Saturday 7 March 2015

Parallel IF for the Vero BITX

Last week's new home for the plug-in modules had a Parallel IF path - but needed some more work...

Specifically, I was actually only running the ordinary Si5351 code - which doesn't support the switching IF frequencies required for the Parallel IF method.

Also, whilst I had two filter modules (as you saw in last week's photos), the 12 MHz unit had only been thrown together without any serious testing.

The technique for direct measurement of receiving response actually makes a nice framework for developing IF filters - because it places them in their intended operating environment (unlike my earlier simplified attempts).

Here's the measured response of the 12 MHz unit as it was first thrown together...

As you see, it is rather wide - in fact it sounded fine (if rather muffled) on voice.

Taking the plug-in rig as an experimental platform, I started messing with an alternative 12 MHz filter on a solderless breadboard (what else!)...

and I soon found a better (i.e. narrower) overall measured receiving response for the entire rig in "CW" mode...

Remember - this isn't just what the IF filter does - it is the response of the whole rig, warts 'n all.

I changed the capacitors on the plug-in module (but kept the crystals the same) and got a better response than the original - but not as good as the one above...

Good enough for who it's for.

A quick change to the software, to incorporate all the Parallel IF bells 'n whistles (especially the on-line alignment facility, so I could optimise the BFO settings for the new filter) and I was done.

Here's the current measured response in SSB mode..

and in CW mode...

Switching between the two is pretty dramatic - especially on a crowded band.

Also, this (today) is the first time I've heard the Parallel IF method running from the Si5351 (Pete, n6qw, has been running it on the left coast).

My original Parallel Rig is still using a pair of AD9850 DDS Modules and they are never PERFECTLY in tune with each other. So, switching from SSB to CW mode whilst listening to CW can cause a tiny pitch change in the incoming CW, which I can sometimes hear.

It is inaudible on voice, but sometimes audible - particularly to those with any musical sensibility - on CW. Some days it is imperceptible. Other days (at different temperatures), it is there.

On the Si5351, where both the clocks are derived ultimately from a single oscillator, there is no "tuning" issue. Joy!

The strip of stripboard really is a nice demo of the Parallel IF method.

It is also a nice demonstration of why you shouldn't mount a computer right next to the front-end of a receiver. It works well and is entirely usable - but you get a little audible tone whenever a control is operated.

Perhaps I will try to persuade audiences that it is an intended feature (which took me ages to code)!!

...-.- de m0xpd

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