Re: USB cable quality and QRM in a spyserver setup with an Airspy Hf+ and Raspberry PI3. 6 microUSB cable test. #airspyhfplus #bestpractice #raspberrypi

Dana Myers
 

On 3/14/2018 7:35 AM, David Ranch wrote:

For a 2-pack of 10' cables [Volutz], $17 on eBay was a deal.
Looks like there's a better deal on Amazon - free same-day shipping
with Prime, $12. Shop around. It's nice having a ridiculously long
cable in my shack, BTW.

Wow.. 10'?  I'd assume there is some pretty serious voltage drop over that!  Regardless, I'll have to give them a look.  Thanks.
Let's do some math, because we're scientists :-). A graphic at the
Volutz website says the power wires in their USB cables is 22 AWG.
I believe we can safely ignore the data wires here; as long as the
cable isn't dropping frames, it's OK.

Consulting Wikipedia:
https://en.wikipedia.org/wiki/American_wire_gauge#Rules_of_thumb

We see that the approximate resistance of  this wire is 16 ohms / 1000'.
So, 10' of the wire (power and return) has about ~ 10/1000 * 16, or
0.16ohms, resistance per leg, or a total of 0.32 ohms.

Assuming a 200mA load (I don't actually know the Airspy HF+ current
consumption), that means the +5V rail at the HF+ drops by 0.032V,
or 32mV, and the ground rail at the Airspy HF+ increases by 32mV. In
other words, the +5V supply to the Airspy HF+ drops by 64mV.

Given that I bet nothing inside the Airspy HF+ runs from higher than a 3.3V
rail, this is plenty of margin for an LDO (you can do worst case math with USB
host power rail of 4.5V and LDO minimum differential of 0.6V and still have
500mV of margin).
Assume I'm wrong  and the HF+ uses 500mA (the max spec for a
USB port, right?). That's 160mV drop to the HF+; still plenty of margin
for a 3.3V LDO (worst-case above would be > 400mV).

The keen observer might have noticed I suggest the DC ground
in the HF+ is ~32mV higher than the USB host ground. Note that
this doesn't matter because the USB signalling is differential data,
not analog.

OK, maybe we're not scientists, we're ham radio operators. In which
case you can disregard the above and say "it works great" (like the way
J-Pole antennas are 3/4-wave verticals :-)).

73,
Dana  K6JQ

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