Lawmate Mods

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The Lawmate systems are made in Taiwan and is a popular choice for FPV use when coupled with RC control systems on a lower frequency band (36MHz, 72MHz, 433MHz). With a few modifications the equipment will become a powerful piece of equipment for any FPV platform.

To get hold of the units used below, I recommend Infinity-hobby as they are located in Taiwan, where Lawmate is situated, and they also offer great shipping options (not sponsored).

Lawmate 2.4GHz transmitter

The TM-240500-LM transmitter has around 500mW RF output power (cooling increases the max.) and is built on a Anadigics AWL6153 power amplifier. The vtx is compact and lightweight, 12 grams, which makes mounting easy. This is the newer model with the frequencies slightly rearranged [1].

The SMA connector is a regular SMA connector (screw threads, female center socket), not RP-SMA as found on most Wifi equipment.

Frequencies (Channel, dip-switch, frequency):

  • CH1 On-On-On 2410MHz
  • CH2 Off-On-On 2430MHz
  • CH3 On-Off-On 2450MHz
  • CH4 Off-Off-On 2470MHz
  • CH5 On-On-Off 2370MHz - outside ISM band
  • CH6 Off-On-Off 2390MHz - outside ISM band
  • CH7 On-Off-Off 2490MHz
  • CH8 Off-Off-Off 2510MHz - outside ISM band

Thermal mod

For maintaining full output power during extended operation and to minimize center frequency drifting, it is essential to keep the unit cool and under thermal control. It is shown as the unit gets hotter the output power decreases [2] (max. amplifier operation temp. +85C). The standard thermal cooling supplied is dismal and lacks physical contact with the casing to dissipate the heat in a proper way. While there is a heat-sink, it is mounted in the wrong side for optimal heat exchange. This causes the heat to be trapped inside the can where it can not easily dissipate to the outside environment.

By opening and removing all the excessive solder, a new thermal path can be built into the casing. I used three small 0.75mm copper pieces soldered together. The pieces were soldered to the casing and a thin layer of silver-based (Arctic Silver 5) thermal paste was applied between the board and the copper pieces.

It is also essential to have a tuned antenna because badly tuned VSWR match means warmer vtx, better VSWR match means cooler vtx (less power reflected back to the amplifier) [3].

Video impedance correcton

The video input impedance was measured to be 342 Ohm, over the expected 75 Ohm. Proper impedance match will optimize brightness and contrast [4]. Some cameras or OSD units might not work if the impedance is out of spec.

By adding a small 100 Ohm resistor in parallel to ground, the input impedance will drop to 1/(1/342+1/100) = 77 Ohm.

Lawmate 2.4GHz receiver

The RX-2460CK receiver contains a Comtech FM2400RTIM8 module with a relatively good receiver sensitivity, -95dBm [5].

Frequencies (Channel, switch number, frequency):

Switch A:

  • CH1 #1 2413MHz
  • CH2 #2 2432MHz
  • CH3 #3 2451MHz
  • CH4 #4 2470MHz

Switch B:

  • CH1 #1 2410MHz
  • CH2 #2 2430MHz
  • CH3 #3 2450MHz
  • CH4 #4 2470MHz
  • CH5 #5 2370MHz - outside ISM band
  • CH6 #6 2390MHz - outside ISM band
  • CH7 #7 2490MHz
  • CH8 #8 2510MHz - outside ISM band

Narrower band filter

The default ComTech module uses a 20MHz or 27MHz SAW (Surface Acoustic Wave) IF filter which is normally used for receiving a signal from a satellite, but for terrestrial use a narrower bandpass filter is preferred to limit the near-band noise and thus improving the signal-to-noise ratio.

By removing the default F480-2 filter and replace it with a new ECS-D480A bandpass filter, should result in a gain in signal-to-noise of be about 1.4dB [6].

RSSI output

The RSSI (Received signal strength indication) output is an analog voltage changing reflecting the relative strength of the received signal. By using multiple receivers a diversity switcher could dynamically pick the one with the best signal.

The image below shows pin 9 which is the RSSI output of the receiver module.

Lawmate comtech rssi output.png

Connect a wire to the pin and another wire to ground and feed the pair out of the casing to a female connector for easy measuring. I used a JST-connector and cut down the sides so that it would fit inside the stylus-pen holder socket.

Readings using a multimeter:

  • Transmitter and receiver 1 meter apart: 4.31V
  • Transmitter turned off, no signal: 0.86V

References

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