I think a continuous carrier be able to heard in a super-heterodyne receiver, since it has it's own oscillator.

  1. Why do we need BFO then?
  2. What's the difference between the two?
  3. So, morse codes would be heard just on super-het receiver?
  • 1
    \$\begingroup\$ 1)No. 2) So you can hear it. 3) The product frequency : typically 455kHz in a superhet, vs something audible like 1 kHz from a BFO. 4) No. (Except on a crappy superhet where the Morse signal modulates the background noise) \$\endgroup\$
    – user16324
    Mar 21, 2021 at 18:11
  • \$\begingroup\$ Thank you for your reply. Can you explain why the product freq. could not be low in super-het rec. like as in BFO? It'd be really helpful if you explain the difference in the principles between the two? \$\endgroup\$ Mar 21, 2021 at 18:16
  • \$\begingroup\$ Because an IF product outside the IF filter bandwidth will be attenuated by said filter. \$\endgroup\$
    – user16324
    Mar 21, 2021 at 18:17
  • \$\begingroup\$ Thank you. Can you suggest me the books/relevant topics that I should study to be understand the topic? I don't have education in electornics/electricals. I've recently started studying super-het rec. Can you please suggest me where to start studying from, as I was unable to understand what you said. \$\endgroup\$ Mar 21, 2021 at 18:28
  • \$\begingroup\$ Too many to name. I have Scott-Taggart, "Manual of Modern Radio" which is very good, but not very up to date. \$\endgroup\$
    – user16324
    Mar 21, 2021 at 18:32

1 Answer 1


Super-heterodyne is short for supersonic heterodyne. It's called that because a super-het receiver has a frequency-selective intermediate frequency (IF) stage tuned to a supersonic band of frequencies. A local oscillator(LO) is used to convert the incoming radio frequency(RF) signal to the IF. You can't hear this conversion directly because the output IF signal is supersonic, i.e. above the audible hearing range.

So in a super-het receiver we need a second oscillator to act as the beat frequency oscillator(BFO). It runs in the intermediate frequency range and mixes with the received signal, now at the IF, to produce an audible audio frequency(AF) signal.

For a Morse signal the BFO is offset from the IF signal to produce the desired audio beat note. If we are receiving a Morse signal which has been converted to an intermediate frequency of exactly 455.0 kHz then we can set our BFO to either 454.2 or 455.8 kHz and we will hear the Morse as audio tones at 800 Hz (a popular frequency for listening to Morse). A continuous unmodulated carrier received in the same way would produce a continuous 800 Hz audio tone.

In summary a super-het used to receive unmodulated carrier or Morse needs two oscillators. The LO is used to convert the incoming RF signal to the IF and a separate BFO is used to convert the IF signal to an audible AF signal.

  • \$\begingroup\$ thanks, makes sense. Can you explain then why it's said that BFO is not needed for N0N A2A Signals? Also, somewhere it's written that BFO is needed for N0N A1A signals for tuning , identification and monitoring, but for N0N A2A, BFO it's needed only for tuning and not for identifying and monitoring. I'm confused. Thank you. \$\endgroup\$ Mar 22, 2021 at 17:56
  • \$\begingroup\$ @SachinChaudhary "Thank you" If I've answered your original question please vote up or accept the answer as described at someone answers. Do you have a link for these claims about signals? The ITU RF signal classifications are described e.g. here. N0N isn't a prefix for A1A or A2A, it's a classification in its own right meaning unmodulated carrier. You need a BFO to hear it. \$\endgroup\$
    – Graham Nye
    Mar 23, 2021 at 0:00
  • \$\begingroup\$ @SachinChaudhary A1A includes Morse where the carrier is keyed on or off, often known as CW. You also need a BFO to hear that. An A2A signal includes an audio frequency tone keyed with Morse. It can be heard on a broadcast AM receiver without needing a BFO. There is an obscure way you might use a BFO to measure the precise frequency of an A2A signal but you don't normally need a BFO. If you can provide a link to this claim I/we can try to explain it. Note you should start a fresh question to ask about a new subject, you should only use comments to clarify your original question here. \$\endgroup\$
    – Graham Nye
    Mar 23, 2021 at 0:13

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