[hpsdr] Hermes 144mhz

[Helmut Oeller]

Hi Bent,

I completely agree with Kjell. You will get no sufficient performance
without changing ALL filters. The loss of LPF between U11/U10 causes too
much necessary gain of an external 144 MHz preamp to achieve a reasonable
total noise figure. But this limits blocking and RX IM behavior
dramatically.

73, Helmut, DC6NY

-----Ursprüngliche Nachricht-----
Von: Hpsdr [mailto:hpsdr-bounces at lists.openhpsdr.org] Im Auftrag von Kjell
Karlsen
Gesendet: Donnerstag, 11. September 2014 10:21
An: hpsdr at lists.openhpsdr.org
Betreff: Re: [hpsdr] Hermes 144mhz

***** High Performance Software Defined Radio Discussion List *****

Hi Bent.

I am not sure if it is possible to just short the LP filter, it may be
necessary to remove it. If just shorted, it will be in parallel to the
signal and can attenuate the 144 MHz signal but I am not sure. The LP filter
between U11 and U10 is is installed to reduce the aliasing noise  from U11
so if it is modified, the S/N on HF will be worse.

There will be a similar problem in the TX. If you just tap the signal
between T1 and FL1, the filter will still be in circuit and may distort or
attenuate the signal at 144 MHz. And the "long" coax to the external board
will be in parallel to the signal when working on HF. It might be possible
to switch in this tap by using a diode in series with the VHF signal and
send a control voltage via the coax on 144 MHz.

I think you must also switch off the Vcc to the TX amplifier when
transmitting on VHF.

It might work but has to be tested.

73, Kjell



På Thu, 11 Sep 2014 02:33:39 +0200, skrev Bent Andersen
<eskolin at gmail.com>:

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