W9GR DSP MODIFICATIONS:
ADJUSTABLE INPUT GAIN FOR THE W9GR DSP.

I BUILT ONE OF THE W9GR DSP (DIGITAL SIGNAL PROCESSOR) UNITS WRITTEN
UP IN QST IN SEPTEMBER OF 1992.  IN USING IT WITH MY TS-440S I NOTED
THAT THE LOW LEVEL AUDIO OUTPUT FROM THE TRANSCEIVER WOULD OPERATE THE
DSP.  THIS WOULD ELIMINATE THE NEED FOR HAVING THE RIG'S AUDIO TURNED UP
HIGH AND THE SPEAKER BLASTING WHEN USING THE DSP UNIT ESPECIALLY FOR

DIGITAL MODES.

ONE SMALL PROBLEM WAS THAT THE LEVEL FROM MY KENWOOD TRANSCEIVER LOW
LEVEL AUDIO OUTPUT WAS A LITTLE LOWER THAN OPTIMUM TO DRIVE THE DSP FOR
MAXIMUM BENEFIT (ONLY LIGHTING ABOUT HALF THE LED BAR GRAPH ON THE STRONGEST
STATIONS). 

A VERY SIMPLE MODIFICATION IS NECESSARY TO MAKE THE LEVELS PRESENTED TO
THE DSP OPTIMUM.  THE BUFFER AMPLIFER ON THE AUDIO INPUT OF THE DSP
(U8A) IS A UNITY GAIN OP AMP.  THE FEEDBACK RESISTOR (R4) IS A 30K FIXED
RESISTOR.  I SIMPLY CLIPPED ONE END OF R4 AND SOLDERED A SMALL
100K POT IN SERIES WITH IT-WIPER TIED TO ONE END OF THE POT TO
ALLOW A MAXIMUM TOTAL RESISTANCE OF 130K FOR A GAIN RANGE OF ABOUT 1 TO 4.

NOW ADJUST THE POT SUCH THAT THE FINAL LED JUST FLICKER OCCASIONALLY
WITH STRONG SIGNALS BEING RECEIVED FROM THE FIXED LEVEL AUDIO SOURCE.
I NOTED A SIGNIFICANT IMPROVEMENT OF THE S/N RATIO OF THE OUTPUT OF THE DSP. 
THIS SLIGHT INCREASE IN GAIN OF THE BUFFER DID NOT SEEM TO EFFECT IN ANY WAY
THE FIDELITY OR LINEARITY OF THE OPERATIONAL AMPLIFIER. 

AN ALTERNATE METHOD FOR ADJUSTING THE POT IS AS FOLLOWS:  I SET UP THE
POT FOR GOOD RECEPTION AND FLICKER OF THE LAST LED ON THE BARGRAPH WITH
MODERATE STRENGTH SIGNALS (RF MAXIMUM AND ATTENUATORS OFF IN THE
RIG).  THEN ADJUST THE RF GAIN OR ATTENUATOR TO AVOID OVERLOAD ON
STRONG SIGNALS.  THIS GIVES EVEN BETTER WEAK SIGNAL RECEPTION WHILE STRONG
SIGNAL RECEPTION IS ACHIEVED BY REDUCING THE RF GAIN AND/OR SWITCHING
IN THE ATTENUATORS IN THE RIG TO PREVENT OVERLOAD OF THE DSP.

THE POT COULD BE OF A TYPE WITH A CONTROL KNOB OR SCREW DRIVER ADJUSTMENT
MOUNTED ON THE FRONT OR REAR PANEL--BE SURE TO USE SHIELDED WIRES BETWEEN
THE BREAK IN R4 AND THE POT IF IT IS NOT SOLDERED IN DIRECTLY--OTHERWISE
NOISE PICKUP INSIDE THE DSP COULD CAUSE PROBLEMS.  CHASSIS MOUNTING
IS ESPECIALLY ADVANTAGEOUS SHOULD YOU ANTICIPATE USING THE DSP WITH DIFFERENT
OR MULTIPLE RIGS SINCE IT ALLOWS EXTERNALLY CONTROLLED COMPENSATION OF
THE FIX AUDIO INPUT LEVELS.


SPEAKER HOOKUP FOR DSP AND NON-DSP MONITORING/COMPARISON:
ANOTHER NIFTY SETUP I USE WITH MY DSP IS A "Y" CONNECTOR FORM THE DSP
OUTPUT--ONE OUTPUT TO THE SPEAKER AND THE OTHER TO MY MULTIMODE TNC.  
THE SPEAKER IS SETUP WITH A SWITCH (DPDT) FOR EITHER SPEAKER CONNECTION OR 8
OHM DUMMY RESISTOR CONNECTION ALLOWING ME TO LISTEN TO THE OUTPUT TO MONITOR 
(AS IN SSB OR CW) OR SWITCH TO THE 8 OHM RESISTOR TO CUT OUT THE DIGITAL
CHATTER WHEN USING DIGITAL MODES--I NOTED BETTER PERFORMANCE WHEN THE 8 OHM
RESISTOR WAS ACROSS THE OUTPUT WITH THE TNC FOR DIGITAL MODE
(AMTOR/PACTOR/RTTY/PACKET) OPERATION.  PERHAPS THIS IS DUE TO PHASE
DISTORTION PROBLEMS CAUSED BY THE SPEAKER WHEN IT WAS SWITCHED INTO THE 
OUTPUT CIRCUIT OF THE DSP. 

I USE THE RIG'S SPEAKER AND AF GAIN CONTROL TO LISTEN AND COMPARE THE
UNPROCESSED ACTIVITY ON THE BAND TO THE DSP OR LISTEN AT A LOW VOLUME TO THE
DIGITAL CHANGE OVER SEQUENCES WHEN OPERATING AMTOR/PACTOR.  A SWITCH (DPDT)
ARRANGEMENT OF THIS TYPE WOULD BE EASY TO CONSTRUCT OR ONE COULD BUY A
SPEAKER SUCH AS THE KENWOOD SP-31 WHICH HAS AN A/B SWITCH WITH LOAD RESISTOR
BUILT-IN.  

THESE SMALL MODIFICATIONS HAVE MADE A GREAT LITTLE UNIT PERFORM EVEN BETTER! 

HAPPY PROCESSING, 73, 

CARL  K5HK/MM2