[hpsdr] Preselector PT-8000

[Klaus Lohmann]

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 2nd 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/) - 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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