RF Attenuator, Low Pass Filter, and Preamplifier Board

The RF Attenuator, Low Pass Filter and Preamplifier Board is a dual use board. As the ExpressPCB mini board service provides 3 boards, it makes sense to design dual use PCBs where possible. Two of these boards are used in the Express Receiver. One board contains the RF Attenuator and Low Pass Filter; a second board is used to mount the RF preamplifier, a Clifton Laboratories Z10043A Surface Mount Norton Amplifier.

RF Attenuator, Low Pass Filter & RF Preamplifier Carrier Schematic
RF Attenuator, Low Pass Filter & RF Preamplifier Board Schematic

The ExpressPCB .pcb file for the board is here: Attenuator & LPF Board_Final

RF Attenuator & Low Pass Filter

The RF Attenuator consists of 4 relay switched pi attenuator sections of -2, -4, -8 and -16 dB; relays RL1 through RL4 select the pi attenuator sections, respectively, providing 0 to 30 db of attenuation in 2 dB steps.

The relays are TE Connectivity type IM06, high frequency RF non-latching relays. These relays are available from Mouser or Digikey. The version used in the RF Attenuator, Low Pass Filter & RF Preamplifier Board is the IM06GR, gull wing style surface mount version.

The relays have 12V coils, and are driven by ULN2803A Relay Drivers on the first I2C I/O Expansion Board. The ULN2803A Relay Drivers include diode inductive spike protection, as such, protective diodes are not included across the relays on this board.

J3 provides connections to the board for the four relay control lines for RL1 through RL4, a relay common, and a ground connection.

The Low Pass Filter is a MiniCircuits PLP-30+ 30-MHz Low Pass Filter. The low pass filter provides image rejection for the 1st Mixer.

All resistors and capacitors are 1206 style surface mount devices, except C12 which is a tantalum capacitor size D.

Top side of the RF Attenuator & LPF Board
Top side of the RF Attenuator & LPF Board

The RF Input connector, J1, is installed on the top side of the board. The RF Output connector, J2, is installed on the bottom of the board to facilitate daisy chained connection between the various boards when they are mounted vertically in the Express Receiver case.

RF Attenuator, LPF & Pre-amp Board bottom
Bottom side of the RF Attenuator, LPF & LPF Board

RF Preamplifier

The Z10043A RF preamplifier is mounted to a second board via four mounting holes. The Z10043A provides 10 dB gain across the nine HF amateur bands with the 12V power supply.

Driven from the first I2C I/O Expansion Board, relay RL5 either inserts or bypasses the RF preamplifier in the RF signal chain. J4 provides connections to the board for the single relay control line for RL5, a relay common, +12V power for the Z10043A, and a ground connection.

The RF preamplifier input is cabled to J7.

RF preamplifier output is cabled to J8.

RF preamplifier power input is cabled to J9.

Top side of the RF Preamp Board
Top side of the RF Preamp Board

The RF Output connector, J6, is installed on the top side of the board, nestled up against the bottom left side of the Z10043A board. The RF Input connector, J5 , is installed on the bottom side of the board to facilitate daisy chained connection between the various boards when they are mounted vertically in the Express Receiver case.

The preamp selection relay, RL5, is underneath the RG-174 coax jumper to the Z10043A RF IN connector.

Note that the “top” of the RF Attenuator & PLF Board is the “bottom” of the RF Preamp Board, and visa versa.

Bottom side of the RF Preamp Board
Bottom side of the RF Preamp Board
RF Chain: RF Attenuator & LPF Board (far left) ; three RF Bandpass Filter Boards; and RF Preamp Board (far right)
RF Chain: RF Attenuator & LPF Board (far left) ; three RF Bandpass Filter Boards; and RF Preamp Board (far right)
RF boards with control cables attached
RF Chain with control cables attached

Refer to RF BandPass Filter Board for a discussion on RF/IF inter-board connections, and control and power inter-board connections.

The RF and IF boards are mounted vertically in the lower deck of the Express Receiver case. All digital boards are mounted horizontally on the upper deck, separating the analog and digital circuitry in the receiver. The RF and IF boards are bolted to lengths of 3/4″ x 1/2″ aluminum angle stock, and then bolted to the bottom deck. 4-40 screw hardware is used throughout the Express Receiver.

RF Attenuator and RF Preamp is controlled via an optical shaft encoder connected to J4 of the Right Display & Control Board. Upon power up of the Express Receiver, the RF attenuator is set to 0dB and the RF preamplifier is set to OFF. Turning the optical shaft encoder CCW increases the RF attenuation in 2db steps from 0 to 30dB. Turning the optical shaft encoder CW from the 0dB position turns ON the RF preamplifier, or decreases the RF attenuation by 2dB if RF attenuation is greater than 0dB. If the RF preamplifier is ON, turning the optical shaft encoder CCW turns the RF preamplifier OFF.

© 2014 – 2015 Rod Gatehouse AD5GH

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