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Created on :
2001-09
Frequency band :
13cm
LO frequency :
1360 MHz








































A 13cm down converter






Description

To receive ATV signals in the 13cm band I designed a down converter.
I intended to use a LO-frequency of 3.36 GHz, but after trying some oscillator designs I found out that oscillating is much harder at 3 GHz than at 1 GHz. So I changed my plans and build a 1360 MHz oscillator.
When the LO runs at 1360 MHz, a signal at 2360 MHz will be converted to 2360 - 1360 = 1000 MHz on the output. This 1000 MHz signal can then be received by a standard analogue satellite receiver.



Block diagram of the converter



The 2.4 GHz input signal is amplified and filtered first, then fed into the mixer circuit with the 1350 MHz LO signal. After the mixer, the signal is filtered by a stripline filter.
Finally the signal is amplified by a MAR-8 followed by a MAR-6.



Input filter and amplifier



Since I don't have the proper equipment to make PCB's I used a plain piece of copper-epoxy board as ground and glued little pieces of it on top of the groud layer to solder the parts on.
The 1/4 wave stripline filters shown in de schematic are made out of pieces of wire of about 2.2cm.
The two FETs are random FETs soldered out of an old 10GHz LNB. Any FET that amplifies 2.4GHz can be used. Before placing the FETs in the circuit, first turn the gate voltage to 0 Volt so you don't blow up the FETs when powering the converter for the first time.
After powering up the converter simply turn the potmeters for maximum signal gain. The coupling of the filters can be tuned by bending the wires towards or apart from each other.

(Click on the images for a larger version)



The mixer



The active component in this mixer is a SMD dual Schottky diode (again) soldered from an old LNB.
(Note that the images down here don't show the 3 turn coil that connects the diode to ground.)



The 1360 MHz Local Oscillator

The following picture shows the oscillator I build in 2001. It doesn't look too charming :) . After a couple of months using the converter I found out this oscillator is heavilly temperature dependent. During sunny days the frequency was 'walking' a couple of 100 MHz. Some winter days the oscillator even stopped.
So I didn't bother drawing a schematic of it :)
I build a better oscillator, but didn't draw a schematic of it yet.




The output filter



The filter consists of three 1/8 wave striplines (at 1000 MHz) tuned by a sky trimmer.
( Note that the pictures of the filter show a coupling capacitor between the mixer-diode and the filter that shouldn't be there. )




The output amplifier



I happened to have a MAR-8 laying around, so I used it as the first amplifier. Two MAR-6 amplifiers would work just as well. But since the MAR-8 has a lower noise-level I put it in front of the MAR-6.
( Note that the pictures below show a capacitor instead of the MAR-6. When building something I tend to place and test parts one-by-one. That way errors can be found very easilly. )




The power supply



All amplifiers and the oscillator need a supply voltage of 5 Volt. Only the oscillator uses 11 Volt to drive the varicap tune. When using a oscillator that doesn't need a tune-voltage above 5 Volt, you could leave the 7806 out of the supply.
The negative supply for the gates of the FETs in the input amplifier is created by a switched capacitor voltage inverter. When the 1u capacitor is lifted to 5 Volt it charges via the upper diode. Then if the capacitor is connected to ground it discharges into the 10u capacitor via the lower diode.
I used a NE555 cut out of a LNB power supply to generate a signal at about 10 kHz. Any oscillator creating a square wave at a couple of kilo Hertz will do.




Some more random pictures




- Created by Daan Vreeken - PA4DAN -


Email me with questions/comments : Daan <Daan @ pa4dan . nl>