Driving RF Decks (R8 value)

The MicroStar Encoder is compatible with most RF decks; in fact I have not found a RF deck that would not work with the encoder. There are a few things you should know and this article provides some needed background.

The RF deck interface is simple requiring only ground, power, and the modulation signal as input. The ground and power are generally the battery supply voltage from the transmitter. I have heard that some 2.4GHz systems that will run fine on 5 volts but I have no experience using lower voltage on any RF decks.

Even thou this interface to the RF deck is simple it has been the subject of a great deal of debate and significant challenges in getting some RF deck to work properly. Below I have a number of recommendations that you should consider when trying to debug a RF deck connected to a MicroStar encoder:

1.) Establish good grounds. This is especially true for the 50MHz and 72 MHz decks. The RF connection to the antenna consists of the signal and ground, make sure this is a low inductance ground connector to the transmitter case, assuming it’s a metal case! You do not want this ground to depend on the power connections and cables through the encoder to get to ground. In the case of 2.4GHz systems, the antenna connections are made through a dedicated coax to the antenna.

2.) R8 should be 100 ohms. On the MicroStar encoder you will find a resistor in series with the signal that drives the modulation input on the RF deck. This is a 0 to 5 volt logic level signal. R8 is a resistor in series with this signal. The purpose of R8 is to limit the drive current from the encoder. The original value of R8 was 2.7K, this is too high for a number of encoders so I later changed the value to 100 ohms. Using 100 ohms is safe for all applications so I recommend you always use 100 ohms.

3.) It is good practice to use a ferrite bead or toridal core to filter the lines going from the encoder to the RF deck. This is most critical in the 50HMz and 72MHz RF decks but good practice in call cases. You can find snap on beads that are easy to install or you can just get a toridal core and wrap the cable through the core a few times. This will keep RF from the RF deck out of the encoder.

4.) Use Dan Thompson’s adapter for plug-in RF decks. This adapter has a buffer transistor to drive the RF deck modulation input and provides a good interface to the plug-in RF deck modules.

While this interface to the RF deck is simple it can be challenging to resolve some of the problems you may encounter. Every custom transmitter is unique and should be carefully tested. Problems where you experience a lot of servo position jittering are generally due to grounding problems of one kind or another.

Feel free to contact me if you are having trouble, I will do my best to help.