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INRAD FT-920 Manual

Installing the inrad roofing filter mod
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FT-920: Installing the Inrad Roofing Filter Mod
The FT-920 roofing filter mod consists of a 6 pole, 4 to 5 kHz wide filter
followed by a high dynamic range feedback amplifier. The amplifier provides
enough gain to overcome the filter insertion loss.
The plot below shows the sweep frequency response of the front end with the
Inrad roofing filter mod in place. For comparison, the OEM filter is about 20
kHz wide at the -6 dB points.
The result of the bandwidth improvement is the reduction of close in
intermodulation from multiple signals. The IMD dynamic range will be
improved up to 20 dB for signal spacings from 2 to 20 kHz. Also, the blocking
dynamic range will be improved for close in signals.
© 2008 International Radio Corporation. Modifications are done at your own risk; seek assistance if you
are not qualified to make these modifications. International Radio assumes no responsibility for any
damages or injuries resulting from improper installation of this modification kit.
Revised 2008-10-15

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Summary of Contents for INRAD FT-920

  • Page 1 FT-920: Installing the Inrad Roofing Filter Mod The FT-920 roofing filter mod consists of a 6 pole, 4 to 5 kHz wide filter followed by a high dynamic range feedback amplifier. The amplifier provides enough gain to overcome the filter insertion loss.
  • Page 2 Frequently Asked Questions 1. What can you expect from this mod? Less IMD in crowded band conditions, particularly from stations at offset frequencies of 2 to 20 kHz on either side of the operating frequency. 2. Will it defeat the noise blanker? There will be some change in the NB performance due to the narrower bandwidth of the mod.
  • Page 3 Preparing the Inrad Mod for Installation Prepare one side of each coax cable by stripping ½ inch of outer covering and shield from the end. Strip approximately 1/8 inch of insulation from the center conductor. Next, prepare the opposite side of each coax cable to accept a TMP connector by stripping ½...
  • Page 4 Connect the coax cable center conductors to the pads left open by C1111. The pad connecting to T1013 will go to J1 on the Inrad mod. The pad connecting to T1014 will go to J2 on the Inrad mod.
  • Page 5 Connect the red wire to the right end of R1305. It is a small power resistor near the center rear of the radio. See Figure 5. Note: If the Inrad Switchboard Mod is installed, the red wire can go directly to the red +12V terminal on that unit.

  • Page 6: Parts List

    2.2K R3,4 Q1,2 MPS5179 FT 920 Roofing Filter Mod W2VJN 3/22/08 Parts List • Assembled Inrad 117 board • 2 RG-178 coax cables, 32” each • 2 male TMP connectors • Red and black wire-18” each, #26 • 2 stick-on standoffs •...
  • Page 7 Figure 1. Preparation of coax cable. Figure 2. Area for modification. Bottom of pc board. © 2008 International Radio Corporation - 7 -...
  • Page 8 Figure 3. View of the board modification. © 2008 International Radio Corporation - 8 -...
  • Page 9 Figure 4. Mounting the modification board. Figure 5. +12V connection point. © 2008 International Radio Corporation - 9 -...