[FlexRadio HelpDesk] Re: SDRs and harmonics

Kermit Lehman

Kermit Lehman
Oct 16, 2020, 9:02 AM CDT
Does an SDR, for example my 6500, generate AF or RF harmonics like an analog transceiver?

Ken, AB1J

Here's what Ken Wells at Flex wrote back to me.  It answered my question. 

Ken, AB1J

Ken Wells (FlexRadio HelpDesk)
Oct 16, 2020, 10:38 AM CDT
Hello Kermit,

I am not entirely sure what your question is getting at, but here are some general principles.

1) Any RF deck, tube or Solid State, will generate SOME RF harmonics.  The big question is how strong are they, and how well they are filtered.  The Flex 6000 series uses excellent quality solid state devices in the RF Amplifier chain and the spurious emission figures exceed FCC specifications for harmonics and spurious emissions.

2) The SSB audio generated in the Flex is digitized shortly after the Mic Input.  All of the processing, filtering, equalizing, and modulation is handled digitally.  Then the signal is generated as an RF signal at the desired frequency. Where it is amplified by the RF amplifier chain.  Our Controlled Envelope Single Sideband Processor adds almost 3 dB of talk power with very little audio distortion, unlike many analog transceivers. 

This results in an extremely clean audio/RF signal on the air.

However, even in SDR's, If you overdrive the audio circuits, you can generate distortion.  This is why we have recommendations that you are careful not to drive the PEAK audio past the recommended levels.

Many operators confuse the AVERAGE audio indicator (the solid line on the Level meter) with the PEAK audio indicator (the small line that dances in front of the solid bar) and improperly adjust the mic gain level, overdriving the audio input.

When operating digital modes, we recommend using DigU or DigL and NOT USB or LSB.  DigU/L turns off the processor and TX EQ, assuring a clean signal, as long as the DAX TX Stream is adjusted properly and the digital signal is not overdriving the audio input.  The Digital signal from your program (WSJT-X for example) is generated digitally and send to the radio over the digital connection, where it is processed and then generated at the desired RF Frequency.  Since the signal remains in the digital domain until converted to RF, the Digital signal on the Flex is very clean when adjusted properly.

Does this help answer your questions?

Ken Wells, NM9P
4616 W Howard Ln Suite 1-150
Austin, TX 78728