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

[John Marvin]

  I was looking at a FAQ on Penelope that addressed why Penelope has no 
filtering on the output. That got me interested in looking at how things 
have changed going from Penelope to Penny Lane to Hermes:

1) Without any filtering I would think that there would have been a fair 
amount of aliased output. In some cases people use that as a feature, 
i.e. deliberately use the aliasing to generate desired signals in ranges 
above the Nyquist rate. That was even mentioned as a possible feature of 
Penelope.  But at the same time it was claimed that at 0.5 W output, 
Penny Lane meets the FCC requirements without additional filtering. How 
far down was the signal in the Fs/2 - Fs range compared to the DC - Fs/2 
range?

2) So, after all the good arguments for no filtering, two filters were 
added to Penny Lane, an RLP-40+ and what appears to be series LC 
band-stop discrete filter. Were the original arguments not valid, or was 
there another reason for doing this?

3) Then yet another LPF filter was added to Hermes (an LFCV-52+). Why 
was this added?

4) The Penelope FAQ claimed that no external filtering was needed at 
0.5watt, yet the user guide for Hermes claims that an external LPF is 
always needed, even with all the additional filtering.  Is that actually 
true? If so, what are the sources of out of band signals that exceed the 
FCC requirements that didn't exist on Penelope?

5) Has anyone done any detailed measurements on Hermes TX output?

6) Has anyone ever tried doing VHF/UHF receive and/or transmit work on 
Mercury and or Penelope/Penny Lane using undersampling/aliasing 
techniques? If so, how and what was done (e.g. filters bypassed, FPGA 
work done, etc.).  Has there ever been any consideration of adding 
jumpers or other features to make it easier to bypass the input and 
output LPF filters in order to explore this possibility (which would 
require external band pass filtering)?

Thanks!

John
AC0ZG