"DC insertion" Unit for AO-40 Down Converters .

The MMDS Units, like the CalAmp 31732 Down Converter, they need a DC-supply of 16-22 Volts in order to operate. The difference with usual Electronics equipments is that the DC voltage must be "inserted" into the RF cable. In other words, they using the same cable for both jobs:

These units are known as "DC insertion" or "DC injection". In practice a "DC-in" unit is a pretty simple circuit. Fig.1 shows the diagram.

Part List L1 = VK 200 L2 = 10 uH C1 = 1nF/63V C2 = 1nF FeedThr C3 = 47uF/35V D1 = 1N4001 F = 0.6A Fuse

FIG.1

The Input-Voltage (16-22 V / 300mA) inserted into +VC plug, following by F Fuse . The D1 is a protection diode. If you connected by accident the "DC-in" Voltage with reverse polarity, the F will "blow-up", in order to protect the Down Converter from any possible damage.
"L1 - C2 - L2 -C3" constituting a LP filter that rejects the high frequencies and allows only the passage of DC-current.

Reversely the C1 capacitor stops the DC-current (DC-blocking capacitor) and allows to pass only the RF-signal.

The unit is shown in Fig.2. It's being constructed into a small metallic box. On the left side is the output to VHF receiver (BNC) and the "Red-Black" cable for the Voltage supply. On the right-side is the "F-type" connector for the Coaxial cable of CalAmp 31732.

My Homebrew "DC-in" unit

Fig.2

P.S: you can replace the VK200 by 4 turns of 0.25mm wire into a small ferrite bead.
Also the 10uH coil by winding 10-15 turns on the body of 100Kohms 1/2 W resistor.

Have Fun
Makis SV1BSX (May 2003)