My "Signal Source" for AO40 mode S .

Various "Signal Source" circuits using rare (or usual) XTALs, blocks etc. as fundamental oscillators to generate through a Fast silicon diode the necessary multiplied frequency for 2,4 GHZ.

My idea was quite simple: a VHF is the most popular equipment into a Ham-shack. Just by using a handheld VHF as generator, we can easily get an output signal around 2,4 GHZ.

For example, by multiplying the 144MHZ x 16 the final F= 2.304 MHZ (13cm).
The AO-40 freq. is 2,4 GHZ, that means 150 x 16 = 2400 MHZ.

My circuit has a few advantages against other circuits:

a) Lower "multiplier" from XTAL circuits.
b) NO "rare" Xtals or Xtal-blocks.
c) Easy contruction
d) Variable coverage depending upon VHF-step, ie if your VHF has 5 KHZ step:

150.000 MHZ x 16 = 2400.000 MHZ
150.005 x 16 = 2400.080
150.010 x 16 = 2400.160
150.015 x 16 = 2400.240
150.020 x 16 = 2400.320

For AO-40 :
150.080 x 16 = 2401.280
150.085 x 16 = 2401.360

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FIG.1 shows the electronic diagram of my 2.4 GHZ signal source. R1 & R2 are 100 Ohms CARBON resistors in parallel. These two resistors constituting a 50 Ohm "Dummy Load" in order to be able to terminate the VHF RF-output correctly. Keep in mind, this Dummy is for Low Power use, depend on resistors power.
The 144-150 MHZ signal driving the 1N4148 diode through CT & L1, which is a serial resonated circuit between (about) 140-150 MHz. The resonance frequency depend on CT capacity.

ATTENTION: DO NOT USE the VHF with "HI" Power output ! In this case, the Dummy-load (R1, R2) & the Diode probably will be destroyed !


1) Connect the RF-out of a handheld VHF into Signal-source input.

2) Set the output to "LOW". An output power of about 500 mW (0.5 W) is ideal.

3)Remove the A-B short-link and replace with a 0-100 mA - meter.

4)Press the PTT on VHF. A deflection must be indicate into mA-meter. Adjust the CT to peak the diode current (at approximately 30 mA). On my Signal-source I've got a value of about 35 mA with 500mW RF Output of my FT-23 handheld VHF.

5) Remove the mA-meter and replace it with the A-B "short-Link" permanently.

After that, if you have your Mode-S DownConverter in working order, press the PTT and adjust the "Tuning-screw" for maximum output... that's all !

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FIG.2 shows the practical construction of my 2.4 GHZ source. The unit has been made on a double-side epoxy PCB which forms the lid of a standard die-cast box (3.6 x 1.5 x 1.2 in) like the most units of this type.

Below they are some fotos from my Web camera to take a view of contruction.

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2g4_source.jpg - 16883 Bytes

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Input - Output = BNC Female chassis connectors
R1, R2 = 100 Ohms carbon resistor, 1-2 W
CT = Push-mica trimmer 10-60 pF
C = 1nF Feedtrough Capacitor
C2 = Tuning-screw (Diam = 3.5mm)
L1 = 5 turns, int. diam 6mm, length = 10mm, wire = 1mm
L2 = RFC (4 turns 0.2 - 0.3mm wire on a small ferrite-bead)
D1 = 1N4148
L3 = 10mm of Diode's warped-leg, 5mm above GND surface, parallel to L4, spacing L3-L4 = 3mm
L4 = 55 mm of bronze-strip, 3mm wide, 5 mm above GND surface
L5 = 10mm length parallel to L4, 5mm above GND, spacing L4-L5 = 3mm.


* A Shield between Input-section (R1,2 CT, L1, L2, C) and L3-L4-L5 is necessary
* 1N4148 diode passes through a small hole drilled to the shield
*The "Tuning-Screw's" hole must be drilled exactly on the CENTER of L4 - space.
*The "Nut" of tuning-screw is soldered on the "Outer" side of Box (NOT between L4 and Ground).

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Have Fun
Makis SV1BSX (July 2002)

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