Customer always has minimum operating distance requirement when we design RF remote controls, sometimes customer will require line of sight distance of approx. 200 meters or more, so aside from using high sensitivity remote control receiver module, we also have to make sure that transmitter side has enough transmit power.
But the problem is, FCC regulation limits transmit power of RF devices, for example, the limit for typical 433Mhz remote control transmitter at base frequency would be 100dBuV/m for peak and 80dBuV/m for average, so we will have to take advantage of this limit and reach maximum transmit power as much as possible.
You may have noticed there is a 20dB difference between peak limit and average limit, that’s where we need to take advantage of, having two limits means you can have 20dB more peak transmit power when designing RF remote controls transmitters, meaning longer remote control distance easier to acquire, because remote control distance only has something to do with peak transmit power, not average, but also having two limits means remote control transmitter has to meet both limit levels, sometimes peak transmit power meet requirement but average one doesn’t, sometimes vice versa.
Before we discuss how to use the 20dB difference, we think it’s better to show how to calculate average transmit power, since peak transmit power is something we can check on spectrum analyzer easily by wave amplitude, but the easiest way to determine average transmit power is to calculate based on duty cycle.
Take the most popular ASK or OOK remote control transmitter for example, you can see the typical data stream is full of 1 and 0, or on and off, the whole length of 1 divide by total transmit length is called duty cycle, a factor to measure how much time in current frequency band the given transmitter occupies, as for FCC requirement, the basic transmit length window is 100ms. Duty cycle is very useful as it serves as important factor in calculating average transmit power from peak transmit power, the complete formula is as follows.
$$P_{Av} = P_{Pk} – 20 \cdot log \left (\frac{T_1}{100ms} \right )$$
T1 means total high bits length in milliseconds, for example, if T1 = 56ms, then the peak and average difference correction would be -5dB, which means to achieve maximum transmit power under FCC regulation, our average power can be 80 while peak power can be 85, also we can easily calculate that to take full advantage of 20dB difference, T1 should be 10ms, which equals to 10% duty cycle.
You may wonder, since smaller duty cycle means better, why we just make it simply as small as possible, the answer is it also has cost, to transmit the same data stream at minimum time, we need to increase data bitrate and lower single data bit timespan, all the change will make remote control distance shorter at the same peak transmit power, also it would require high performance and more costy RF receiver module to match, since most low cost receiver module can only decode data stream up to 10kbps, so we have to balance between duty cycle and bit rate limit, to achieve optimal solution.
Take Microchip HCS301 encoder format for example, the standard TE length would be 400us by default, corresponding data rate is 2.5kbps, and if TE length is 200us, data rate would be 5kbps, and if we choose TE length as 100us, data rate would be 10kbps, which is maximum for most middle class superheterodyne modules, so usually we will choose at most 5kbps to achieve the best balanced performance.
Based on HCS301 encoder format and above calculation formula, transmit power difference would be -5dB at 400us in worst case, -11dB at 200us in worst case, and -17dB at 100us in worst case, the above data is assuming only one data stream is transmitted in 100ms FCC observation window.
We can see for HCS301 format, we need to pay attention to average transmit power, and make sure it won’t exceed 80, fortunately this won’t be problem for most handheld transmitters using PCB small loop antenna design, since its peak transmit power would be somewhere around 90 due to low efficiency of small loop antenna, so if we choose 200us data rate when designing hand held transmitter, it would easily pass FCC ID lab test.
However, if we design remote control transmitter with external antenna, then due to high efficiency, the peak transmit power may achieve around 98 – 100 in best case even with 0dB gain antenna, in this case, even we choose 100us data rate, the average transmit power would slightly exceed 80, which means we have to lower the peak transmit power output by chip intentionally, thus shorten the remote control distance.
In our latest development project as above photo, we have designed a 433Mhz remote control transmitter that works around 200m in open space, and in order to meet FCC regulations, we have lowered peak transmit power to approx. 91 as well as use 200us data rate, the average transmit power measured is kept around 80, also we have used high sensitivity super-heterodyne receiver to make sure operating distance achieve the goal.
In conclusion, to achieve best operating distance under FCC ID labs regulation, we have to carefully consider the following elements, chip output power, antenna efficiency and transmitting data duty cycle in 100ms window, sometimes we have to lower duty cycle to certain level, to take full advantage of 20dB bonus.
Also please note the above data is based on 433Mhz remote control transmitters, if your transmitter is at another frequency such as 315Mhz, then the FCC limit value will be different, we will write about that in following blog post series.
Here at Solidremote Technologies Ltd, we can design and manufacture all kinds of RF remote control transmitters and receivers to meet FCC ID and CE R&TTE regulations, as well as achieve excellent performance and stability, the line of sight distance can normally achieve 200 meters, please feel free to contact us if you have any enquiries.