Diplexers [was: Re: [Premium-Rx] Ten-Tec RX-340 - Amateur Use]

[Karl-Arne Markström]

The filter bandwidth comes very much into play in diplexer design.

If the requirement for a diplexer is to provide the mixer with a constant
resistive termination impedance regardless of impedance variations in the filter
transition and stop bands, the Q of the impedances in the diplexer will have to be in the neighborhood
of the Q of the filter impedances. (Rough estimate is Q = center frequency/filter bandwidth)

Otherwise, the mixer will be subjected to the same impedance variations near the passband 
as if the diplexer was not in the signal path at all.

>From a circuit theory standpoint, filters can be either absorptive or reflective.
An absorptive filter provides a nearly constant impedance both in the passband and in the stopbands, but
reflective filters attain their filtering action by presenting a mismatch to incoming signals outside the passband.

All crystal or mechanical filters that I know of are reflective filters.

The impedance variations of a subsequent IF filter near its passband is one of the limiting factors for the 
close-in distortion properties of any mixer, but by using circuits that convert filter impedances outside the passbands
to magnitudes and phases that are less detrimental to mixer IM performance, this influence can be minimized.

73/

Karl-Arne
SM0AOM

----- Original Message ----- 
From: "Chuck Hutton" <charlesh3 at msn.com>
To: <premium-rx at ml.skirrow.org>
Sent: Wednesday, February 09, 2005 5:28 AM
Subject: RE: [Premium-Rx] Ten-Tec RX-340 - Amateur Use


> That's not the idea I have, for whatever that means. As I see it, the
> diplexer design is driven by the frequencies of the IM products generated by
> the mixer and the bandwidth of the filter is not part of the design per se.
> 
> Chuck
> 
> -----Original Message-----
> From: premium-rx-bounces at ml.skirrow.org
> [mailto:premium-rx-bounces at ml.skirrow.org] On Behalf Of Carcia, Francis A HS
> Sent: Tuesday, February 08, 2005 10:31 AM
> To: premium-rx at ml.skirrow.org
> Subject: RE: [Premium-Rx] Ten-Tec RX-340 - Amateur Use
> 
> The tighter the filter the more you need a good diplexer. Easy to build
> though.
> 
> -----Original Message-----
> From: John Miles [mailto:jmiles at pop.net]
> Sent: Tuesday, February 08, 2005 12:55 PM
> To: premium-rx at ml.skirrow.org
> Subject: RE: [Premium-Rx] Ten-Tec RX-340 - Amateur Use
> 
> 
> Keep in mind, too, that simply adding a 3-kHz filter after the first mixer
> does not guarantee improved real-world performance.  You may also need a
> diplexer after the first mixer to present a 50-ohm (or whatever) load across
> the board.  A mixer that isn't terminated in a *broadband* load won't be all
> it can be in the IMD department.
> 
> -- john KE5FX
> 
> >
> > First, as I mentioned, making a narrow filter at high frequencies is not
> > trivial.  You think paying $100 for a typical 9 MHz xtal filter is bad...
> > try and price a 40.455 3 KHz xtal filter!
> >
> > And I can think of many receivers costing much more than the TenTec that
> > don't have them either.
> >
> > > Tell me then, why don't all high end HF rigs have this capability?  The
> > > Ten Tec 340 roofing filter mod looked pretty straight forward and
> > > inexpensive (using a KIWA 4 kc filter).  Others have done this
> > mod and are
> > > happy, but I wasn't due to the loss of fidelity.  Guy Atkins and I had
> > > problems when we tried added a switch to the mod.  Somewhere along the
> > > line we lost a lot of signal strength, which was not the case if the
> > > filter was wired in permanently.  I've been wondering about
> > this problem
> > > for quite a while.  I don't understand why a $4k receiver (ie the 340)
> > > wouldn't have such a capability.  I re-iterate again that the
> > Ten Tec is a
> > > bit of a dog on MW in it's unaltered state.......Walt.
> > >
> >
> 
> 
> _______________________________________________
> 
> Premium-Rx Mailing List
> To Post: premium-rx at ml.skirrow.org
> For Info: http://ml.islandnet.com/mailman/listinfo/premium-rx
> Visit the Website: http://kahuna.sdsu.edu/~mechtron/PremRxPage/
> 
> _______________________________________________
> 
> Premium-Rx Mailing List
> To Post: premium-rx at ml.skirrow.org
> For Info: http://ml.islandnet.com/mailman/listinfo/premium-rx
> Visit the Website: http://kahuna.sdsu.edu/~mechtron/PremRxPage/
> 
> _______________________________________________
> 
> Premium-Rx Mailing List
> To Post: premium-rx at ml.skirrow.org
> For Info: http://ml.islandnet.com/mailman/listinfo/premium-rx
> Visit the Website: http://kahuna.sdsu.edu/~mechtron/PremRxPage/
> 
> 
> -- 
> No virus found in this incoming message.
> Checked by AVG Anti-Virus.
> Version: 7.0.300 / Virus Database: 265.8.6 - Release Date: 2005-02-07
> 
> 


-- 
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.300 / Virus Database: 265.8.6 - Release Date: 2005-02-07