Re: [Xylo-SDR] Janus board block diagram

["Phil Harman" <pvharman@arach.net.au>]

Hi Ahti,

Initially the Janus board will simply act as a USB sound card that you can 
connect the outputs of the SDR1000 INA163's to.

I intend to keep the inputs to the WM8785 (or PCM2402) at high level to 
maintain the S/N ratio at their input so had not intended connecting the QSD 
capacitors directly to it.  I'm not sure that would work without additional 
gain anyway but if not we could always use a higher performance amplifier 
(with differential outputs) than the INA163.

In fact at the moment I don't intend to use a QSD at all. We still have not 
found a way to overcome the serious noise figure problem of the QSD above 
about 18MHz and the  current theory says its not fixable.

I have breadboarded a new bus switch based mixer designed by I7SWX and that 
works up to 70MHz without any NF problems.  It does have a conversion loss 
of ~6dB so we will need to follow it with a low noise audio pre-amp ahead of 
the A/D converter.

It seems to me that we need to feed these pre-amps at a much higher 
impedance than 50 ohms in order to achieve the NF they are specified at. My 
idea at the moment is to follow the mixer with a step-up audio transformer 
and fix any phase shift issues with software.  My previous experience with 
such transformers is they tend to pick up all sorts of hum and noise so some 
experimentation is required.

I really hope Phil C's idea of using a LTC2208 high  speed A/D converter 
works and we can then ignore these mixers altogether. Plenty to do and lots 
of fun to be had!

73's  Phil....VK6APH


----- Original Message ----- 
From: "Ahti Aintila" <ahti@attocon.com>
To: "Xylo-SDR Discussion" <xylo-sdr@lists.ae5k.us>
Sent: Wednesday, January 25, 2006 3:03 PM
Subject: Re: [Xylo-SDR] Janus board block diagram


> Phil,
>
> Thanks! Looks good.
> Do you intend to connect WM8785 directly to the QSD capacitors without
> any buffer opamps in between? (I hate the INA163.)
>
> 73, Ahti OH2RZ
>
> ----- Original Message ----- 
> From: <pvharman@arach.net.au>
> To: "Xylo-SDR Discussion" <xylo-sdr@lists.ae5k.us>
> Sent: Wednesday, January 25, 2006 8:28 AM
> Subject: Re: [Xylo-SDR] Janus board block diagram
>
>
> > OK - the Janus block diagram is on www.hamsdr.com in the public
> > folders area
> > in the directory Janus. Is in both Visio and GIF forms.
> >
> > Let the feedback begin!
> >
> > Phil...VK6APH
> >
> >
> >
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