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While shopping at Microcenter, I found a “bargain” wireless mouse and keyboard combination on sale for $12.99. I wanted to add something like this to my media center and the price was really low so I decided to give it a try. The first attempt at using this proved that you really do get what you pay for. Both the mouse and keyboard work fine if they are only a foot or two from the receiver, however, the mouse stops working intermittently as the range is increased. I opened the receiver up and noticed that it was an inductively coupled system running at around 27MHz. The receiver has a pair of MC3361 (one to receive keyboard and one to receive mouse),  low power FM IC and a single loop antenna. The mouse has a MA6221-S7K IC for a transmitter. The documentation for these chips has been very scarce, but what is available is at the bottom. The keyboard worked fine at large distances so I assumed it had a good antenna and didn’t bother taking it apart. It looks like the whole system is based on a reference design from Mosart, a Chinese company. Without having direct access to a network analyzer or standing wave ratio meter or lock-in amplifier, the two improvements that I could think of making were to “upgrade” the antenna and to change the squelch setting on the receiver ICs.

After playing with a few different designs, I settled on a 15-turn bundle of insulated wire to be attached in series with the single turn antenna on the board. Since I did not change the capacitors in the LC circuit, assuming the inductance was increased 16-fold, the resonant frequency of the circuit decreased to 1/4th. Again, without proper instrumentation, it is very hard to determine accurately what the actual resonant frequency is. My hypothesis was that even though the resonant frequency was lowered, the operating frequency was still a harmonic of the resonant frequency whose gain would be higher than what the original antenna circuit was. A better design here would be to replace the fixed capacitors with a variable 5-100pF cap and try to tune the antenna for better performance, but I didn’t have one laying around. The only thing that I could test was the effective range improvement which almost doubled making this device usable a few yards from the TV. This is acceptable for now, however, the next step is to try to determine if the squelch setting is hardwired for the device or if it can be made more sensitive by changing some biasing. Again, there are a-lot of ifs here, so your millage may vary.

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( mc3361.pdf ) ( ma6221-s7k.pdf )

 
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