(taken from the installation instructions)
The APO Cancel module is a retrofit modification to the FF-800 repeater controller
that improves control autopatch performance. Audio from the controller that is fed
to the telephone line (tones, speech, or repeater traffic) can interfere with the DTMF
decoder during the control autopatch monitor function (cntl406). Under certain
circumstances, this can cause the FF-800 to miss DTMF codes entered by a control
operator over the phone line. The APO Cancel module reduces the possibility of
interference to the DTMF decoder by subtracting the controller audio from the patch
audio that is fed to the DTMF receiver.
Theory of Operation
The APO Cancel module uses an inverting op-amp to "subtract" the controller audio
from the control patch audio that is fed to the DTMF decoder. Refer to the FF-800
schematic included in the FF-800 Operators Manual. The repeater audio for output
on the phone line is derived from U17 pin 14. This is the "APO" signal that drives
the phone line interface transformer via R62 and C10. The APO Cancel module taps
this signal using the U17 socket-header that is provided with the APO Cancel module
(note: on factory installed modules, the U17 tap point is at R62). The APO signal
is then fed through a POT on the APO Cancel module to an inverting op-amp with a gain
of 2.5. The POT is used to balance the canceling effect which requires user adjustment
upon installation. The output of the inverting op-amp is fed through a 100K resistor
to U18 pin 6 to form a summing network at the input of the U18 phone line RX op-amp.
The inverted repeater signal is thus added to the signal from the phone line interface
The input to the phone line transformer will have incoming and outgoing audio superimposed.
By adjusting the POT on the cancel module, the repeater audio that is present at the phone
line transformer secondary is effectively removed from the phone line audio (API) that is
sent to the rest of the controller. Of course, the canceling effect is not precise. This
is partially due to the fact that the impeadance of the phone line is reflected onto the
secondary of the phone line interface transformer. This impeadance is not simply resistive
(it will generally appear to be inductive due to the long cable length to the central office
equipment) and forms part of the divider network with R62 that is used to generate the cancel
effect. However, this canceling circuit is able to achieve enough attenuation of the repeater
audio to prevent DTMF decoder interference.
Refer to Figure 1 to identify the module components. This circuit modification is
accomplished by removing the IC at U18 and installing the APO module in its place.
U18 is then installed in the module. Additionally, the IC at U17 is removed and the
socket-header assembly is installed at U17. This IC is then replaced at the U17
Figure 1. APO Cancel module layout, field installed version.
The cancel module must be adjusted to achieve optimum performance. Furthermore, the
adjustment procedure must be carried out with the FF-800 plugged into the actual phone
line that it will normally use. This is because variations in the phone line impeadance
can have a significant impact on the adjustment.
A noise source is required to perform the adjustment. The best source is to simply open
the squelch on the repeater receiver. However, if you can not open the squelch (i.e., if
your receiver has built-in CTCSS decoder) you may have to obtain another receiver or noise
source to perform this adjustment.
You must also be able to monitor the DTMF valid signal. This signal is visible on the
FF-8010 display. If your system does not have a display installed, you will have to
monitor the DTMF valid signal at the DTMF RX I.C. (U21 pin 15). DTMF is valid when
this signal is approximately 5V.
Using a separate phone line, call the FF-800. You may need to get someone to assist
with this step. Note that you can not use a local telephone on the same line as the
controller because any other telephone sets that are off hook on the FF-800 line during
this adjustment will alter the phone line impeadance and the adjustment will not be correct.
Thus, all of the control commands must be entered from the calling telephone set (this
could be a cell phone, or a local telephone on a separate phone line).
If you have difficulty locating a null with the APO pot, you may have to reduce the
control patch monitor level. This adjustment is fixed at 1/4 amplitude for firmware
versions 3.07 and earlier. V3.08 introduced a command to allow adjustment of the
monitor level: "cnfg855*". Note that the v3.08 configuration defaults to full monitor
level. To reduce the level (to 1/2 amplitude), enter "cnfg855*1", re-enter the CAP
Monitor command, and repeat the pot adjustment. If this does not result in a
sufficient null, try the 1/4 amplitude setting, "cnfg855*2". The monitor amplitude
is stored in EEPROM and is not a part of the state settings. Once modified, the
change will be retained automatically.
- When the controller answers, enter the control patch logon command (107#*) followed by
the control unlock command (100#*).
- Next, enter the CAP Monitor command (004061#*). This will allow the repeater audio to
be heard on the telephone. Verify that the monitor function is active.
- Press and hold a DTMF digit on the phone line. Verify that you have a DTMF valid signal.
- Open the repeater squelch or activate your noise source. Observe the DTMF valid signal
and adjust the APO Cancel POT until the DTMF valid is active. If possible, adjust the
APO POT up and down to locate the "active" region and center the POT on this region.
Check the DTMF validity with several different DTMF digits to make sure they all detect properly.
- Remove the noise source and log off of the control patch. The APO Cancel module adjustment is now complete.
APO Frequently Asked Questions
Why do I need this adapter?
The APO module is specifically targeted at users who use the Control Patch Monitor or the Receiver via Patch
features. If you don't use or need either of these features, you don't need the module.
APO Module Pricing