RFM98PW radio module modifications

[storborg]

Hi @maholli, I read this page on the PyCubed site discussing the removal of the linear regulator from the RFM98PW, and was hoping to ask a few questions.

Given that the PyCubed mainboard supplies 3.3V to the module, I assume that this modification has been flown on all PyCubed missions?

I noticed that the motherboard still powers the 5V input pin, even with the regulator (presumably?) gone. Is this is because the RF power amplifier still runs off the 5V supply?

On the PyCubed mainboard v05, there is a jumper to select between 3.3V and 5V/adj. On the PyCubed-Mini mainboard, the 5V pin is supplied with 3.3V. Can you comment on the tradeoffs of supply voltage choice here, and/or which configurations have been proven? Is there a tradeoff of transmit power or linearity that comes with the reduced voltage?

Related to all of this: do you know what RF power amp is used on the various amplified HopeRF modules?

Thanks!

[Mikefly123]

Hey @storborg, you got it right! When using an RFM98PW the onboard PA requires a 5V input to generate the +30dBm transmit power. The PW module will also regulate that 5V down to a 3.3V rail to be used as the logic level by the transceiver and the datasheet actually calls the 3.3V pin on the module an output that you should use to power the MCU. Since there is already a switching 3.3V regulator on the PyCubed it's a good idea to remove that internal LDO to improve reliability and also protect against possible reverse current problems from having two regulators on the same rail.

Non RFM98PW modules only take in 3.3V since they generally only get up to +23dBm.

[storborg]

Thanks @Mikefly123. Do you know what configuration(s) have been flown? And/or any idea what the tradeoffs are of supplying the 5V PA rail with only 3.3V?

[polygnomial]

It has flown many times with the TPS7A suppling 5V to the radio with the LDO on the radio removed.

If you supply VDD on the radio with lower voltage, you will likely get less output power, possibly not enough to hear it from orbit depending on the rest of your RF system and your ground station setup. Try it, measure it, and let us know!