[hpsdr] Preselector PT-8000

[Graham Haddock]

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:
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>
> 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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