[Skimmertalk] RP-16 front end tenderness

[Bob Wilson, N6TV]

I overlooked something in my description below (TNX to WA6KBL for off-list
conversation). Yes the 1:4 transformer steps up 1V p-p RF input to 4V p-p,
but the center tap of both TRF3 and TRF4 is at RF ground, so only half of
that voltage is fed to the two analog inputs of the ADC chip (Ain+ and
Ain-), so 2V, not 4V.

It seems safe to speculate that the ADC chip is an LTC2185.  Data sheet is
here:
https://www.analog.com/media/en/technical-documentation/data-sheets/218543f.pdf

This Data Sheet has an important note that should alleviate concerns about
damage to the device being caused by a 2V p-p input or more, as long as a
DXE front end protector is used on the antenna input.

Analog Input Voltage (A9in+, AIin,PAR/SER, SENSE) (Note 3)........... –0.3V
to (Vdd + 0.2V)
Vdd = 1.8V
So the maximum analog input voltage would be 2V, or 1V on the antenna
connector, but then there is Note 3:

Note 3: When these pin voltages are taken below GND or above Vdd, *they
will be clamped by internal diodes*. *This product can handle input
currents of greater than 100mA below GND or above Vdd without latchup.*

100ma into 100 ohms would be 10V, or 5V on the RF input.  100 ma into 200
ohms would be 20V.  This is way more than the measure output of the front
end protector.

In sum, as said before, as long as you're using a DXE Front End Protector
or equivalent at all times on the input to the Red Pitaya SDR 122.88-16, I
don't think any other protection is required (other than lightning
protection).  The "1V max" is a maximum operating voltage, not the voltage
level where damage can occur.

73,
Bob, N6TV

On Thu, Jul 11, 2019 at 5:00 PM Bob Wilson, N6TV <n6tv at arrl.net> wrote:

> On Thu, Jul 11, 2019 at 4:48 PM Jeffrey Pawlan <jpawlan at pawlan.com> wrote:
>
>> The version 1 schematic that Bob sent me says +/-  1v max.
>>
>
> It also says input impedance (Rin) is 1 Meg Ohm, which is copied from the
> old RP schematic, and clearly no longer applies.
>
> However if that 1:4 transformer is stepping up the RF voltage by a factor
> for 4, and the ADC chip can't handle more than 4V RF input, them maybe the
> 1V number is correct.  What's typical for a 16 bit ADC sampling RF?
>
> The old RP-14 schematic said 30V max, but that was in it's factory
> configuration with large input resistors, not a radio with the impedance
> pad bypassed.  It then says 30 ma Max on the amplifier and filter, no
> longer present in the RP-16 schematic.
>
> I was trying to evaluate the ratings of all the components in the input
> chain, one by one, as I have already done for the transformers.  1V seems
> quite low for those.
>
> 73,
> Bob, N6TV
>
>
>
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