Re: RD16HHF1 in the uBITX #ubitx

Jerry Gaffke
 

Excellent find!
Power into Q90 of the uBitx is less than 1dBm (best guess, perhaps -7dBm?) and at 50 ohms.
The stage at Q90 might have a gain of 15dBm, I'm not sure without checking my simulation.
So this attenuator would never see 10dBm, spec says absolute max (where damage occurs) is +30dBm  = 1 Watt.

I think I'd leave RV1 in place to set max power, and then place this attenuator between RV1 and C84.
Assuming it works well with impedances that are in the ballpark but not exactly 50 ohms.

Disclaimer:  I'm a digital guy too.

Jerry, KE7ER


On Tue, Feb 20, 2018 at 11:40 am, RCBoatGuy wrote:
I ran across this web page earlier today: https://www.sv1afn.com/rfattenuator.html .   This design uses a 6-bit, 50 ohm, 1-4000 MHz digital RF attenuator chip from Peregrine Semiconductor to vary RF attenuation from 0dB to 31.5dB in 0.5dB steps via an SPI serial interface.  Could we place something like this in between two of the earlier uBitX TX gain stages and use this to control our TX drive level digitally rather than using RV1 alone?  The newer PE4312 chip (the replacement for the obsolete PE4306 used in the design above) goes for less than $5 in small quantities (less than 100 units).

Please note from the PE4312 datasheet (http://www.psemi.com/pdf/datasheets/pe4312ds.pdf) that the maximum allowed input RF power is much lower for HF frequencies, going from roughly 11.5db @ 1MHz to about 23.5dB @ 50MHz (see Figure 4).  The PE4312 also allows control via a parallel interface for those that would rather control it that way, perhaps using an I2C digital I/O expander chip to control the attenuation rather than using SPI to save pins on the Raduino.

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