Return-Path: Received: (qmail 19854 invoked from network); 26 Apr 2003 14:25:36 -0000 Received: from warrior.services.quay.plus.net (212.159.14.227) by mailstore with SMTP; 26 Apr 2003 14:25:36 -0000 Received: (qmail 25740 invoked from network); 26 Apr 2003 14:19:00 -0000 Received: from post.thorcom.com (193.82.116.70) by warrior.services.quay.plus.net with SMTP; 26 Apr 2003 14:18:59 -0000 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 4.14) id 199QUu-0002u7-ND for rsgb_lf_group-outgoing@blacksheep.org; Sat, 26 Apr 2003 15:17:48 +0100 Received: from [212.1.130.143] (helo=smtp-1.visp.telinco.net) by post.thorcom.com with esmtp (Exim 4.14) id 199QUp-0002ty-Od for rsgb_lf_group@blacksheep.org; Sat, 26 Apr 2003 15:17:43 +0100 Received: from [80.225.212.85] (helo=RSGB613192) by smtp-1.visp.telinco.net with smtp (Exim 3.36 #1) id 199QUo-00061Y-00 for rsgb_lf_group@blacksheep.org; Sat, 26 Apr 2003 15:17:42 +0100 Message-ID: <005901c30bfe$836774e0$55d4e150@RSGB613192> From: "Andy Talbot" To: rsgb_lf_group@blacksheep.org References: <146.fffb845.2bdb149c@aol.com> Date: Sat, 26 Apr 2003 15:17:03 +0100 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2615.200 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Subject: Re: LF: Conundrum - image cancelling mixer Content-Type: text/html; charset=windows-1252 X-Spam-Status: No, hits=-5.5 required=5.0tests=HTML_10_20,HTML_MESSAGE,REFERENCESversion=2.53 X-Spam-Checker-Version: SpamAssassin 2.53 (1.174.2.15-2003-03-30-exp) X-SA-Exim-Scanned: Yes Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rsgb_lf_group-outgoing@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false Content-transfer-encoding: 8bit
I think all three versions are functionally similar, and the cancelling of the input sideband is something that falls out of the maths.  If you consider the 90 degree phase shift to operate only at the freqeuncies you are interested in, then look at the signs of the two image responses ie.  IF = RF1- LO  and IF = LO - RF2 then the technique works just the same as visualising it backwards.  By then reversing teh signal flow you end up with my first solution.
 
After I posted the conundrum (spot my silly spelling mistake in the subject!) we had a long session on the whiteboard at (work with a mathematician along as well), and there is a reasonably straightforward answer as to why the maths appears to fail.  There is one fundamental property of the sin and cos functions which have to be remembered.   Wolf nearly got there.   I'll post our solution on Monday when I get back into work as it is still up on the board and will be easier to copy..
----- Original Message -----
From: MarkusVester@aol.com
To: rsgb_lf_group@blacksheep.org
Sent: Friday, April 25, 2003 11:45 PM
Subject: Re: LF: Conundrum - image cancelling mixer

Dear Andy and LF group,

we had that very same discussion at work about a year ago... can the  multi-octave IF quadrature network not be replaced by a simple narrowband RF phase-shifter? No, it can't.

As you say, the first circuit with quadrature RF and LO drives will actually cancel one of the IF output frequencies, eg. the sum frequency, and deliver only the difference. But that is not what you normally want, because it does not differentiate between the two possible RF input frequencies, signal and image. With low IF, these are closely spaced and difficult to separate by a preselector filter.

Example: Direct conversion receiver for 14.2 MHz USB voice. Circuit Nr. 1 is not a true SSB RX, it will convert both 14199 and 14201 kHz to 1 kHz audio. It would however cancel the IF sums near 28.4 MHz, but this is much more easily done with even the most simple IF lowpass. - Circuit Nr.2 is right, it will accept 14201 and produce both 1 and 28401 from it, and it will reject any input at 14199 kHz.

For a transmitter, the same holds true. Circuit 1 would be insensitive at 28.4 MHz "audio", but not be able to split between RF outputs at USB and LSB.

There is a third variant with quadrature LO and IF which will also provide a true SSB mixer. This is advantageous if you need a large tuning range, as the LO is only a single frequency at a time, and digital LO-divider schemes can easily provide matched 90-degree outputs. Also, thanks to mixer saturation, amplitude balance is less critical.

73 de Markus, DF6NM