[hpsdr] Pennywhistle completed at last

[Phil Harman]

Hi Chris,

John's suggestion is the correct way to set the gain since we don't have any 
feedback from the PA we can't automate the process for now.  Once Alex is 
available we can close the loop.

If your PA strip requires little drive  then you may want to fit an 
attenuator between the output of Penny and the input of your PA.  It's best 
to run Penny at say 200mW minimum since otherwise you degrade your Tx S/N 
ratio.

We found a way to improve this and Hermes (and PennyLane) uses the full 14 
bits of the output DAC at any power level.

73's Phil...VK6APH


----- Original Message ----- 
From: "John Petrich" <petrich at u.washington.edu>
To: <hpsdr at openhpsdr.org>
Sent: Sunday, March 21, 2010 9:05 AM
Subject: [hpsdr] Pennywhistle completed at last


> ***** High Performance Software Defined Radio Discussion List *****
>
> Chris,
>
> You might be over thinking this a bit.  Given the same situation  with my
> AN762 amplifier, I just set the Power SDR PA settings at some arbitrary 
> high
> number such as "56" and then gradually decremented the setting to get the
> desired output per band.  Simple.  No calculations.  And, like you I'm
> running my PA about 3 dB down from maximum, 90 watts, to provide head room
> on SSB to keep the IMD reasonable.
>
> Hope this helps,
> John Petrich, W7FU
>
>
> Hi
>
> Despite taking delivery of PW before Christmas, I've only just completed 
> the
> kit. My excuse is that I have a (virtually) unheated shack in the back
> garden and the weather in this part of south-east England has been 
> dreadful
> since Christmas. The day-time temps have only recently risen above 0C or 
> 32F
> in old money.
>
> Now, bearing in mind that I haven't taken the action with regard to the 
> O/P
> transformer on Penelope mentioned in the errata, I was wondering what the
> gains in the PowerSDR PA Settings should be?
>
> I have Pennywhistle followed by the 140W version of the Motorola AN762 PA.
> I'm assuming that the dB figures in the box for each band is the combined
> gain of PW plus any additional PA required to produce a fixed o/p, perhaps
> 100W, at the antenna socket. Is this correct?
>
> The gain figure quoted for the 140W version of the AN762 amp is 13-15 dB 
> and
> Pennywhistle has a gain of approx 19 dB - a combined gain of 32-34 dB. 
> Let's
> say 32 dB.
>
> In light of the reduced o/p from Penelope (320mW instead of 500mW), which 
> in
> my simple mind represents  a power loss of just under 1 dB, if I'm aiming
> for an o/p of 100W i reckon the boxes should be set to around 33-34dB.
> However 38dB seems to be the minimum setting. However, I'm sure this would
> result in a dreadful over-driving of the final.
>
> If I set the Tune Power to 20W in the Transmit window, I get 100W out into 
> a
> dummy load on Tune.
>
> So, I'm confused. Can anyone put me right please? I'd prefer to under-run
> the PA at 100W with a little in reserve than push it to max o/p.
>
> Cheers & 73
>
> Chris
> G4NUX
>
>
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>
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