A 10 GHz Dual-Conversion High-Side LO Transverter
from Surplus Qualcomm OmniTracks Units

By Kerry Banke, N6IZW

Author's Note:  The original version of this article was presented at the Microwave Update '99 Conference in Plano, Texas, October 1999, and published by ARRL in the Proceedings of that conference.  Due my problems in submitting the original photos for publication in that document, the details of the modifications were difficult to discern in the published photos.  Since there has been significant interest by others in modifying these units, I am providing all the photos, plus corrections noted since the conference in these web pages.  I hope this will help many of you in successfully converting these units for amateur use.

The web version of this article is presented in three parts, plus figures for each.

Part One - Overall Concept and Design

The previously published1 Qualcomm X-band conversion project required considerable mechanical as well as electrical modifications and was based on replacing the original stripline filters with pipecap filters.  The pipecap filters were required to provide sufficient LO and image rejection at 10 GHz which the original stripline filters could not provide for a two meter IF.  This version uses a somewhat smaller, more recent OmniTracks unit which contains the power supply and synthesizer on the same assembly as the RF board and utilizes dual-conversion high-side LO to allow use of the stripline filters.  The filter modification has been proven to work well by extending the filter elements to specified lengths.  Some additional tuning of the TX output stages appears to be required for maximum output.

The synthesizer VCO operates at 2272 MHz which multiplied by 5 becomes 11,360 MHz for the first LO.  The first IF frequency is 992 MHz which is near the original internal IF frequency of 1 GHz.  The second LO is derived from the synthesizer prescaler which divides the VCO frequency by two to produce 1136 MHz.  Other second IF frequencies may be calculated using the relationship: (RF - IF2) / 0.9 = LO1, where RF is the 10 GHz operating frequency (10,368 MHz), IF2 is the second IF frequency, and LO1 is the first LO frequency.  The synthesizer output frequency is then LO1 divided by 5.  Figure 1 shows the Excel spreadsheet which is used to calculate the synthesizer programming.

3216 PLL Calcs for X-Band Xverter with 144 MHz 2nd IF, 1st LO 11360 MHz, 1st IF 992 MHz

Ref MHz: 2         # Ref MHz can be 10 MHz divided by any integer from 1-16
VCO MHz: 2272
PLL MHz: 1136      # PLL in MHz is VCO/2 & must be an integer multiple of Ref MHz
      N: 568       
      M: 55
      A: 8
      R: 4

             M6 (Pin 15) M5 (Pin 14) M4 (Pin 13) M3 (Pin 10) M2 (Pin 9) M1 (Pin 8) M0 (Pin 7)
             0           1           1           0           1          1          1 
Board As Is  0           0           0           0           0          0          0

             A3 (Pin 21) A2 (Pin 20) A1 (Pin 19) A0 (Pin 18)
             1           0           0           0
Board As Is  0           0           0           0

             R3 (Pin 5) R2 (Pin 4) R1 (Pin 3) R0 (Pin 2)
             0          1          0          0
Board As Is  0          0          0          0

Lift Pin 22

Add 1 pF to VCO to lower frequency

Reference suppression filter mods parallel these caps with the following values:

Ref MHz        C1        C2, C3      
5              None      None     
2              1000 pF   3000 pF
1              4700 pF   6800 pF

Figure 1: Synthesizer Calculations

The second conversion stage consists of a second LO amplifier (1136 MHz) and SRA-11 mixer converting the 992 MHz 1st IF to the 144 2nd IF.  A 992 MHz filter is required between two conversion stages.  Both Evanescent mode and coaxial ceramic filters have been used.

SRA-11 Mixer Pin Diagram

The conversion yields a reasonably high-performance transverter with a noise figure of about 1.5 dB and a power output of +8 dBm, frequency locked to a stable 10 MHz reference.  Powered required is +12 VDC with a current consumption of about 0.5 amps in receive and 0.6 amps in transmit (about 1.5 amps total in transmit when including the 1 watt PA).

