Thursday 11 December 2014

Parallel IF

Now that the January 2015 number of RadCom has started hitting the newsstands (even though I'm still looking forward to Christmas), I guess the cat is out of the bag...

The journal contains my article "A parallel filter architecture for flexible IF processing" (page 78) which - in turn - prompts me to reveal the true identity of the rig which I have been struggling to conceal in these pages for the past year.

What I have been clumsily calling the "multi-band rig" or (worse still) the "multi-band, BITX-inspired development platform" is, in fact, the Parallel IF rig...

I explain in the RadCom article how and why two ordinary crystal filters with disjoint pass-bands can sit next to each other - in parallel - quite comfortably. If these filters have different bandwidths, the whole rig can change receiving bandwidth simply by using the appropriate IF frequency required to select one or other of the filters. NO SWITCHING of any kind is required - just a change of oscillator frequencies.

You can imagine how easy this change of oscillator frequencies is to achieve with the RF Generator scheme that Pete Juliano and I already have published in SPRAT 158 and described in many blog posts, as summarised on this page.

The Parallel IF rig pictured above has two such crystal filters, between two of the ordinary bidirectional amplifiers familiar to builders of the BITX...

You can see my embodiments of these filters in the following photo, in which you'll note the absence of switches, relays and the like...

The software of the Parallel IF rig is a super-set of the m0xpd "Dual DDS" code - it includes an additional menu option to switch between the two IF frequencies required to operate one or other of the IF filters.

It is presented (of course) simply as a choice between the two receiving bandwidths...

The scheme works - very well.

Two weeks ago I presented a method for directly visualising overall receiving response - from RF to audio (magnitude) frequency response. Now you understand why!

With the disclosure of my new Parallel IF architecture, it is now time to use the new measurement method in anger...

Here's the measured response of the Parallel IF rig in SSB mode (a result you've already seen)...

Here's what happens if you keep the tuning the same but "switch" to the alternative IF path, giving a tighter overall response, appropriate to CW...

Formally, this is what might usually be called CW(R) mode as it is in lower side band; CW is more usually received in USB - this is just a convenience associated with the technique I've used to send CW from my rig, as described in SPRAT 159. The method works just as well with CW in USB - but would not allow a direct comparison with LSB reception in 40m, as above.

RSGB members can learn more about the method from the article in their copy of RadCom - other readers will be hearing very much more about the method over the coming months.

I am honoured to be working with Pete Juliano, n6qw, on the development of a new rig - a project specifically to showcase this "Parallel IF" principle. We will be publishing construction details of this new rig in 2015, hopefully in QRP Quarterly (subject to confirmation).

I am further honoured to have been invited to speak at QRP ARCI's Four Days in May 2015 - "the biggest and best QRP event in the World" (at least it was, before they invited me along HI HI).

You will not be surprised to hear that my subject will be this new method of achieving variable receive bandwidth in simple superhet rigs at very low cost.

I'm excited about it!

...-.- de m0xpd


  1. Very cool, great job, and brilliant solution!

  2. Looking Forward to meeting you and seeing the Rev Dobbs again at FDIM.
    73 KC4URI Steve