Re: [Freetel-codec2] SM1000 phantom power

[Matthew C. <vk...@bi...>]

David,

Provided people understand the limitations of phantom power then you're
good.  I have only implemented phantom power on transceiver cables that
have an A+ rail (13.8V@1A).   The majority of transceivers seem to have an
8V@30mA rail for biasing eletret microphones, these should be avoided at
all costs.

If Rick and yourself are worried about people inadvertently using this 8V
line and smoking a radio, then simply place a voltage lockout circuit on
the phantom power patching input.  This is as simple as one P-channel
MOSFET, resistor, trimmer, cap and a zener.   If the threshold was set to
9V, this is well above a mic power supply and lower than a flat car battery
(10V).   If you were to diode-OR the existing jack with this phantom power
input just prior to the switch, you can also stop the user doing something
really dumb like powering the DC jack with phantom power enabled and back
feeding volts into the radio via the SM1000.  There are a plethora of good
p-channel fets in SOT23 with low RDSon, so we're talking less than 1cm^2 of
PCB real estate (3/8"^2 for our imperial friends).  About $0.80 in
components :-)

However when employing phantom power up a cable it is very important that
the digital DC ground from the SM1000 regulator and the Microphone/AF
outputs are not commoned at the SM1000.   I've made this mistake already
this week in a caffine induced stupor.  Instead a separate wire for the
digital DC ground is required, which is then commoned at the radio
connector with the PTT ground et al. The line-in and line-out signals can
also then be kept separate and likewise commoned at the transceiver.

Tonight I hope to improve the K3 cable I've made and once perfected I'll
send you the circuit.   Along with the PCB design for the jumper block PCB
to match ;-)

73

Matthew
VK5ZM

On 10 September 2015 at 09:23, David Rowe <da...@ro...> wrote:

> Hello list,
>
> A few people have modified their SM1000 to draw power from their radio via
> the RJ45 connector.
>
> Rick and I have considered this a few times but to date have decided NOT
> to provide a power on the patch panel to date - as we are concerned about
> (i) possible damage to radios and (ii) variable operation of the SM1000.
> The type of power (e.g 8V, 12V, current) available varies widely.
>
> The flip side is it simplifies set up of the SM1000 - just one cable, and
> the RJ45 locks positively compared to the DC connector.
>
> Would be interested in your thoughts!  Rick and I are currently working on
> Rev F so now is the time to speak up.
>
> Thanks,
>
> David
>
> On 10/09/15 07:16, David Rowe wrote:
>
>> Hi Alfred,
>>
>> codec2-dev is likely to change at any time.  I need that ability for my
>> development. However the stable releases and FreeDV API version number
>> are there to control exactly the situation you are concerned with.
>>
>> Cheers,
>>
>> David
>>
>> On 09/09/15 16:45, wully wrote:
>>
>>> Hi David
>>>
>>> Very nice! But please don't change the interface to often, because we
>>> are just at the point to test 700B in the wide field (current conds are
>>> very bad, so this will take time). It is very important, to have a
>>> stable environment to be sure about the effects on the air. When
>>> experimenting and not knowing wether the other station has the same
>>> mode, we will not come to positive conclusions.
>>>
>>> Thank you for all your efforts!
>>> 73, hb9epu, Alfred
>>>
>>> On 08.09.2015 22:53, David Rowe wrote:
>>>
>>>> James Ahlstrom has submitted a fine patch to enable FreeDV 700 to run at
>>>> a sample rate of 8000 Hz rather than the rather obscure 7500 Hz.  I
>>>> thought it would be useful to document the testing here:
>>>>
>>>>       http://www.rowetel.com/blog/?p=4561
>>>>
>>>> This means some changes to the FreeDV GUI software (freedv-dev won't
>>>> build atm), which I'll clean up and test over the next week or so.
>>>>
>>>> - David
>>>>
>>>>
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>>>
>>>
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