Having Reception Issues with Digital Wireless?
Larry Fisher has observed that many users of Full Digital Wireless are having problems getting good reception.
The crew at Lectro has recently been getting a fair number of reported problems concerning performance issues with digital transmitters. In almost every case it has been traced back to amplified antennas or antenna distribution systems that have not followed good design principles.
Typically, the system is being used in an environment that has strong local RF signals. To pinpoint the problem, we’ve asked the users to eliminate their amplified antenna or distribution amp and simply try a whip antenna to see if the transmitter and receiver are working properly.
Full digital systems are particularly vulnerable to overload in the RF signal chain since the full digital modulation has both FM and AM components. If that RF signal passes through an overloaded amplifier, the AM part of the signal is compressed (clipped) and the AM information is distorted or lost. Antenna systems that may work satisfactorily with FM systems can fall apart when full digital signals are used in the presence of other strong RF signals.
So what designs work properly?
Those that do the following:
1. Use a band pass filter in front of the RF amplifier to remove strong out of band signals.
2. Use a high current, relatively high power amplifiers in the RF amp stages to handle high RF signals without overload.
3. Use low gain amplifiers in the RF amp stages to keep from overloading the output of that same amplifier.
4. Use only enough gain to overcome splitter losses and cable losses. This goes along with #3 above.
Why wouldn’t a designer follow these rules?
1. Bandpass filters add cost and reduce the theoretical sensitivity due to signal losses of the desired signal.
2. Higher cost of the amplifier plus associated power supply parts.
3. Low gain amplifiers typically have higher noise figures. Higher noise figures reduce the receiver sensitivity. It is difficult to satisfy both low gain, low noise and low cost.
What can the user do?
1.Buy an antenna or antenna system with the minimum frequency range that will satisfy your needs. A system with a response from DC to light is wrong for your purposes.
2. The addition of a passive filter just after the antenna can work wonders (such as Lectro PF25, PF50 or equivalent). This does restrict your frequency of operation but different filters can be swapped in or out as you work in different locations.
3. Stay away from low current/low power systems. The designer doesn’t understand today’s RF jungle.
4. Beware of systems that have fantastic noise figure specs like 0.8 dB or 1 dB. The design is probably built around a high gain, low current, low cost part.
5. Beware of systems that don’t quote a third order intercept. The designer will not have taken digital signals and/or strong interfering signals into consideration.
You as a user can help the situation by only using enough amplifier gain to overcome cable or splitter losses. Don’t turn up the gain of an amplified antenna to greater than that necessary to overcome cable and splitter losses. Excess gain will also upset the automatic gain leveling stages that exist in most digital receivers. Gain in an antenna amplifier increases not only the desired signal but also noise and any interfering signals.
As an aside, antenna gain is a different animal since a hi gain antenna is always directional and therefore rejects background noise and interfering signals. So, hi gain amps are bad except to overcome cable and splitter losses; hi gain antennas are good (pointed in the right direction).
as posted to JW Sound by Larry Fisher, Past President Lectrosonics Inc and RF Guru