[hpsdr] Preselector PT-8000

Mon Mar 12 08:07:45 PDT 2007

Graham,

When you lower the impedance of the load and source, the shape of the series 
resonance is much more pronounced. You can get in the order of 20 dB of 
selectivity improvement. This forces you to have better tracking accuracy. i 
have been playing with these topologies for about a year and they look 
really promissing. All of the new radios are incorporating some sort of 
tunable preselector based on the same idea. Yaesu tunes the inductor with a 
stepper motor. I have been leaning toward switching the taps on the inductor 
and adjusting the caps. The design I use is very similar to the PT8000, but 
it has 2 cores. a T200-2 and a T200-6 for the higher frequencies. I might be 
able to get it working with just the T200-2 as in the PT8000 design.

Pieter, N4IP

>From: Graham Haddock <grahamh at verizon.net>
>Reply-To: grahamh at verizon.net
>To: Klaus Lohmann <Klaus_Lohmann at t-online.de>,'High Performance Software 
>Defined Radio Discussion List' <hpsdr at hpsdr.org>
>Subject: Re: [hpsdr] Preselector PT-8000
>Date: Mon, 12 Mar 2007 09:27:04 -0500
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>***** High Performance Software Defined Radio Discussion List *****
>
>Klaus:
>
>Great information, thanks.
>
>Phil and I are considering/researching continuous tracking filters
>as an alternative to switched filter banks.
>
>Do you know what the advantage is of lowering the impedance at
>which the filter operates? I can think of both good and bad results
>from lowering the impedance, but there must have been at least one very
>important good reason.
>
>Thanks,
>--- Graham / KE9H
>
>==
>
>Klaus Lohmann wrote:
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> >
> >
> > ------------------------------------------------------------------------
> >
> > Hi Phil and all,
> >
> > the preselector of the PT-8000 is a series tuned circuit from 1.8 to
> > 30MHz. The main inductance is based on a T-200 toroid. Researches have
> > revealed that the IMD characteristics correlate with the mass of the
> > material and the transformation ratio used. The IP3 of this
> > preselector was measured with +50dBm. The series circuit is tuned
> > through software by 5 inductor taps (5 bit) and an 8 bit capacitor
> > set. A noise generator is used to tune and store all settings in a
> > table. In order to enhance the filter characteristics the circuit is
> > operating at approximately 3 ohms – there are down- and up
> > transformers at in- and output. We should have in mind the design goal
> > for this solution: Hans Hilberling, DK7LG, wanted to get a preselector
> > with approximately 20 to 30dB out of band suppression. 20dB will give
> > you 60dB enhancement in IMD/IP3 figures. If we look for more
> > suppression to avoid 2^nd harmonic mixer products especially with the
> > analogue mixers (QSD) this might not be sufficient. I am working with
> > 7 LPF for the entire HF spectrum. A HPF for (BC)signals below 1.5Mhz
> > might be added.
> >
> > 73s – Klaus, DK7XL
> >
> > >
> > > Message: 1
> > > Date: Sat, 10 Mar 2007 10:02:50 +0900
> > > From: "Phil Harman" <mailto:pvharman at arach.net.au>
> > > Subject: [hpsdr] ALEX
> > > To: <mailto:hpsdr at hpsdr.org>
> > > Message-ID: <mailto:014c01c762af$d358cec0$0201a8c0 at phil>
> > > Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> > > reply-type=original
> > >
> > > Thanks for all the great feedback regarding Alex. We are now looking a
> > > making a prototype board so we can actually measure the IP3 etc of
> > some of
> > > the potential filters we are considering.
> > >
> > > Rather than a bank of Bandpass filters another alternative is a
> > tracking
> > > filter. I think that the new FT9000 series uses one of these and also
> > > perhaps the IC7800. Does anyone have one of these receivers and can 
>see
> > > from the circuit diagram what is being used?
> > >
> > > There is also an intersting filter used in the the PT-8000 (
> > > http://www.hilberling.de/) <http://www.hilberling.de/%29> - have a
> > look at the photo of the preselector. It
> > > seems to be using a relay switched 5 bit variable inductor mated with 
>8
> > > relay switched fixed capacitors. Given the large size of the toroid
> > used I
> > > would expect the IP3 to be quite good. Not sure how this is
> > configured, a
> > > series tuned circuit perhaps, but it would be an interesting
> > solution since
> > > we could cover the SW bands as well. If anyone has experience or 
>futher
> > > information on these type of preselectors then please pass it on.
> > >
> > > 73's Phil....VK6APH
> > >
> >
> > ------------------------------------------------------------------------
> >
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