/decade divider.See photo C for this modified synthesizer on my workbench.

This is the modification to the ORO in order to get it to go sig-nificantly higher in frequency (see photo 0 ). The ORO is the bigblock at about 8 0' clock in this crude view through an eye loupe.The VCO transistor is the device about in the center. This modwas accomplished by lifting the coupling chip capacitor into theair and putting a solder bead over the pad with the transistor. Iremoved the pad below the ORO connecti on and put a large beadon the ORO center pad. The chip capacitor now bridges the twolow-inductance solder beads. With this technique I was able toget the free-run frequency up to 2718 MHz. This made the oscil-lator easy to pull in to 2688 with tuning voltage.

It is good to know that the ORO can be pushed this far in fre-quency, because there are a lot of useful freque ncies if the oscil-lator can be made to lock above the design center of 2620 MHz.A signal source at 47088127 = 2616 MHz is possible without

Red +12 vdcRFOSCDRO

RF AMPSHIELDCOVER

Black ioxorz TCXOWhite led to gnd

a

DRO

oGRAY RF 2620 Mhz +1ODBM

_~=~=f~COPPER STRIPLINE MOD

SYNTH MC145l 52

DRODOT

Figure 1. Connections for the test on 2620 MHz to verify thesynthes izer works as a stock programmed unit. A lso shown aredetails of the copper stripline added between the DRO centerdot and the transisto r to obtain afree-runningfrequency slight-ly above the desired f requency bef ore prog ramm ing the

synthesizer.

64. CQ VHF. Winter 2005 Visit Our Web Site

mods to the DRO , and if the DRO can bepushed to 28 16, a signal source for 76032can be made. Lots of LO possibilitie sexist. However, for the most part they willrequire some creative mods to the DROor the removal of the DRO , replacing itwith a Johan son trimmer of small pFvalue and a strip of tweaked-to-frequen -cy copper or a coil for lower frequencies .The oscillator should be made to free runabout 20 MHz above the operating fre-quency to have a suitable lock voltage.The DRO is easy to remove by discon-necting the solder connection and heat-ing up the body. This modificatio n pro-cedure by John, WB2BYP , can be foundon the web at with John ' s con-version data and more details of theQualcomm DRO synthesizer.

ApplicationsWhile the DRO synthesizer does have

limitations, John has greatly improved theadaptability in his conversion by changingto a more u seful reference frequency. Ihave done lots of different applicationssticking with the 1.25-MHz reference fre-quency and its limited operation. Forinstance, we were able to use this synthe-sizer on our 47-GHz transceiver. We cutone pin open (TTL high) and grounded twoother pins (TTL low) on the synthesizerchip, allowing operation on 2640 MHz.The idea was to use a Verticom very-high-quality BCD (binary coded decimal) con-trolled agile synthesizer opera ting at10.44250 MHz. This LO drove a PECOMTX Module (multiply by 2 =20.885 GHz+ the LO drive to the IF port at 2.640 GHzfor a output at 23.525 GHz). Doubling infrequency in our final harmonic mixer pro-duced47.05GHz. Using an IF of 145MHzproduced 47.195 GHz. While the rig is nota barnburner, it has shown to be a reliableand simple rig to put together. Of course,the use of the MTS2000 Verticom synthe-sizer, the step frequency of which is I kHzfrom 8.7 GHz to 10.7 GHz, was a key ele-ment in making the system's main localoscillator easy to use.

The system ' s IF drive LO: The DROsynthesizer also proved quite versatile inits easy conversion to 2640 MHz becausethe stock synthesize r frequency steps areat 20 MHz, making its use in this appli-cation a no-brainer. I thought I would tryto get the DRO oscillator to free run (nolock) somewhere in the 2592 frequencyband. Remembe r, this is 2592 times 4,which equals 10368 MHz, a great test

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shot to demon strate what I had to do toget there at this frequency of 2592 MHz.

First I removed the chip capacitor fromthe DRO center dot and the transistor onthe PC board . Then, using a solder suck-er, I removed the excessive solder fromthe DRO cente r connection to the PCboard trace to leave the DRO center dotfloating (isolated). I re-attached the chipcapacitor stand ing up on the transistorside of the now open PC board trace andsoldered it to this PC board trace. I fabri-cated a small section of copper tape thatwas a few thousands of an inch thick andsoldered one end of it to the DRO centerdot. I soldered the other end to the chipcapacitor (top) while it was standing onend, thus inserting a new inductance cou-pled by the series chip capacitor betweenthe transistor and the DRO center dot. AsI write this, a half-hour has gone by andthe frequency of the free-running oscilla-tor is still 2593.73 MHz as read on my fre-quency counter. Not bad for free running!

The first frequency I came up with was2400 MHz, while the oscillator was freerunn ing. That meant my copper induc-tance was too large. Changing the copperinductor (a short piece of copp er tapeabout 3/16 by 1/4 inch was use for firsttrial) by removing some length and widtha bit at a time brought the free-run ningoscillator up to 2593.73 MHz. During theprocess of soldering and unsold ering, thechip capacitor got lost when it went fly-ing from the tool that held it. I replacedit with a 1.1-pF chip capacitor (not a sur-face-mount standard type , but a micro-wave type from ATC [Americ an techni-cal Cerami cs], .050-size l.l-pF chipcapacitor to reach the 2592-MHz range).I found I could go down to approximate-ly 2100 MHz by replacing the 1-pF chipcapacitor with one in the 5-pF range.

The bottom line is first adjust the chipcapacitor or change the dimensions of thecopper stripline inductance to achieve afrequency close to the one in which youare interested. Then set the synthesizerchip pin for pin programmin g to achievesynthesizer lock. For an LED indicator,tie the white wire to one side of the LEDand ground the other. If you want to goup in frequency , take some copper off thecopper bit; to go down in frequen cy, adda small bit of copper to the stripline trace,making line dimensions larger.

Check out the synthesizer first by con-nect ing a crystal IO-MHz source to theblack-shielded lead, +12 volts to the redlead, and the LED to the white lead. Thesynthesizer should fire up and lock on

2620 MHz. If the LED is not on, reversethe connections to the LED-no lockLED off; lock okay LED bright (see fig-ure 1). For connect ion power lock and RFinput output wiring.

The configure the 7490 for dividing by10, the pinout configuration is: IO-MHzinput pin is 1; tie pin 11 to pin 14; groundpins 2, 3, 6, 7, 10; positive VDC on pin5; and 1 MHz output on pin 12.

SummaryThe synthesizers are available from me,

WB6IGP. Two DRO synthesizers are $22postpaid, U.S. dollars. The synthesizerrequires a IO-MHz crystal source, whichis not supplied in this package. Normally,a 10-MHz TCXO is used forthis function.I provide some schematic detail and acopyof the programming spreadsheet that Johndeveloped, plus some other conversiondetails that were too numerous to includein this column. I will be happy to answerquestions related to the synthesizer orother microwave-related querie s. Pleasee-mail me at clhough@pacbell.net for aquick response.

In closing, I would like to thank JohnStevens, WB2BYP, allowing us to pub-lish his modification procedure and con-version data in this column .

73, Chuck, WB6IGP

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