K.Banke – N6IZW – San Diego Microwave Group (Revised 7/19/2000)

The previously published* 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 11360 MHz for the fist 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 (10368 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 spread sheet which is used to calculate the synthesizer programming.

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.

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 1.5amps total in transmit when including the 1 W PA).

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

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) & four 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 4 capacitors added.

8. 2nd LO amplifier , mixer and 1^st 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.

* (Page 1, 1st paragraph) Refers to modifications and hardware earlier than those discussed in the 1999 Microwave Update article.