[hpsdr] Penny PTT unkeying spike - T/R sequencing?

[Phil Harman]

Hi Tom,

Fine on the use of a 20dB pad on the output of Penny. This is exactly the 
right thing to do ( as apposed to turning the drive level down) since it 
maximises the S/N out of Penny.

The new PennyLane board uses a different technique to vary the power output 
in order to maintain maximum S/N even with just a few mW of output.

I don't understand your need to place a 50 ohm resistor across the output of 
Penny. I would have expected you to have placed the 20dB pad directly on the 
output of Penny. In which case this would provide a solid 50 ohm termination 
at all times.

How do you have your system configured?

73 Phil... VK6APH

----- Original Message ----- 
From: "Tom Cathey" <k1jj at comcast.net>
To: <phil at pharman.org>
Cc: <hpsdr at lists.openhpsdr.org>
Sent: Saturday, February 05, 2011 6:11 AM
Subject: RE: [hpsdr] Penny PTT unkeying spike - T/R sequencing?


> Phil,
>
> Another "discovery"....
>
> I was testing an MRI  10mw in, 100w out  linear amplifier today with 
> Penny.
> Penny drove it well with a 20 db pad in-between.
>
> However, I started getting those lockups again where Penny puts out 
> spurious
> signals and will not shut off until the power is killed. Penny has been
> stable into a good dummy load with just an occasional lockup.
>
> I added a 50 ohm resistor across Penny's RF output. This cured the problem
> and is now stable with at least two amps I've tried.
>
> I think that during unkey, there is a short time where Penny has no 50 ohm
> load due to the external  PTT relay. The existing unkey spike shock I have
> seems to exaggerate the condition and Penny takes off. The delay from TX 
> to
> RX probably sees Penny still on with an infinite load for an instant. 
> The
> added constant 50 ohm resistor keeps a load on at all times.  I have more
> drive than needed so this is not a problem.
>
> That said, I'm finding Penny's output IMD to be so clean, I'm having 
> trouble
> finding a comparable pre-linear amplifier to use. I've tried MRF-150's and
> might build up a class A, two-stage  tube linear with negative RF 
> feedback.
> All I need is about 30W pep to drive my KW amp system, which is already
> about -40db 3rd IMD. The run of the mill -30db IMD SS amps don't seem to 
> cut
> it. Running them in full class A requires huge heat sinking and I'm 
> thinking
> it can be done easier with tubes.
>
> 73,
> Tom, K1JJ
>
>
>
>
>
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