Figure 2: Block Diagram

Figure 2 is a block diagram of the modified unit.  The unmodified circuit has a synthesizer output of 2620 MHz providing an LO of 13.1 GHz.  The original TX frequency was around 14.5 GHz with 1 watt output, and the RX was near 12 GHz.  Unfortunately the integrated PA in the original configuration provides no useful output below 12 GHz and is not modifiable and so has been removed for the 10 GHz conversion.  The TX & RX IF preamplifiers makes the TX input requirement low (-10 dBm) and provides high overall transverter receive gain.

Figure 3: Photo of Modified Transverter with 1 Watt PA and 10 MHz TCXO

Figure 3 is a picture of the modified transverter, 1 watt amplifier and 10 MHz TCXO.

Figure 4: Photo Showing Functional Areas of Modified Transverter

Figure 4 is a picture indicating the locations of the various functions.

The following is an outline of the conversion procedure:

  1. Marking location of RF connectors and removal of circuit boards.
  2. Base plate modification for mounting two SMA connectors (10 GHz RX & TX) and fou SMA connectors installed (2 RF + 1 IF &10 MHz reference input).
  3. Clearing of SMA connector pin areas in PCB ground plane.
  4. Remounting of PCBs.
  5. Cuts made to PCB and coupling capacitors installed.
  6. Stripline filter elements extended and tuning stubs added.
  7. Synthesizer reprogrammed and four capacitors added.
  8. 2nd LO amplifier, mixer, and 1st IF filter added.
  9. Power and TX/RX control wires added.
  10. Test of all biasing.
  11. Synthesizer and RX test.
  12. TX test and output stage tuning.

Note 1:  Refers to modifications and hardware earlier than those discussed in the 1999 Microwave Update article.

Part Two - Modification Procedures

1.  Mark the location of RF connectors and board cuts for coupling capacitors.

Before removing the boards from the baseplate, Carefully drill through the board in the two places shown using a 0.050-inch diameter drill just deep enough to mark the baseplate.  These are the locations for the RX & TX RF SMA connectors.  The upper-connector hole (TX) is located 0.5 inches to the left of the transistor case edge.  The lower hole (RX) is located 0.4 inches to the left of the transistor case edge.  Make the cuts as shown in Figure 1 using a sharp Exacto knife.

Figure 1

Figure 2

2.  Baseplate removal, modification, and connector installation.

After making the holes and cuts, remove all screws and lift the boards off of the baseplate.  (Note:  The original antenna connector pin must be desoldered to remove the board.)  Once the boards are removed, drill through the plate in the two locations marked using a 0.161-inch drill to clear the Teflon insulator of the SMA connectors.  Use a milling tool to remove enough material on the back side of the baseplate (see Figure 3) to clear the two SMA connector locations, taking the thickness down to about 0.125 inches (may vary depending on available SMA connector pin length).  Locate, drill and tap the baseplate for two #2-56 mounting screws at each connector.  Mount the SMA connectors on the baseplate and cut the Teflon insulator flush with the top-side of the baseplate (circuit board side).  Carefully clear the ground plane around the two connector holes on the bottom side of the circuit board to prevent the SMA probe from being shorted (using about a 0.125-inch drill rotated between your fingers).  Reinstall the circuit boards onto the baseplate.

Figure 3

Figure 4

3.  Add coupling capacitors

Add the three capacitors along with the additional microstrip pieces to modify as shown in Figure 2.

4.  Extend the TX LO filter elements to the total length shown in Figure 4.  Filter extensions are made by cutting 0.003 to 0.005-inch copper shim stock into strips about 0.07-inch wide and tinning both sides of the strip shaking off excess solder.  No additional solder is normally needed when attaching the extensions as the tinning reflows when touched by the soldering iron.  The length of the top element (0.21-inch) is measured between the marks as shown.

5.  Extend the LO filter elements as shown in Figure 5.  Again, total element lengths are shown except for the right-most element which has additional dimensions.

Figure 5

6.  Extend the RX filter elements as shown in Figure 6.  Dimensions shown are total element length.

Figure 6

7.  Extend the TX filter elements as shown in Figure 7.  Dimensions shown are total element length.

Figure 7

7a.  Add the tuning stubs to the x5 multiplier stage as shown in Part Three of this web page article.  This step was inadvertently omitted from the original article in the Microwave Update '99 Proceedings.

