Monday, January 17, 2011

28322 Beacon Net Receiver

With the prospect of the upcoming activity in the present solar cycle, it is about time to think of a receiver for the (Italian) 28322(kHz) beacon network.
Due to the nature of those transmitters, the frequency range we want to be looking at is something like 3kHz, maybe 4kHz, i.e. 28320 to 28324kHz.


Now that the task is defined, let's move on and look at the obvious design involving inexpensive parts.

The xx322kHz frequency immediately makes me think of 14.318MHz crystals to form a filter for the intermediate frequency.

With an intermediate frequency of 14.318MHz, a local oscillator should create a frequency of 14.00xMHz. A local oscillator that close to the intermediate frequency will however put unnecessary strain on the IF-xtal-filter and even could end up clogging up a/the IF amplifier. I further believe that LO and BFO being so close is not such a good idea.
Solution to said problems: a local oscillator at 7.00xMHz (crystal easily available) hooked up to a subharmonic first mixer (pair of anti-parallel diodes). The intermediate frequency stage would be blind to 7.0MHz LO stray.

With the mixing all sorted, the next thoughts need to be spent on filter design. A bandwidth of 3, maybe 4kHz, makes a ladder filter a hard task, in particular since such a ladder filter would require quite some amount of poles. Even being harder to make, I figure a lattice filter would be the best option here. Lattice filters however require pairs of matched crystals being a some kHz apart. That is where the work sits in. One pair of 14.318MHz xtals can be selected by measuring/matching the series frequency of stock xtals. The other pair will have be to created by penning down two xtals to the exact same frequency.

For the BFO and the product detector the most obvious choice would be the NE612, just the way one would use it anyway.


Bored of QRSS?
There could be another use for the setup: an SSB phone RX, TX or even TRX. The important bit here, the bandwidth of the crystal filter should be around 2.4kHz. Such a bandwidth is easily available with a ladder filter, however, a lattice filter would give a better response.
Very obviously having a single channel SSB radio at a frequency where beacons beep around the clock is not the best of ideas. So, the LO will have to employ a different frequency. Luckily, many crystals are available for frequencies in the 40m band, e.g. 7030kHz, 7040kHz etc., hence, channelized or VXOed rig is no problem at all. Taking things further, a VFO could be on the wish list. And there is just a perfect option. The famous NE612 (SA612,NE602,SA602) can be configured to operate as a frequency doubling ceramic resonator oscillator. With a pulled down 3.58MHz ceramic resonator (avoid 3.58MHz!), a good portion of the 10m SSB range will be available.

At this place, I would like to thank Jan (PA9QV/OZ9QV) for triggering my thoughts about a 10m upper side-band design with the simple question "do you know a combination for 28322?" :-))

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