F91120 Field Radio BER Test Set

©Brooke Clarke, N6GCE


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General Information

This is a test made for the Marines by Codalex for testing the digital Bit Error Rate (BER) on the X-mode connector of  field radios like the RT-524/VRC, RT-524/VRC,  RT-442/VRC, RT-841/PRC-77, MRC-135A and others.  This is in support of the TSEC/KY-57, KG-84A and similar voice cipher equipment that needs a digital data format in a voice channel and multichannel telephone systems using radio links.  The case for the F91120 test set may be the same size as a TSEC/KY-57 and some of the internal parts may be the same.  There is a place on the case bottom for a BB-588/U or BA-5588/U battery.  My 68BA should work here.

Maybe the PRC-68 series of radios can be tested in support of the TSEC/KYV-2A?

This unit can be powered from normal 115/230 Volt mains power, an internal battery or an external DC source.
ID Plate:
F91120 Test Unit
Field, Bit-Error-Rate
NSN <none> Ser. No. 001
Contract No. M67854-94-C-2009 P/N F91120 CAGE Code 96344

96344 gives Codalex Ltd, Montreal, Quebec, Canada

Front Panel Controls and Indicators




A multiplier of 1, 10 pr 100 applied to the ERRORS that are displayed.


The ON or OFF switch.





Wire Binding Posts


Will start a BER test, although the set is continously testing so you can just wait for the next result.  The first result after making a change will be in error and a few seconds can be saved by pressing START.


This 26 socket connector allows cabling the test set to various radios to facilitate testing them.


Self Test

Internal Battery

This appears to be the same as is used on the PRC-68 series radios, like the BB-388 or others that are used in the battery box.

Two Test Sets

For most of the tests you need two of these test sets, one to send the data pattern and the other to check it for errors.
The transmit and receive portions of the F91100 are seperate, so you can run a test with a single unit.  For example the F91100 output in the audio mode could  be used to modulate a signal generator which could drive a receiver and the receiver X-mode digital output could be tested for errors.  Or a wire line system could be tested if there was a loop back capability at the far end.  Another way is to use two radios whose antenna ports are connected by an attenuator and use the X-mode connector for input to the Tx radio and the X-mode connector on the Rx radio.
When Testing:

Parts List for F91100 Test Set

BER Test Unit Connector
UUT Connector(s)
BER Test Unit    F91120  
1 & 2
Canvas Bag "96344/F91100"  
Cable Ass'y A  96344/F91100    RT-524/VRC
MS 3116J16-26P
MS 3116E12-10P
MS 3116JF12-14P 
Cable Ass'y B  96344/F91100 
MS 3116J16-26P
MS 3111J14-19P 
Cable Ass'y C 96344/F91100
KY-90 data port
MS 3116J16-26P
MS 27467T17835S 
Cable Ass'y D 96344/F91100
MS 3116J16-26P
 M 55116/4-4 (U-183)
Cable Ass'y PWR  96344/F91100
 24 VDC
MS 3116J10-6S
9/16" Alligator Clips
Cable Ass'y DC  96344/F91100
115 or 230 VAC
50, 60 or 400 Hz
IEC Line Cord
American 3 prong Plug

J1 Connector Pin Out

A = same as RCV bind post, B = same as XMT bind post, C = Code Word
The FUNCTION switch has three wafers whose contacts are brought out to J1.  This allows controlling an external devices mode of operation.  In order:
Wiper = E, P, G, OFF = D, N, T, AUD= C, M, S, DATA= B (= XMT), M, R, TEST= (nc), (nc), (nc)
K (may=) same functin as TRIG?
Related to Z1 (data generator) are: u, F (Ext Clock In), H (Bipolar Tx Clock), J
+5 VDC = b

Module Functions

Z1 = 100 K bit/frame data generator, Z2 = RCV clock Phase Lock Loop, Z3 = Error Counter, Z4 = Power Supply, Z5 = Baud Rate Generator, Z6 = Error Counter Range divider, Z7 = LED Display


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