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I have quite simple question about topology of superheterodyne. I am facing with adjacent channel selectivity problem.

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The problem is that I can't obtain selectivity more than 40 dB. F.e. when wanted channel frequency signal had ampl. level 1 dBuV and adjacent channel frequency ampl. have more than 40 dBuV receiving process is broken. Both frequencies are provided to antenna circuit by sumator. Similar problem - 20 dBuV (wanted freq.) and 60 dBuV adjacent freq.

It is not a issue of IF filters stage - problem doesnt't occur when IF frequencies are directly modulated and provided to IF stage skipping first mixer and LNA stage. Then almost 85 db selectivity is obtained.

The problem is VCO (to first mixer) frequency reference from crystal. Due to phase noises output frequency (from VCO) to first stage mixer have only 40 dB difference of amplitude between center frequency and + / - 20 kHz.

So, if any adjacent channel frequency have amplitude 40 dB bigger than wanted freq. channel it's mixed with phase noises from crystal / VCO and have higher amplitude than wanted frequency (so it is totally corrupted).

The question: Is there any topologies or f.e. limiters / attentuators at the RF antenna signal stage to limit unwanted signal without limiting wanted 40 db lower signal?

Thanks for any answers.

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    \$\begingroup\$ You've pretty clearly described a phase noise problem which needs correction. \$\endgroup\$
    – Chris Stratton
    Commented Dec 2, 2019 at 14:07
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    \$\begingroup\$ If front-end frequency-selective filter could separate adjacent from desired signal, superhet is not needed. VCO is the problem...fix this weakest point first. Greater than -40 dBc at 20 kHz offset shouldn't be difficult to achieve unless this is an exotic oscillator with other requirements. \$\endgroup\$
    – glen_geek
    Commented Dec 2, 2019 at 14:12
  • \$\begingroup\$ First, verify that it's really a problem with the local oscillator by replacing it with something more stable, like a crystal oscillator. Test it at the receive frequency and see how it does. Then, if you're sure it's the local oscillator -- fix that. There is no superheterodyne topology that will fix a local oscillator phase noise problem. \$\endgroup\$
    – TimWescott
    Commented Dec 2, 2019 at 15:42
  • \$\begingroup\$ @TimWescott I have tested many high quality crystals f.e. I533-2P3-12.8000 - 2ppm an phase noise levels -146 dBc/Hz @ 10 Khz. According to phase noise to jitter converter ( changpuak.ch/electronics/phase_noise_jitter_conversion.php and reverse conversion: changpuak.ch/electronics/… ) I have obtained correct values according to datasheet - crystal center frequency is nearly 40 db above +/- 10 kHz noise. It is difficult to find better crystal. Is any topology that could improve phase noise issues of crystal? \$\endgroup\$
    – Fasset
    Commented Dec 2, 2019 at 18:47
  • \$\begingroup\$ You say you've tested, but you point to a theoretical web site. If your crystal oscillator has phase noise that's 146 dB below carrier, that means that the spurious mixing at that frequency offset, from your oscillator, has an attenuation of 146dB -- that's a lot more than the 40dB that you're reporting. \$\endgroup\$
    – TimWescott
    Commented Dec 2, 2019 at 19:59

1 Answer 1

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A fix at RF front end will cost more than fixing your VCO.

But yes. One (uselessly expensive) possibility would be to used a crystal fixed frequency oscillator and mixer, downconvert, use an agile-frequency DSP filter, then up convert back to RF to feed your noisy VCO/mixer front-end.

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  • \$\begingroup\$ I have tested many high quality crystals f.e. I533-2P3-12.8000 - 2ppm an phase noise levels -146 dBc/Hz @ 10 Khz. According to phase noise to jitter converter ( changpuak.ch/electronics/phase_noise_jitter_conversion.php and reverse conversion: changpuak.ch/electronics/… ) I have obtained correct values according to datasheet - crystal center frequency is nearly 40 db above +/- 10 kHz noise. It is difficult to find better crystal. Is any topology that could improve phase noise issues of crystal? \$\endgroup\$
    – Fasset
    Commented Dec 2, 2019 at 18:48

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