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Add tone filter tuning info and some minor cosmetic adjustments
The most I have discovered amounts to what I have seen regarding FCC filings from the 1986 to 1990 under the company name (under grantee code '''F49'''<ref>https://fccid.io/F49</ref>) and what I have seen in various sources of media from Google images and YouTube videos.
The FCC ID's associated with F49 are as follows:
*'''F49LCR-2'''<ref>https://fccid.io/F49LCR-2</ref> registered on 02/09/90 with an operating frequency range of 150-174 MHz
*'''F49LCR-1'''<ref>https://fccid.io/F49LCR-1</ref> registered on 02/09/90 with an operating frequency range of 25-50 MHz
===Speculation===
My current research shows the existence of at least 2 or three types of these devices. Obviously there are more devices produced by the company, but I have zero clue what they are.
* This model that I own, which is always civil defense yellow and measures 325x232x92mm a few times on the internet in videos and photos
** Appears to always have it's radio receiver on the motherboard with descrete components.
* A larger more rectangular one, which I have acquired (yay!) that can be either silver or civil defense yellow.
** This unit is even more modular and can contain either a descrete radio receiver board or a maxon data radio board (which is reprogrammable)
** These units have built-in local activation buttons and no terminals for remote activation. Just AC in and one or two relay contacts
** These units also can do multiple signals including alert (steady), attack (wail), and fire (not quite sure what it means, fast wail maybe?) and have more tone decoders and timer cards
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* P4 - Logic NOT signal out (default high, drops low when signal is in passband)
* P5 - Logic Out (Not used on this board, but is the opposite of P4)
====Tuning====
Tuning the onboard pots was a little odd at first, but it appears <b>CCW is higher frequency, CW is lower frequency.</b> It seems that out of circuit (or at least with my crackpot test setup) the logic NOT output of the chip appeared to not function. I thought I had broken the chip at first, but after using the standard logic output to retune back to the original frequency, things worked when back in the main board. So, moral of the story is to use the standard logic output when tuning and just be careful.
Looking at the datasheet for the XR2211A, the signal input on pin 2 can be anywhere from 10mV RMS to 3V RMS which is quite nice. Keep that in mind when injecting a signal from a function generator.
=====Searching for tone filter max/min/current frequency=====
I tested filter A2 and the highest seems to be ~1.44kHz and lowest at ~1.12kHz with its components. I suspect that due to this small range, there may be either multiple model numbers (other than 031-0392-000) which have slightly different component values or possibly a different potentiometer. One other possibility is that this model just cant go outside those ranges, but I highly doubt that since many times these has to work with existing fire department/police department paging systems.
With that being said, you may want to test your own filters. Here is how I tested mine:
# Hook up your function generator or audio source (pure sine wave) on pin P1, VCC (12v DC) on pin P2, GND on P3, and your oscilloscope or voltmeter on pin P5.
#* You may also want to hook a second oscilloscope channel to the function generator output.
# Without touching the onboard pot, slowly sweep through frequencies from about 300Hz to 2.5kHz. Most paging systems are probably within these.
# Watch for the logic output of the filter to go high, indicating you have found the proper frequency.
#* There is a certain amount of passband in the filter, so tune back and forth slowly around the points where the filter triggers to find the upper and lower bounds, then you can calculate the approximate center frequency from there.
# You may want to re-silicone the pot and also mark the set frequency on the PCB in marker.
=====Tuning filters to a different frequency=====
If you aren't searching for the frequency bounds like I was, I suggest tuning the filters in the following way:
# Hook up your function generator or audio source (pure sine wave) on pin P1, VCC (12v DC) on pin P2, GND on P3, and your oscilloscope or voltmeter on pin P5.
#* You may also want to hook a second oscilloscope channel to the function generator output.
# Turn on your function generator and set it to the frequency you wish for the module to respond to.
# If you haven't already, dig the silicone out of the pot and make sure its clean. VERY slowly rotate CCW for a higher frequency or CW for a lower frequency.
# Once you have the pot tuned properly, the logic output of the filter will go high. You are within the bandwidth of the filter now.
# Test the bandwidth by changing frequency slightly above and below your desired set point. Slowly adjust the pot to center in the passband to your desired frequency.
# You may want to re-silicone the pot and also mark the set frequency on the PCB in marker.
