[hpsdr] Hermes Sweep Generator?

[John Marvin]

  John,

What is possible may depend on what your use case is. As Phil has 
already mentioned, with special FPGA code anything is possible. His VNA 
support allows for the possibility of a wideband sweep.  Using the 
current non VNA features in the FPGA limits the current possibilities.  
The transmit sampling rate is 48 Khz, which would allow for a narrowband 
sweep anywhere within the 0-60Mhz range, i.e. you could choose any 
starting frequency in the 0-60 Mhz range, but then could only sweep up 
to a frequency ~48 Khz higher; which may be more than enough for many 
particular use cases.

I did try a few experiments to see what else might be possible.  I was 
curious about what happened when changing the TX center frequency while 
transmitting a fixed sine wave. I wasn't sure if that would produce 
glitches or not. But that actually worked. The center frequency 
resolution is 1 Hz, so the sweep isn't purely linear, and there are 
other limitations, but that might allow for some other possibilities.

I then went on to see if the two methods could be combined, although I 
would have been surprised if this had worked. I'm not sure when a TX 
center frequency change takes effect relative to the samples contained 
within the same packet, and that will be dependent on the FPGA code 
(i.e. it could take effect X samples in, depending on delay in the dsp 
algorithm being used). This whole idea would be a fairly unstable method 
of doing things anyway.  But I tried jumping the digital oscillator 
frequency up 5 Khz at the beginning of a packet (but not restarting the 
oscillator, i.e. maintaining the current phase) while at the same time 
dropping the TX center frequency by 5 Khz (which would result in the 
output frequency of the sine wave staying constant). I toggled that 
behaviour back and forth  (i.e. up then down) roughly ever 2 seconds 
(again on a packet boundary) to see if there were glitches. There were 
glitches, which I would have been very surprised if that was not the 
case.  It's possible with more internal knowledge that this could be 
made to more or less work, but I'm not sure it would be worth 
implementing something that depended so heavily on internal 
implementation. The purpose of this test was to see if it might be 
possible to do a sweep within a 48 Khz "window" while periodically 
sliding the window to allow for any possible programmable behaviour over 
a wide range of frequencies (without special FPGA code).

John
AC0ZG

On 12/19/2012 1:37 PM, John wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
>
>
> Has anybody written a program to use Hermes as a sweep generator?  I 
> need to align the IF in my vintage HQ-129X and it occurred to me that 
> it might be possible to use Hermes.  Maybe even briefly pause the 
> sweep at certain frequencies and boost the output to make markers.
>
> John,  WoGN
>
>
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