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@arrl.net> wrote:

On Thu, Jul 11, 2019 at 4:48 PM Jeffrey Pawlan <jpawlan@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