Re: [Freetel-codec2] VHF Frontend for SDR !!!!!

glen english Thu, 23 Apr 2015 15:00:09 -0700

Hi Brady How much gain you need is pretty easily calculated from calculating your SDR noise figure, and then knowing your desired noise figure. Add a few dB for something you didnt take into account ! I think you can just relax that (tight) filter. if there is an interference / overload issue, make that a separate problem to your prototype - IE dont worry about it now. In prototype you can rarely cover all bases. rather like over optimizing too early in software writing... maybe put down 2 x pads for UFL connectors and a 0805 0R link between them, so you could connect an external 50 ohm filter if you wanted it (and not fit the resistor).... I use these : DK P/N # ARF1579CT-ND I'll be very interested to see how the '32 ADC goes on 10.7 meg under sampling . good project. cheers

On 24/04/2015 7:31 AM, Brady O'Brien wrote:

Glen, Shane, Bruce, and everyone else on the list:

Thanks for looking over the frontend and giving me these suggestions!

Based on what everyone's suggested, I have quite a few changes to make:

- Replace PIN diode rf switches and biasing with 'canned' RF switches (SKY13351-378LF ?)

- Remove 2n5109 amplifier

--- I probably don't need the extra gain right now and can figure it out later if I do

- Replace 10v vtx/vrx with 5v vtx/vrx

--- MMIC Amplifiers don't need 10v; the only thing keeping it there was the 2n5109

- Change the mixer BPF

--- Switch to topology that supports looser components

--- or use filter module

I'll probably keep the MMIC amplifiers I'm using now as the are not too expensive and unconditionally stable, although, it might not hurt to use something with a lower NF on the receive side.

A couple more notes about the frontend and IF side:

- I plan to use a 10.7 Mhz IF, as narrow monolithic filters are available for that frequency and it's possible to reach from the STM32 through undersampling

- This board will be fabricated by OSH Park on their 4 layer stackup. All of the RF carrying tracks are (in the current layout) 50 ohm microstrips

Thanks,

Brady O'Brien KC9TPA

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Re: [Freetel-codec2] VHF Frontend for SDR !!!!!

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