[hpsdr] Getting PowerSDR to display SWR

[Jim Sanford]

All:

For what it's worth . . ..

Crenshaw's book, Math Toolkit for Real Time Programming has some neat 
and efficient algorithms for all kinds of functions -- on small, integer 
computers like used to navigate Apollo to the moon.  (AND, it has some 
interesting history as well...)

There might be something in there for SQRT that would allow "good 
enough" simple calculations which could be implemented in the FPGA.  
Might simplify latency/delay issues....

All the best and very 73,
Jim
wb4gcs at amsat.org


On 3/8/2012 9:31 PM, Phil Harman wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> John,
>
> Preferable to do the calculation in the PC rather than the FPGA. There are
> approximations to SQRT that would be good enough for calculating SWR and
> are easy to implement in the FPGA.
>
> This should be straightforward to implement in PowerSDR. It's sure to work
> for a steady carrier but latency issues my cause a problem with modes that
> have amplitude variations.
>
> 73 Phil...VK6APH
>
>
>> HPSDR,
>>
>> I think it makes more sense for PowerSDR to do this computation than doing
>> it on PennyLane - both the Forward and Reflected Power are transmitted to
>> PowerSDR in the packet transmission.
>> (assuming the SWR cable and Alex is being used; of course.)
>>
>> Since doing fast floating point on the PC is so much simpler and faster -
>> why wouldn't we want to do the computation there?
>>
>> SWR = (1 + SQRT(ABS(Pr/Pf))  /  ( 1 - SQRT(ABS(Pr/Pf)      rho =
>> SQRT(ABS(Pr/Pf) - then
>> SWR = ( 1 + rho ) / ( 1 - rho )
>>
>> The ABS may be superfluous in this case since the ADC can't read negative
>> I
>> suppose.
>>
>> Seems simpler to do on the PC side with the current architecture.  Am I
>> missing something here?
>>
>> 73's,
>> John
>> AJ6BC
>>
>> On Thu, Mar 8, 2012 at 11:45 AM, John Westmoreland<
>> john at westmorelandengineering.com>  wrote:
>>
>>> Phil&  HPSDR,
>>>
>>> OK - took a look at the code and have some test code implemented - issue
>>> is using real #'s in the Verilog environment for the FPGA; specifically
>>> -
>>> two functions, square root and absolute value are needed to compute SWR
>>> correctly from the current hardware configuration.  Another possibility
>>> is
>>> if Voltage or Current can be extracted - then that would make the
>>> calculation simpler.
>>>  From some rough testing I did sending an SWR test value over vs. the
>>> Reflected Power - looks like it was working.  To be sure I will want to
>>> review the code for PowerSDR though.
>>>
>>> 73's,
>>> John
>>> AJ6BC
>>>
>>>
>>> On Wed, Mar 7, 2012 at 6:07 PM, John Westmoreland<
>>> john at westmorelandengineering.com>  wrote:
>>>
>>>> Hello Phil,
>>>>
>>>> If this can be implemented in PennyLane then that is great.
>>>>
>>>> I will take a look at the source and see what it will take.
>>>>
>>>> 73's,
>>>> John
>>>> AJ6BC
>>>>
>>>>
>>>> On Wed, Mar 7, 2012 at 4:39 PM, Phil Harman<phil at pharman.org>  wrote:
>>>>
>>>>>    Hi John,
>>>>>
>>>>> Your correct, we did look at this before.  From memory the issues is
>>>>> that there is a delay between the FWD and REV samples that means for
>>>>> other
>>>>> than a steady carrier the SWR reading is meaningless.  I think this
>>>>> was a
>>>>> problem with the Flex hardware but not sure if we will have the same
>>>>> problem.
>>>>>
>>>>> The solution may be to calculate the SWR in the FPGA on Penny(Lane)
>>>>> since that will have the least sample delays.
>>>>>
>>>>> 73 Phil...VK6APH
>>>>>
>>>>>
>>>>>
>>>>>   *From:* John Westmoreland<john at westmorelandengineering.com>
>>>>> *Sent:* Thursday, March 08, 2012 3:37 AM
>>>>> *To:* HPSDR list<hpsdr at lists.openhpsdr.org>
>>>>> *Subject:* [hpsdr] Getting PowerSDR to display SWR
>>>>>
>>>>> ***** High Performance Software Defined Radio Discussion List *****
>>>>>
>>>>>   ------------------------------
>>>>> HPSDR,
>>>>>
>>>>> I am sure this has been discussed before - but I can't seem to find it
>>>>> in the archives.
>>>>>
>>>>> Is there a way to get PowerSDR to display SWR on the TX Meter?  In the
>>>>> configuration settings there is a box to pick for this - but when you
>>>>> run
>>>>> there is not a way to select SWR.
>>>>> Reflected Power is there - but not SWR.
>>>>>
>>>>> Thanks in Advance.
>>>>>
>>>>> 73's,
>>>>> John
>>>>> AJ6BC
>>>>>
>>>>> ------------------------------
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>>>>>
>
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