[Skimmertalk] RedPitaya 125-10 vs 125-14

[Paul Higginson]

On 24/11/2017 13:55, Bob Wilson, N6TV wrote:
> On Fri, Nov 24, 2017 at 3:50 AM, Paul <gw8izr at gmail.com> wrote:
>
>>  From your list Bob and for technical completion, it is quite a lot less
>> sensitive than the QS1R even with the matching transformer, certainly here
>> I would need an external preamp or active band preselector, this would
>> further degrade the already worse large signal handling capability. The
>> return loss at the input will make it more difficult to match to external
>> filters.
>>
> I'm not following you here.  We have:
>
> 50 ohm antenna -> 50 ohm lowpass filter (opt) -> 50 ohm pre-amp -> 1:14
> transformer -> High impedance input of Red Pitaya
>
> So where is the high SWR / high return loss?  The transformer board is
> located right on the input connector.

This is quite simple Bob, the return loss of the RP board is not 
constant over its used frequency range therefore the return loss 
transformed by the turns ratio will also vary, its a compromise. From 
the measured data, above 10MHz that delta becomes quite significant. Of 
course I accept having two antenna ports is a big advantage here as its 
possible to tailor the match, I believe a different matching topology 
would be better from 20m upward if you want to get the best from this RP 
board. This *may* be part of the sensitivity reduction as well as it 
won't be simple to measure with this transformer.

It may be close enough if the RP is being fed by an active antenna which 
is not too fussy about its match but if you have a more complex 
requirement such as a series of filters to use at a multi band contest 
station or you suffer large levels of SW BC signals its going to make 
terminating those preamps and filters a bit harder.

> This is the first time anyone has reported the the QS1R is "quite a lot"
> more sensitive than the Red Pitaya.  What frequencies are we talking
> about?  How was the measurement made?  Both the QS1R and the Red Pitaya
> benefit from external filtering and pre-amps in the same way.

Its not my data to share but the figures stack up. The measurements are 
comparisons of the RP and a QS1R, neither data set has any big outliners 
so I think the data is good.. So right now I've seen one set of data for 
the RP and it fits with my suspicion, that's all. If anyone else has 
measured data then of course I'll consider it. The whole point here is I 
don't know and I don't particularly want to buy one just to measure it 
and confirm it won't be good enough for me.

If others are happy that its good enough that's fine.

>
> Pretty much what we should expect when comparing a $750 receiver with $300
>> piece of test equipment. (or whatever the current prices are)
>>
> Well, another advantage of the Red Pitaya is that it *can also* do many
> other things:  dual trace oscilloscope, dual channel signal generator,
> Vector Network Analyzer) VNA, etc.  See http://bazaar.redpitaya.com/ .
>
> 73,
> Bob, N6TV

OK Bob your obviously a great fan of the RP board, excellent I'm happy 
your pleased with it. I'm not saying it isnt a good product but I can't 
accept that its as good as a QS1R or Perseus etc on the basis of the 
information available. I'm happy to be proven wrong, that's all.