Ing. Miroslav Gola

Simple system for Monitoring Weather Satellites:

FM Receiver For 134 - 141MHz

(A double conversion superhetrodyne with pll)

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Miroslav Gola OK2UGS

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UP-MODE-DOWN

Control of function of the receiver RX134141MHz (setting of operating mode and frequency) can be done in one of two manners. The first way was described in the chapter 3.8 (Communication with PC – setting of frequency of received signal .. CD-ROM) and it follows the setting by the push-button MODE.

Push-button MODE is functional after switching-on of the receiver and initial test. The following texts are progressively shown on LCD display:

After termination of receiver’s initial test there will be displayed the text "..SCAN WXTOIMG.."

In the mode "..SCAN WXTOIMG.." the receiver RX134141MHz expects at its input RS232 a code from decoding program WXtoIMG.

After reception of the code the display show current frequency in the following format – e.g. 137.20MHz or 137.9100MHz:

Program WXtoIMG sends codes only for frequencies 137.20 - 137.10 - 137.40 - 137.50 -137.62 - 137.91 - 137.30 - 137.70 - 137.80 - 137.85 MHz.

Since the design of the RX134141MHz receiver will only accept frequency steps of 10 KHz, the new 137.9125 frequency cannot be exactly programmed, so I chose the closest values above and below that frequency. The error is only 2.5 KHz, so either channel should be able to receive the APT signal with minimum degredation. Due to doppler shift, the recption may be slightly better on one channel while the satellite is approaching, and it may be better on the other channel as the satellite moves away from your location.

By pressing of the push-button MODE you will get from the mode "..SCAN WXTOIMG.." into operating mode "..FIX MAN TUNG.."

By pressing the keys UP or DOWN you can set the frequencies 141.00 - 137.20 - 137.10 - 137.40 - 137.50 - 137.62 - 137.91 - 137.30 - 137.70 - 137.80 - 137.85 - 134.00 MHz.

By next pressing of the push-button MODE you will get from the mode "..FIX MAN TUNG.." into operating mode "..SCANNING..", when the receiver is tuned progressively to the frequencies 141.00 - 137.20 - 137.10 - 137.40 - 137.50 - 137.62 - 137.91 - 137.30 - 137.70 - 137.80 - 137.85 - 134.00 MHz:

Scanning is stopped on the current frequency, when the voltage level "L" .. 0 volt appears at the input IC3 SQ_OUT. This happens when high-frequency signal of current frequency with amplitude 0,5uV and modulation 2400Hz is present at the receiver’s input. The same effect is produced by the signal SQL, depending on setting of the potentiometer P1. Selection of scanning by modulation or squelch is controlled by the switch JP3.

By next pressing of the push-button MODE you will get from the mode "..SCANNING.." into another mode "137.5000 MHz" (operating mode "STEP10KHZ"). After pressing of the key UP or DOWN the displayed value increases or decreases by one step equal to 10 kHz (mode "STEP10KHZ" in the range from 134 to 141MHz).

By next pressing of the push-button MODE you switch to another mode "..SCAN WXTOIMG..", it means to the beginning of the MODE cycle.