Sunday 15 March 2015

Suddenly Direct Conversion

As mentioned recently, there has been a resurgence of interest in the receiver section of the Occam's Microcontroller system - which itself was inspired by George, g3rjv's Sudden...

This has inspired Dennis g6ybc and I to dust off my prototype PCB and make some new ones, which I populated yesterday.

The PCB is in the format of an Arduino Shield - but it will run happily enough as a stand-alone receiver - which was the mode in which I first tested it yesterday.

The oscillator signal was furnished by the Kanga VFO system, and I listened on 20m.

Today I listened to the fun and games leading up to Guenter, dj2xb's valiant attempts to read the news on 7.127 MHz. To do that, I mounted the receiver, as intended, atop a stack formed of an Arduino, an Si5351 shield and the new receiver...

The eagle-eyed may notice that there are a few components missing from the PCB in the picture of the shield - these missing components are explained by the facts that i) they're not required to get the receiver running and ii) I'm planning to do some optimisation in these areas.

The key areas are highlighted below...

Receiver  muting is a little noisy in the prototype and uses a transistor which isn't so easy to source. The CW sidetone in the Occam's transceivers is a squarewave generated by the Arduino - I would like to check my first attempts to filter it. Finally, the Occam projects were conceived back in the AD9850 DDS era, when the RF oscillator was (nearly) a sinusoid. Whilst both the Tx and Rx shield run FB on the squarewave signals from the Si5351 shield, I'd like to look at optimising level and (potentially) some filtering of the oscillator signal to the 602 on the Rx shield.

The stack is also seen here...

The Si5351 shield is actually a pre-production prototype, the PCB for which also arrived this week "hot off the press". It has a jumper to select input logic level (3v3 or 5v) and a jumper to power up the amplifiers (or leave them powered down to save power or power them from an external voltage source)...

It also features (in this clumsy build, at least) toroids which are way TOO BIG! Yesterday morning I was listening to a nice BBC radio 4 documentary, in which some of the technology predictions of Douglas Adams were being put to the test as background entertainment whilst I was stuffing the Si5351 board and - being distracted by this interesting material - I didn't notice the "terrible miscalculation of scale".

The little DC receiver sounds fine - apparently g6ybc has his receiver running too.

This morning I was happily listening to able to hear dj2xb reading the RSGB news on 7.127 until dk5dr started chatting to g0jmz on 7.130 and I remembered just what "fun" wide-open direct conversion receivers are!

...-.- de m0xpd

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