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Homebrew QRSS beacon working

March 9, 2009
tags: ,

Frequency display

Frequency display

Not had too much spare time recently, but when I had some I was building my QRSS beacon.  The display above shows it working in the 100Hz QRSS region of 10.1400 MHz to 10.1401MHz (note the leading ‘1’ is missing off the frequency counter due to the higher resolution setting).

The beacon has been constructed ‘ugly’ style, first on one copper board then onto two, as I wanted to separate the oscillator from the amplifier stage, since the oscillator was to be boxed in polystyrene packing for thermal and consequently frequency stability.  There are five stages to the beacon,

  • a switch, which is driven by the controller,
  • the oscillator, which has a 5 Hz shift depending on the switch setting,
  • a buffer amplifier, this gives gain and  isolation from the power amplifier stage,
  • the power amplifier stage,
  • a filter, which is done in two stages, first an impedance shifting circuit for better matching to 50Ω (which is also a low pass filter) and then a seven element low pass filter.
QRSS oscillator

QRSS oscillator

QRSS amplifier stage

QRSS amplifier stage

The photographs above show the two boards, shortly after the oscillator was removed from the amplifier board and before the seven element filter was added. (It is called ‘ugly’ style for a reason!) The push switch was included so I could manually activate the switching circuit during testing, this was later replaced by a 3.5mm socket for the cable from the beacon controller.
Power out from this setup was around 250mW into 50Ω, which dropped to about 160mW when the filter was added.  I was surprised with the power drop but was still happy with the 160mW.

Output is nice and clean as seen below (taken before the filter was added).

Output on an oscilloscope

Output on an oscilloscope

Initial tests on Saturday showed it on the Vernon’s grabber, but unfortunately where there were other signals on almost the same frequency, so I have shifted the frequency down a little.  I went on to box the unit up and now running it to see whether it shows up on any grabbers.

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