[1000mp] MP vs MkV vs K2

Sun, 30 Jun 2002 12:19:20 -0600

Thanks, Tom (W8JI) , for the detailed response.  I think you have confirmed
what I suspected...that the MP IM problems are generated prior to the IF
filters.  Almost tempts me to put a little disconnect switch on the noise
blanker FET.  I  don't use it often, and when it's off, it would be nice if
it didn't serve as an unintentional limiter!

As an aside, and lest we don't get too upset at our MPs...   I worked a lot
of CW on field day last weekend.  Due to little equipment  problems, we
ended up using an IC706, with 500 Hz. CW filter, for our CW rig.  Boy was it
nice to get back home to my MP after that.  I got a reewed appreciation for
my MP, with its 500, 400, and dual 250 filters.  It's still a great rig, and
I love it.  I"m sure most of us feel that way, even though we like to push
the envelope, and grouse a bit along the way!

73,
Tom / W7GT

----- Original Message -----
From: "Tom Rauch" <[email protected]>
To: <[email protected]>
Sent: Sunday, June 30, 2002 5:10 AM
Subject: Re: [1000mp] MP vs MkV vs K2

> > Where are the non-linear products in the MP being generated?  In
> > particular, are they in the early stages, prior to the 8.2 and 455 IF
> > filters?
>
> In virtually all systems IM products are generated in the early
> stages of the receivers, before reaching narrow filters. IM
> generation requires non-linear signal amplitude response, so filters
> are almost perfect up to large fractions of a volt or more. Almost
> exclusively, the active devices used in mixers and amplifiers are the
> fault.
>
> In the FT1000MP and D series, the first mixer is a well-designed
> strong mixer. The RF amp is pretty good also. Yaesu unfortunately
> hangs a dual-gate FET right off the 70MHz IF just after the roofing
> filter, and it is by far the worse problem. The FET is used for the
> noise blanker, and it saturates and back-feeds IM products into the
> 70 MHz IF even when the noise blanker is OFF.
>
> It's as if the designer's only goal was looking good in a 20kHz test,
> and as if they didn't care a bit about good operating performance.
>
> After that the FET, the second mixer and RF amp are the next poorest
> stages for IM and blocking dynamic range in the 1000 D and MP. The
> synthesizer has a bit of noise that results in unwanted noise
> production, but that is minor compared to the other problems.
>
> The small signal diodes used to switch circuits in the receiver have
> no effect at all on performance. Changing them is a waste of time and
> money, like taking a sugar pill for an infection.
>
> The K2 actually is old simple technology, with no special effort to
> be good. The mostly just didn't do anything grossly wrong, rather
> than actually having an exceptional design. It would be quite easy to
> beat the K2, or improve it.
>
> What they did right is use a diode passive mixer, and a reasonable
> post-mixer amplifier after the mixer and before the filter. Just by
> changing the post-mixer amp, the numbers could increase 5dB. For
> about another twenty dollars or so in cost, you could push IM3 and
> blocking up another ten or fifteen dB.
>
> Yeasu, like most manufacturers, screwed up after the roofing
> filter...acting like no one would ever operate within 10kHz of the
> user. The same "careful design thought" obviously went into the CW
> keying waveform, like it does on many other rigs.
>
> Generally harmonics and intermod products are generated in
> > the latter stages, where the signal levels are the highest.  And
> > woujld be after one or both IF filters.
>
> That is not correct, unless you are talking about signals within the
> passband of the filters. Even at that, such IM and harmonic
> distortion is not deleterious to communications. The only exception
> is when broadband noise mixers with a noise-floor CW signal and
> muddies it up. That can make it difficult for people with "good CW
> ears" to dig out the signal.
>
> The real problem is in front of the first **narrow** filters.
>
> > The point I'm heading for is:  What is the significance of  20 KHz.
> > spaced signals, or even 5 KHz spaced signals, if the nonlinear
> > products are generated after the IF filters, where you won't have both
> > of those strong signals simultaneously present?
>
> Because the products are NOT generated after the narrow filters, and
> NOT generated IN the narrow filters, and the because the
> receiver has a very wide roofing filter than can make a poor receiver
> look good at wide spacings, test-spacing is critical.
>
> Manufacturers would love to have all receivers tested at 20kHz or
> wider, because it makes the system look better than it is when a
> roofing filter is employed. Users on the other hand would find 1kHz
> spacing more useful for CW, and 3kHz for SSB.
>
> After all, stop and think about it. Who bothers you most often
> (keyclicks aside)? Is it a guy 3kHz up or down on SSB, or a guy 20kHz
> away?
>
> The 20kHz test obviously thought up by someone who cared less what
> the end-application would actually be.
>
> > The assumption in the measurements must be that the nonlinear products
> > are generated pre-IF filters.  Does anyone know this for sure?
>
> Absolutely!!
>
> Without any doubt at all the most destructive IM products and
> blocking problems are generated **ahead** of narrow filters. Not in
> them, not after them.73, Tom W8JI
> [email protected]
>
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