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

[Gary Geissinger]

On our X-band systems at work we specify the designs to include resistive attenuators around the mixer ports.  That way we can guarantee that our mixers see mostly resistive terminations on their ports.  I imagine that the RX-340 probably can't stand the extra insertion loss.  The diplexer approach seems like a better fit for the RX-340.

I'm really disappointed in the RX-340.  I really wanted to buy one.  When I talked to TenTec I got the message that they aren't going to come out with an upgrade/new version...just firmware fixes.  You know, if it cost an extra $500 or $1000 for a better set, I would have been glad to spend the money.

This reminds me of the NRD-545.  when I talked to JRC I got the message that they aren't going to fix this receiver either.

With the 24 bit A/D conveters that are on the market, a swapout of the A/D and DSP engine could really improve the performance of some of these sets.  I'd still like the narrow roofing filter, though.

Gary WA0SPM



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-----Original Message-----
From: premium-rx-bounces at ml.skirrow.org [mailto:premium-rx-bounces at ml.skirrow.org] On Behalf Of Karl-Arne Markström
Sent: Wednesday, February 09, 2005 11:18 AM
To: Chuck Hutton; premium-rx at ml.skirrow.org
Subject: Diplexers [was: Re: [Premium-Rx] Ten-Tec RX-340 - Amateur Use]


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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