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

Wed Feb 9 14:39:34 EST 2005

Karl-Arne:

You are of course quite correct.

My point is that if I give you only the filter bandwidth, you don't have 
enough information to design a diplexer that is guaranteed to be optimum. 
You'd really need a frequency vs. impedance curve for the filter or at least 
to know what general type of filter you are working with.

That's why I said "bandwidth of the filter is not part of the design per 
se".

In addition to the filter characteristics, it's good to know what 
frequencies (if any) are of interest coming out of the mixer. For example, 
if you know our LO leakage is -40 dB and that level is a problem, you'd want 
to make sure the diplexer works well at that frequency.

Chuck

>From: Karl-Arne Markström <sm0aom at telia.com>
>Reply-To: Karl-Arne Markström <sm0aom at telia.com>
>To: "Chuck Hutton" <charlesh3 at msn.com>, <premium-rx at ml.skirrow.org>
>Subject: Diplexers [was: Re: [Premium-Rx] Ten-Tec RX-340 - Amateur Use]
>Date: Wed, 9 Feb 2005 19:18:18 +0100
>
>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.
> > > >
> > >
> >
> >
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