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Buffered IF tap board for Panadapter / SDR in a Yaesu FT-991A
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Buffered IF tap board for Panadapter / SDR in a Yaesu FT-991A

Yaesu FT-991A Custom Panoramic Adapter Buffered TAP Board for external SDR / Spectrum Analyser

This project is a remix of OE2DOR's perfect and neatly crafted FT-991A Panadapter board.

The final objective is to actually embed a RTL-SDR inside a FT-991A, without drilling holes, external cables, connectors or whatsoever: The FT-991A sports a USB hub (which feeds the sound card and the CAT interface). By replacing the existing 2-port USB hub chip with a 4-port variant (same SMD footprint and compatible pinout), the SDR will be installed in the radio, and it will be exposed to the computer via the existing USB port, conveniently routed through the radio's USB hub. So, plug the radio's USB port and the computer will see three devices: the virtual sound card, the CAT port and the SDR.

This fork adds on-board latch control, by using a AND gate with two inputs that controls a RF switch: The IF signal is only connected to the SDR if both signals (on my project, a SDR GPIO signal and the SCPON signal from radio's Main Unit) are present. Otherwise, the SDR is left isolated to the ground and no signal is tapped.

The schematics are in Autodesk Eagle EDA format. Check the Schematic folder. The BOM (containing Digikey parts) and a few pictures are available in Design folder.

The project tracking, evolution and discussion is on QRZ Forum: A FT-991A IF tap for Panadapter / RTL-SDR inside the radio

Board reprints in OSHPark

This project is not compatible with the non-A model FT-991.

Current state:

  • Giving it a few days to mature prior to order/produce

Next steps:

  • Order the board, the components
  • Build and test the board
  • Document the install and tests

Changelog: (PY2RAF)

07/Apr/2019 - Revision I

  • Created USB interconnection with the SDR
  • Proper power management: Three voltage regulators in the board (9V, 5V, 3.3V)
  • Power is only enabled with signaling coming from USB hub; otherwise, board quiesce
  • Updated BOM

29/Mar/2019 - Revision H

  • Revised routes in Cheb Filter - Made the tracks fatter and straighter
  • Repositioned Q2 for better signal routing
  • Better unit spelling in silk screen for resistors
  • Flipped IF/SCP ports
  • A few routing changes.

27/Mar/2019 - Revision G

  • Removed a lingering DC blocker capacitor in filter in after RF switch.
  • Added chokes in DC IN lines (3.3V and 5V) (yet another Gordon trick)

27/Mar/2019 - Revision F

  • Added 1 nF capacitors in RF lines in PE4529 (except RF2, goes with a 470 pF to GND)
  • Got rid of the VCC via which was crossing the RF line (argh)
  • Finally got the courage to change a bit the board layout and escape from the thin strip that I was working previously.
  • And another THANK YOU to Gordon AD5GG for his invaluable feedback

25/Mar/2019 - Revision E

  • Changed the switch from a analog one (was a SN74LVC1G66) to a RF one (now: PE4259) (thanks to Gordon Hudson AD5GG)
  • Now using 3.3V (because of PE4529)
  • The switch default state now grounds the SDR
  • Updated the list of contributors in PCB Silk Screen.

20/Mar/2019 - Revision D

  • Moved to OE2DOR amazing board design
  • Added provision for two external inputs for latching control (actuating on a RF switch), isolating the SDR receiver when the two inputs are not in HIGH state. This is a bit of overkill in FT-991A, but I want to be able to control the IF tap by software and be able to unplug it completely when not in use, without having to open the radio ;-)

History (OE2DOR)


  • Board Layout changed to fit neatly inside FT-991A
  • Connection to IF using existing U.FL jumper cable
  • Connection to FT991A SCP-UNIT using additional short U.FL jumper cable
  • Connection to SDR/SpectrumAnalyser using new U.FL to SMA cable
  • Mounting using M3/10mm PCB Spacer soldered to head of existing PCB mounting screw (since PCB mounting screw has no metric thread)
  • Fixation using M3 Plastic nut & screw sticking into tuning hole of shielding box
  • All but one resitor (1206) changed to 0805 to allow for smaller PCB layout

History (G4HUP/W3AXL) Updated G4HUP / W3AXL panadapter tap Simple revision to the G4HUP high-impedance IF tap with specific components and updated filters.


  • Revised to add quantity of parts needed.


  • Up and running on a Yaesu FT-991A


  • Revised component values to values that you can actually obtain. Added digikey parts list.


  • Second revision testing complete and satisfactory. Up and running on a Yaesu FT-450D


  • Schematic revised to fix capacitor placement error in original schematic. New boards sent out for fabrication and testing will be done when they get back.
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