DetLog module application example

DetLog EME station controller

f5jwf april 2010

The DetLog module can be integrated in microwave station to monitor the output power as shown in Figure 1.

An external semi rigid coupler probes the forward and the reverse signal. The DetLog measures these two values, linearizes them and display the results either on RS232 and/or on RS485. In case of excessive reverse power (antenna mismatch) the DetLog open the PTT (Push To Talk) line which stop immediately transmission. The maximum value for return loss is configured in the module (RL_LIMIT command) with RS232.

The temperature of the power amplifier (T1) and driver (T2) is measured by using KTY-10 silicon sensor. Two ADC channel is reserved to measure the station voltage. The +28VDC of LDMOS amplifier and the general purpose +12V. Temperature and voltage is displayed in serial interface but no action is done on PTT in case of abnormal situation.

Both branch of the coupled line can be used for forward and reverse as the return loss of the DetLog is better than 20dB. This value limits the measurement directivity (with coupler directivity itself) but 20dB is accurate enough for station power monitoring. Of course dual coupler can be used to improve directivity.

When return loss cross the limit, the PTT line is opened for few seconds (typ. 5s) then re authorized to check if the problem still present.

Figure 1: DetLog in 13cm EME station

The RS232 interface is mainly use for configuration and for local measurements. When the EME station is installed in the antenna around 30m from the shack the two wires RS485 is used. This protocol allows transmission along few hundred meters over a single twisted pair. A MODBUS framing has been added to the transmission. This software layer allows reliable transmission (error detection), multiple access of modules over the bus by adressing each of them.

Each devices are connected in parallele (Figure 2) on the bus and the last device terminate the bus with 120ohms load resistor.

Figure 2: Devices connections on RS485

A small Windows XP software allows to communicate with the device. So far two modules have been integrated inside the software:

. DetLog

. Synfox

The synfox is a fractional synthetizer which is used as local oscillator of the EME station transverter. The frequency is adjusted according to the band to work:

. Eu: RF = 2320MHz, OL= 2176MHz

. US: RF = 2304MHz, OL= 2160MHz

. VK: RF = 2301MHz, OL= 2157MHz

An additional RX mixer is used for Japanese band:

. JA: RF = 2424MHz, OL= 1992MHz (FI=432MHz)

Figure 3: RS485 Windows interface