Saturday, February 12, 2011

[HamBrewers] An FM Receiver for the 144MHz (2m) Amateur Band


This receiver is a six channel crystal controlled design. An AFC facility is available on two channels, since the doppler frequency shift on the 2 meter beacon is about + /-3 kHz which will cause a significant degradation to the signal if a fixed frequency receiver is used. The AFC range is just sufficient to cope with the +/- 15 kHz doppler shift found on the 70cm beacon. A suitable low noise 70cm converter was described in reference (5).

The low noise figure and high gain of the 3SK88 RF and mixer stages are exploited to give the receiver a sensitivity exceeding O.15uV (-l24dBm) for 12dB SINAD. The heart of the receiver is an MC3359/ULN3859 IF IC, which is a tidied-up and slightly improved version of the MC3357. The use of this IC simplifies the design of the IF considerably, and helps achieve the overall excellent price/performance of the unit. A nine times multiplication is used in the local oscillator chain to give the required range from the VXO - which also means that a readily available crystal may be used.


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