[hpsdr] Questions re internal and external LPF on Penelope, Penny Lane and Hermes

[John Marvin]

Kjell,

Wow, no need to be sorry! Thanks so much for looking into this further. 
It would seem that with the right software one could actually do the 
measurements on a single hermes with the transmit output connected to 
the receiver (with the 20db attenuator enabled). As already mentioned in 
the VNA setup documentation, it might be better to use a 6db external 
attenuator when connecting output to input, not so much for the 
attenuation but to ensure that a proper impedance is seen by the 
transmitter.

I haven't played with the two receiver mode of PowerSDR. What does it do 
when transmitting? Is the second receiver still enabled during transmit 
(I seem to remember that on older versions of PowerSDR the main window 
switched to a transmit view while transmitting). Potentially you could 
send the two-tone test and use the second receiver to move around the 
spectrum looking at the various possible harmonics. But there may be 
something that prevents that since I haven't tried it. I'll look into 
that tonight. Since PowerSDR doesn't support the wide bandwidth view I 
would likely miss anything non harmonically related.

Of course, using Hermes to measure itself could have a variety of 
issues, so that such measurements may not be completely trustworthy. But 
it still seems like an interesting exercise to me, given that I have no 
other way of doing such a measurement.

Thanks,

John

On 11/7/2012 4:27 AM, Kjell Karlsen wrote:
> Hi John.
>
> As I mentioned, I have never measured the output from Penny so today I
> did it and found that the output is much cleaner than I measured from my
> Hermes. I also found that the output from Penny is less than from Hermes
> so I tested Hermes again. Then I found that if I reduced the output to
> the same level as from Penny, the spectrum was also the same. I was
> driving Hermes too hard, around 600 mW PEP but at 500 mW as the
> specifications are, the output is clean and all harmonics are > -50 dBc
> up to 17 m and > 40 dBc above.
>
> I have experimented with the gain and also some filtering in this Hermes
> so I have obviously left it with a higher than necessary gain in the
> last stage. The Gain Per Band settings I ended up with today without
> over driving the PA is between 41 and 43 dB.
>
> I do not have any of the production boards to measure on so if someone
> can do that on one, we can have this results confirmed.
>
> If you use the two-tone test in PowerSDR and measure with a good
> Spectrum Analyzer (or Mercury or another Hermes) adjust the GPB settings
> until the IM3 products are just visible at >-50 dBc. Then do a broadband
> measurement and look for the 2.nd and 3.rd harmonic on the different bands.
>
> I am sorry for this miss yesterday but I am trying to learn a new CAD
> program so this "multitasking" must have made me dull!
>
> 73, Kjell
>
>

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