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Re: [Xylo-SDR] Janus board block diagram



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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