Cleveland RAYNET Group
PC Battery Charger Noise
Radio to PC data interface circuits using transformer coupling should remove the problem of some PC battery chargers which introduce hum and other noise onto the transmitted data signal. Depending on the battery charger circuit, the noise on the transmitted signal could be hum at 50 Hz or 100 Hz. It could also be noise at multiple audio frequencies. Desktop PCs don't suffer from this problem. Some RAYNET operation will be from a car using a laptop PC and either a 12V DC to 240V AC mains inverter or a 12V DC to 20V DC battery charger. Transformer coupled data interface circuits will remove the requirement to disconnect the battery charger while transmitting.
Choice Of Transformer
Small cheap audio transformers may not have a good flat output voltage versus frequency response. There is usually a large price jump between simple basic transformers and the better ones. After the transformer coupled data interface circuit was designed and tested using good quality transformers, a source of cheap and easily available transformers in the UK was investigated. The one chosen is sold by Maplin. It is a miniature audio driver transformer type LT44, product code HX82D. The price is about £1.89.
The Crash program is described elsewhere on this website. It was written to record RAYNET off-air networks as an audio data file. The trigger which starts recording a radio transmission is user configurable. The default is the presence of audio at the mike input. As it takes a certain amount of time for each recording section to start, using the COM port's RFS pin voltage as the recording trigger will give fewer problems of the first syllable being missed.
These data interface circuits also provide the recording trigger for the Crash program. If you have no intention of ever using Crash, omit that part of the circuit.
If you don't intend to make any construction mistakes, there is no need to provide any test points in the circuit. If you are human, test points may be a good idea. The test points are small pieces of wire soldered at certain places of the circuit to allow a test meter's crocodile clip to connect.
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Page updated on 09 January 2017