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
On Thu, Apr 16, 2015 at 3:42 PM, glen english <[email protected]> wrote:
> Bradly
>
> You must define what are the enemies of the receiver.
>
> A zero frequency SDR receiver has no IF images.
>
> but it has alias responses, so the reality is that the baseband filters
> (I and Q) should be >90dB down outside the passband and at the aliases.
>
> My experience is that this is where effort needs to be spent.
>
> As long as the front end does not overload (hit the P1dB) then you are
> pretty much OK in that part unless you land a unfortunate / unlucky
> intermod.
>
> My experience is for this receiver you just need to keep strong FM and
> TV stations from overloading the receiver. if another ham overloads the
> receiver, well so be it.
>
> The other thing you need to be concerned about are mixer spurs,
> harmonics of the mixer LO, harmonics of the products etc.
> Hence the good IQ LPFs.
>
> or if you have a low IF receiver, just watch the converter aliases.
>
>
>
> if you do need a front end bandpass
>
> try this one Bradly
>
> probably an overkill for a mobile/portable.
>
> http://www.temwell.com.tw/PDF/TT67709B-146M-Performance.pdf
>
> or
>
> suggest triple tuned circuit, top coupled C, using T25-2s and fixed
> caps with cap divider at each end.
> adjust by squeezing the toroid inductors. you should be able to operate
> it at a loaded Q of >15 with reasonable loss.
>
>
>
> regards
>
> On 14/04/2015 12:37 PM, Brady O'Brien wrote:
> > Hi everybody,
> >
> > I've been helping out with the development of VHF and up FreeDV and the
> minimalist VHF SDR. I've gotten to the point, on the analog side of things,
> that it seems appropriate to get a board made. What I've designed is a VHF
> frontend for further experimentation. It's based heavily on the VHF section
> of the DSP-10
>
>
>
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