Thread: R1392 Crystals
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Old 18th Mar 2012, 9:37 pm   #11
Radio Wrangler
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Default Re: R1392 Crystals

Hi Chris

DDS = Direct Digital Synthesiser

An ordinary oscillator, VFO or crystal usually makes a sine wave voltage in pretty much the same way as a pendulum moves sinusiodally.

A DDS does things the hard way. It *computes* what the voltage of the wanted frequency would be at regular time intervals, and it instructs a digital to analogue converter to make that voltage. The stepped voltage levels from the converter need filtering to construct the real sine wave.

It sounds difficult, but the whole thing can now be fitted onto a single chip in a package less than 10mm square.

The frequency out is programmable in ludicrously small increments. The DDS still needs a crystal oscillator to give it a timing/frequency reference.

The hard part is that DDSs normally do binary arithmetic and you need to convert your nice frequency number into base-2, and then squirt it serially onto the DDS chip.

However, there are loads of kits where you get a small board with a keypad for typing in numbers, or a rotary knob and a little LCD display. The board has a small microprocessor to juggle the digits, and it also has the DDS chip and a crystal reference oscillator.

You get crystal stability AND VFO versatility.

One problem is that in an air band receiver with such a low IF, the front-end will be tuned to get a few adjacent channels and to reject things elsewhere in the band. You still won't get all 760 channels (now being increased by 3 as 8.3333kHz channel spacing is being introduced to replace 25kHz spacing)

The R1392 is intended to not only have a crystal fitted for the intended channel, but also to have its RF stages peaked for that channel. It then sits at an airfield monitoring that channel only. If they want another channel, they have another receiver. Only if frequency assignments were changed would a receiver need re-channeling by fitting a new crystal and realigning the receiver for that frequency.

Having a DDS or a bank of crystals doesn't get you around the realignment issue, I'm afraid. You'll be limited to a group of channels.

My current day job is designing aircraft radios. I've got to cover the full band and get full certified performance on every channel, and minimise current consumption for people running from batteries in gliders or balloons.

Sorry,
David
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