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One problem I observe with my homebrew superhet receivers/transceivers is the constant presence of several "birdies". From what I know this happens due to the great sensitivity of the receiver and the presence of at least two (depending on the number of IFs) strong signals - BFO and VFO. Due to these factors, it's not uncommon to hear, for example, the 8th harmonic of the VFO on some random frequency.

I experimented with shielding and extra filtering in my latest project (2 IF all-HF bands superhet) and these measures didn't seem to help much. What I ended up doing was modifying the firmware so that if there is a birdie on 7.033.400 Hz the rig tunes to 7033450 instead. This approach helped me to eliminate all the "birdies".

Still, I can't get rid of the feeling that this is more like a "hack" rather than a proper solution. Is it possible to build a propper 2 IF all-HF superhet receiver without "birdies"? If so, what are the best practices I should take into account in order to achieve this? Or maybe the presence of "birdies" is an inevitable artifact of the superheterodyne design and changing the design (to DC with sideband suppression or SDR maybe?) is the only way to get rid of "birdies" completely?

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Since true perfect linearity does not exist when using any real physical radio components, there will always be birdies somewhere, due to various non-linearities. Made worse by the multiplicity of signals inside a superhet.

With superhet SDRs (RTL-SDR, SDRPlay's, mcHF/RS918, et.al.), it's a common practice to tune the physical mixer oscillator(s) to various offsets away from "centering" the baseband on the desired Rx frequency if necessary to get away from oscillator birdies, mixer imbalances, and other IMD. And then using DSP (IQ mixing and filtering in software), to retune a wider band IF capture back to the desired Rx frequency as an offset within that IF bandwidth (since high precision arithmetic software has a much reduced "birdie" issue.)

e.g. if you can move the birdies to where they don't bother you, do so.

Fixed frequency oscillators (DC-DC switching supplies in a superhet, digital IO interfaces, etc.) and their harmonics will still be a problem. Some radios are reportedly even capable of varying the frequency of any voltage converters needed in the box to avoid this issue.

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In the simplest manner, "birdies" in superhet designs are often an unwanted result of combined harmonics between the BFO and the VFO. A careful choice of IF to avoid harmonics in the BFO that could interact in this way with the VFO for whatever frequencies it tunes across is the usual workaround. The larger the range the VFO tunes the harder this gets, of course.

So, your approach is more or less in line with that. Most SDR software out there, for example, tweaks the VFO/tuning to minimize these problems with complicated mixers.

A direct-conversion model sidesteps this and other problems associated with superhet conversion (but introduces other problems to be solved).

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