8.  Modify the 2nd LO amp board, mount onto transverter and connect 1136 MHz LO input through 1 pF coupling capacitor as shown in Figures 8-10.  Figure 8 shows the overall second IF converter which is mounted using two grounding lugs soldered to the top-edge of the LO amp board and secured by two of the screws which mount the main transverter board.  Figure 9 shows the coax connected to the 1136 MHz point on the synthesizer through a series 1 pF capacitor.  Figure 10 shows the mounting and wiring of the SRA-11 mixer onto the LO amp board.  Note the cut on the original amplifier output trace after the connecting point to the mixer.  The mixer case is carefully soldered directly to the LO amp board ground plane.  The IF SMA connectors are mounted by carefully soldering them directly to the top of the mixer case.

Figure 8

Figure 9

Figure 10

9.  Program the synthesizer as shown in Figure 11 by carefully lifting the pins shown with an Exacto knife.  Ground pin-10 connecting it to pin-6 which is ground.  Add the two 3,000 pF and 1,000 pF in parallel with the existing reference filter capacitors as shown in Figure 12.

Figure 11

Figure 12

10.  Add a 1 pF capacitor as shown in Figure 13 to lower the VCO frequency

Figure 13

11.  Add three TX mixer tuning stubs as shown in Figure 14.

Figure 14

12.  The TX/RX control is connected as shown in Figure 15.  Grounding the control line places the transverter in the TX mode.  The control can be open or taken to +5 VDC to place the transverter in the RX mode.

Figure 15

13.  The +12 VDC power input is connected to the point as shown in Figure 16.  The original air core coil with one end connected to that point has been removed from the board.  (This choke was originally used to supply +12 VDC to the transverter through the 1st IF port.)

Figure 16

14.  Powering up the transverter.

Apply +12 VDC to the power connector and verify that the current draw in RX mode is about 0.5 amp.  Connect the 10 MHz reference to the transverter board.  Pin-43 of the synthesizer IC should be HIGH when locked.  If available, use a spectrum analyzer to check (sniff using a short probe connected by coax) the synthesizer output frequency and spectrum.  The synthesizer should be operating on 2272 MHz with no 2 MHz, or other spurs, should be visible.  Carefully probe the drain of each FET in the LO multiplier, LO amp, and LNA to verify biases are approximately +2 to +3 VDC.  A drain voltage of near 0 VDC or +5 VDC probably indicates a problem with that stage.  Place the transverter in the TX mode and verify the biasing on the TX LO amp and Tx output amp stages.

Tune the 992 MHz 1st IF filter (not part of the transverter board) and connect it between the 1st IF ports on the transverter board and second IF converter.  The RX noise level at the 2nd IF port on the 2nd converter should be very noticeable on a 2-meter SSB receiver.

A weak 10,368 MHz signal can then be connected to the RX RF input connector and monitored on the 2-meter SSB receiver.  The overall gain from RX RF input to 2nd IF output should be roughly 35 to 45 dB.

Place the transverter into TX mode and connect about -10 dBm at 144 MHz to the 2nd IF port.  Monitor the power level at the TX RF output port and add/move the TX amp tuning stubs shown in Figure 14 as required for maximum output.  Typical TX output will be about +8 dBm.  This is considerably more than required to drive the 1 watt amp to full power.

Part Three - Corrections and Updates

These corrections and updates apply to the original article presented at the Microwave Update'99 Conference in Plano, Texas and published in the Proceedings of that Conference.

The original article inadvertently omitted a necessary modification to the x5 multiplier which requires the addition of two tuning stubs.  Lack of this modification will result in low LO output to the TX and RX mixers.

A section on the modification of the x5 multiplier stage was inadvertently omitted from the article.  This stage is located directly to the left of the LO filter which is shown in Figure 5 of the article on page 133 of the Microwave Update '99 Proceedings.  The gate of the x5 multiplier stage requires addition of two stripline stubs, as shown in the following photo.  Lack of this modification results in inadequate LO drive to the transverter TX and Rx mixers causing low TX output and poor RX performance, as reported by several amateurs who have followed the original article.