The FCC requires devices such as the field phone to be tested against radiated (radio) emissions and conducted (into the power line) emissions standards, and this system passed the final portion of the testing yesterday, 2 Sep 2016. This version of the board will require some external filtering for the power inputs, as mentioned below, but there are now no more obstacles to us selling this product.
The following is a brief layman's overview of FCC testing requirements; we refer you to the FCC website for more detailed descriptions regarding Part 15 compliance. This information is for educational purposes, and is intended to help people who may wish to expand their application of the field phone components as a homebuilt system. The FCC allows generous exceptions to testing and compliance requirements for homebuilt and mobile applications, and other certain exemptions for communication during emergencies. Regardless, it is a good idea to try to remain as low emission as possible so as to not interfere with other equipment, communication and otherwise, that you and others may be trying to operate.
Although the homebuilt and mobile user enjoys certain exceptions to the regulations, manufacturers and vendors of electronic equipment (SoftBaugh is both an original equipment manufacturer, or OEM, and a vendor) must strictly adhere to the relevant emissions regulations. To do this, we have to identify the likely worst-case typical configurations, and then test these configurations against the standards. In the case of the field phone, the major difference between these configurations is how external power is supplied.
All configurations involve the following features, each of which is essential to typical field phone system deployment:
- Phones connected to all lines.
- Phones off-hook.
- Call indication via the ring interface modules.
- LiFePO4 18650 battery installed and supplying operational power.
As tested, all configurations also featured the optional audio output cable, connected to a laptop PC. While not an essential feature, this audio connection helps support training by allowing conversations to be recorded for later after-action analysis. Some users will find this a nice feature, but for those who don't need it, an immediate 1.5 dB gain in compliance margin will result simply by unplugging the audio cable.
With these fundamental features in place across all configurations, our FCC compliance lab tested the following external power configurations:
1. No external power, system running completely from the on-board LiFePO4 battery. The system easily passed testing in this configuration.
2. External power supplied by a 6v SoftBaugh AC adapter. Testing showed that this configuration requires a ferrite to be installed at the equipment end of the power cable. Our lab tested many other supplies, including high-end supplies, and none of those tested were able to pass, with or without the ferrite installed. Fortunately, we had this supply developed years ago while shipping ISM-band radio evaluation boards, and have sufficient stock of these remaining to support the field phone project.
3. External power supplied by a 12v lead-acid battery. Testing showed that this configuration requires a toroidal common mode choke to be installed at the equipment end of the power cable, as well as a 0.01 uF capacitor installed at the external power screw terminals. Other common lead-acid battery or battery array voltages, such as 6v, 8v or 24v, were not tested.
As a result of this testing, all field phone units will ship with the requisite toroidal choke and the capacitor to allow external charging via a 12v battery. Field phone bundles which include the SoftBaugh 6v AC adapter will ship with the ferrite. A separate post will show photos of the compliant configuration and instructions for installing the external components, as will the user's manual PDF.
Note that there is a difference between a technical capability and regulatory compliance. As designed, the field phone host module can accept DC input power, at either the barrel jack or the terminal block, from 6v to 30v. But, we can only market the system and sell bundles which have been verified as being in compliance, even if these other configurations would pass compliance testing.
For example, consider a typical 10W, 12v solar panel, which generates a peak voltage of about 18v and usable power starting around 6v or so in low illumination. While this panel might actually generate usable power capable of recharging the field phone host module battery, we cannot market this panel for this purpose, nor can we bundle it with the field phone host module, unless we have that configuration tested. Given that such testing is very expensive (we've spent thousands of dollars on FCC testing alone for this system to this point before selling a single unit), we probably won't have external solar power testing done unless the field phone system sells well.
If the technically-proficient homebuilt user did want to experiment with external power configurations that our lab has not tested for compliance, then we make the following recommendations:
a. Always make sure that the LiFePO4 battery is installed, which greatly stabilizes transients.
b. Make sure that the external DC source does not exceed 30v. It may be necessary to verify 24v truck or heavy equipment battery arrays to be sure this requirement is met while charging from an industrial-capacity alternator, as these voltages can be surprisingly higher. A slightly higher voltage is acceptable, but be sure to monitor the temperature of D1, a 30v zener diode which protects the charger IC. If an additional half-volt or so must be tolerated, the D10 Schottky diode can be replaced with a silicon diode.
c. Use the power supply toroidal choke and external 0.01uF capacitor at the equipment end of the power cable just as you would with the compliant 12v battery configuration.
d. If you have the ability to monitor emissions, look for marginal emissions in the 100 MHz to 200 MHz range, particularly around 140 MHz and 190 MHz. These emissions only appear when the unit is charging; while the unit is not charging the system is exceptionally quiet from an emissions perspective.
e. Avoid using an external audio cable. As mentioned above, this alone buys 1.5 dB of margin.
f. Avoid grounding the system as grounding is an important principle in more efficient radio emissions, which is what we do not want with the field phone system. However, if grounding is important for safety, then opt for safety first.
g. If your modifications are causing interference with anyone, stop operating immediately. Out of all the regulatory categories who get to use the RF spectrum, homebuilt users are at the bottom of the food chain.
Related to power input is the amount of power the system can emit over the phone lines. The field phone system was tested with a 7' cables to the master phone, 7' cables between ring interface modules and remote phones, and 25' cables between the field phone host module and the remote ring interface modules. Other wiring combinations create homebuilt configurations. In addition, armed with the component datasheet, the LT3909 current source driver can be reconfigured by a technically proficient user for additional current to support additional phones, or additional voltage for additional range. We believe these modifications are less risky from a regulatory perspective as the charging side is where we see compliance issues, but have not tested this belief in the lab. However, any such modifications would invalidate our testing and make the end user solely responsible for FCC compliance with such homebuilt units. For most purposes, the stock currents and voltages will be fine anyway.
We also did not have the field phone system tested for CE or related international standards. This testing is hideously expensive compared to FCC testing, so this won't happen unless the units sell exceptionally well. As a result, we have to restrict shipments to US (and possibly Canadian once our final report has been received) customers.
The good news is that we are now ready to begin accepting orders for this system, and can begin shipping orders once the chokes and capacitors arrive later this week or the next.
Thanks again to everyone who supported the Kickstarter promotion, and although that failed to reach its goal, we will make sure those supporters can take advantage of the special offers we promised back in May.
The Field Phone project is your own personal telephone company! This project allows you to use normal wired telephones for communication within and between buildings and shelters, either for everyday convenience or for security in a crisis when the normal telephone system might not be available.
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