MFJ MFJ-1708B-SDR, MFJ-1708B-SDRS Instruction Manual

The MFJ-1708B-SDR and the MFJ-1708B-SDRS are RF sensed TR switches with a
receive splitter designed to be used with a transceiver and a separate receiver such as
the popular SDR receivers. The splitter give high isolation between the transceiver and
SDR Receive port to reduce crosstalk and interference between the two ports. The SDR
port has a protection circuit to help prevent damage from transmit RF. The MFJ-1708B­SDR has a Stripline PCB reducing the SWR up through 450MHz and improved isolation
for better receiver protection.
MFJ-1708B-SDRS has an SMA connector for ease of connection to SDR dongle
receivers.

Features

 RF Sensing for radios that do not have a keying output when transmitting.
 Good isolation between transmit and receive ports.
 Excellent isolation between Transceiver and SDR Receive ports.
 Good SWR characteristics up to 450MHz.
 Receiver protection circuit to limit any transmit RF spikes.
 Control Line input for radios that have a key line output.
 An AUX output that can be set to ground to receive or ground to transmit.
 Power off bypass sets the transceiver to the antenna.
 Adjustable receive delay 50ms to 4 seconds.

Connections and Controls

 REC Connection for the receiver.
 ANT Connection to the antenna.
 XCVR Connection to the transceiver.
 AUX Connection to the receiver mute terminal.
 CTRL Connection to the transmitter PTT or KEY line.
 +12V Power Connector
 DELAY Receive delay adjustment.

+12V

+

-

Receiver TR Switch

DELAY

Model MFJ-170 8B-SDR

PWR TX

RF Sense SDR

CTRL

CTRL

+12V

+

DELAY

-

Receiver TR Switch

Model MFJ-170 8B-SDR-S

AUX

ANTSDR REC XCVR

PWR TX

RF Sense SDR

AUX

ANTSDR REC XCVR

SDR

Reciever

1. Connect the transceiver to the XCVR connector.

2. Connect the antenna, antenna tuner, or linear amplifier input to the ANT
connector.

3. Connect the SDR or second receiver to the SDR REC connector. DO NOT
TRANSMIT into the SDR REC input.

4. The CTRL terminal may be used in place of or in addition to the RF sensing
circuit by connecting to the output to key a linear amplifier to energize the MFJ­1708B-SDR.

5. If a linear is used then connect the AUX output to the jack that keys the
transmitter. Set the jumper JP1 to ground when keyed.

6. Connect a 12V DC power source to the power terminal. The MFJ-1312D is ideal
for the MFJ-1708D.

7. The DELAY control is used to adjust the time it takes to go back to receive. Full
CCW is minimum delay which is around 40-50ms. Full CW is maximum which is
around 2-4 seconds.

Note Do Not use the MFJ-1708B-SDR with transmitters over 200W or place it on the
output of linear amplifiers over 200W. If the MFJ-1708B-SDR is used in conjunction with
a linear amplifier place it in between the amplifier and the exciter. The AUX connection
can be used to key the amplifier and the output from the exciter can be connected to the
CTRL connector. In this mode there will not be a mute connection for the receiver.

At VHF and UHF frequencies the transmit SWR and receive losses can be improved by
removing the jumper JP2 if the RF Sense function is not required. In this case the CTRL
connector is required to be connected to a key line out of the transmitter.