For ease of construction, low cost and frequency stability, a superhet using a VXO (super VXO) with a ladder or lattice XTAL-filter.
The arising questions here:
1) Which QRGs are used by the most popular (useful) nets?
2) Which (standard) crystal are available?
3) Is there a chance for regular ham-radio-operations?
4) Any design tricks?
There are a couple of networks, some frequencies are most popular:
- 14300kHz - Maritime Mobile Service Net, Seafarer's Net, PAC Maritime Net, Intercon Net
- 14313kHz - Maritime Mobile Service Net, Seafarer's Net, International MM Net, Med. Sea MM et, German MM Net, Intercon Net
Here, the thinking starts, and that would be y solution. Standars crystals are available for 4194,812kHz and 4194,304kHz. The first idea that pops up would be an intermediate frequency of 4194,812kHz, using a pulled down BFO using a 4194,304kHz-crystal.
Filtering could be done by ladder (the easy choice) or lattice. The latter option would use crystals of both frequencies, this option needs further investigation, i.e. bandwidth.
VXO-frequencies that would mix to the above NET-QRGs (a tiny bit of pulling required):
- 10106kHz - 30m QRP-crystal
- 10120kHz - 30m crystal
Pulling a bit further (up) other networks could be reached:
- 14303kHz - UK Maritime Net, Swedish Maritime Net
- 14305kHz - Confusion Net
- 14315kHz - Tony's Net
- 14316kHz - Indian Ocean MM Net, South African MM Net
Other networks will be available with other standard crystals, such as 10115kHz, 10125kHz and 10145kHz.
Yes, there are options for regular, non maritime that is, ham-radio-operations. A 9830,4kHz-crystal would result in 14024kHz, thus A1A-operations. For J3E, there are 10000kHz rocks available, well ... add 4194kHz, that's were one ends for USB. For operations at those frequencies, and some better ranges, I will go for a super-VXO for the A1A-part. For the voice, there is choice: super-VXO using crystals or even a greater range using a ceramic resonator.
Not a trick yet, more a design idea at the present stage.
My 30m MEPT is using the PWM-output of a PICAXE to apply an analogue signal to a varactor (well, the MEPT actually uses a rectifier diode as varactor).
Advantages a suitable (PICAXE-)µ-controller offers:
- channel selection/switching by digital i/o-ports
- RIT/XIT and/or VXO pulling voltages provided by PWM
- VFO/RIT/XIT controls by using on-chip ADCs
- memories by selection of crystals and pulling voltages
- sweeping VXO pulling frequencies
- squelch by use of an on-chip ADC
- channel/memory scanning with halt/stop by squelch
Besides µ-controllers, there's some more to think of, such as power amplifiers, data traffic, maritime aerials.... Not to mention position reporting and distress alerting. For the latter I probably would rather rely on GMDSS-gear/procedures....
Open questions here, with an I.F. of 4194kHz, would there be any obvious choice of standard-xtal to use for PACTOR? Please let me know....