CCWN 75:41
                                                        
         The Coherent Filter To adjust the coherent filter, apply
+12  volts and the 4 Khz drive signal to the filter. The input
should be  jumpered to ground, and any lead carrying a 1 Khz
signal removed from the  vicinity, since the input is high
impedance and will easily pick up  signals with its passband.
Observing the outputs of the buffers (U4a pin 1  and U4b pin 7)
adjust the balance controls for a value as close to the  enter
voltage as possible (approximately 6.2 volts). This equals the
levels  applied to U5, which is alternately driven by the
processed drive pulses  from -he 4 Khz input. U5 acts 25 a
balanced modulator, much in the same  manner as a self-rectifying
mechanical vibrator. This equalization will  minimize the 1 Khz
tone appearing at the output, so tune for a null in the  output.
If adequate suppression of the 1 :Khz tone is not obtained, place 
additional sections of the output filter as shown, and the audio
level  increased through an outboard audio amplifier, if needed.  
        Remove the jumper from the input, and place a 10 K 
variable resistor across the input. Apply a 1 Xhz signal from     
    the TTL standard through this network         and observe the
output of the filter                with a scope or VTVM. Vary
the in         put level and observe the saturation         point
where the filter output no         longer increases when the
signal         becomes larger. Reduce the in         put
approximately 10% and note         the level for future
reference.          The next step is to provide a CCW keyed input
into the filter,  rather than just a single tone. This may be
accomplished by using the CCW  keyer to activate the IC chip in
the K4EEU standard that provides a  non-symmetrical output,
although this will not demonstrate the best  performance of the
filter. An alternative is to have a CCW equipped amateur  send
you a test signal through the stabilized TEN-TEC receiver. This
input  should be applied to the filter and the keyer shorted in
the dit position.  The 10 position phase adjust switch on the
filter may now be adjusted for  maximum contrast and crispness.
Once this is achieved, the keyer can be  used to send test
signals through the filter.          Testing The QRM Rejecting
Capability of the Filter: With the filter  adjusted as above and
the desired signal input, preferably a series of  dits being
applied at the pre-determined level 10% below saturation, it 
will be instructive to take a ham band receiver and practice
tuning  interference over the input of the filter while observing
.he filter  input with a scope.           The prototype required
a 300 millivolt input signal for  saturation. Using a .1 ufd
capacitor and coupling the output of the  station receiver into
the filter while CCW test signals were being  sent, undesired
signals of 6.0 volts (p-p) could be placed on the  filter before
objectional falsing of the filter occurred. At this