Is there a technique to remove the heterodyne tone from a Superhet?

Space Varmint

New Member
Does anyone now of a way to remove the beat note from a received superhet signal? It would seem there is because the beat not is one frequency as long as the receiver tuner is stable. I understand that if you zero beat the frequency there will be no tone. But because the tone is stable and only the modulation changes frequency it would seem that possible a slow acting phase inversion being fed back to a mixer of some sort should remove it. The reason I ask is I recently constructed a simple Direct Conversion receiver. Just something small and cheap. I really lie allot of the advantages a DC receiver has over say a regenerative receiver or a TRF receiver. Stable carrier through tuning range, high selectivity and good sensitivity. The problem is, it would be nice to listen to standard AM broadcast without constantly trying to zero beat the carrier or tune far enough off frequency if the signal is strong enough. Thanks for response.
 
You're discovering why direct conversion isn't a popular method.

If the transmitter is stable, then a digitally tuned receiver can reduce the beat to subsonic frequencies. But, by the time you've introduced that complexity, there are other good designs that are easier and perform better.
 

Yeah, figured as much. Dumb question. If I had a good high Q antenna tuner I could just shut the beat oscillator off. Right now I got a parallel tuned input and the bandwidth is nil.
 
Why not phase lock the carrier with your oscillator using a PLL? That'll get rid of the beat frequency.
 
Bob Scott said:
Why not phase lock the carrier with your oscillator using a PLL? That'll get rid of the beat frequency.
Click to expand...

Trying to keep it simple and low power. Think I will have another look into this regenerative stuff. I got one working for both but that feedback in the regenerative is so frequency dependent. In fact I am about to try another AGC approach to see if I can lock that level. Other than that I extremely impressed with it's AM reception.

Do you do much PLL stuff? I have a project I'm finishing up now that uses the MC145170. It's fairly easy to use with serial interface and "bit grabber" technology where you don't need to do much with the latching control signals.
 
Perhaps Synchronous detection helps improve Audio of Superhets. I may refer to synchronous detection as employed in SW radio capture article of Elektor UK Dec2006.
 
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Nigel Goodwin said:
Misleading title - Superhets don't whistle - Direct Conversion is only suitable for SSB or CW, not for AM (for this very reason).
Click to expand...
Please see the IQ demodulators used in Software defined radio published in Elektor UK May2007 by Burkhard kainka. Ofcourse they use computer controlled local oscillator using Cypress CY27EE16. we could also use first LO with DDS usng AD9851.
Direct conversion is used here for AM and also for DRM. I agree that the audio is generated using sound card as DSP and it has facilites for better control of active bandwidth, selection of side bands, and/or CW.
Perhaps it is pertinent to mention SDRadio software by Alberto (I2PHD)which could be seen at www.weaksignals.com
 
Why do you listen to AM radio?
It picks up interference and its bandwidth is so low that it sounds awful.
 
audioguru said:
Why do you listen to AM radio?
It picks up interference and its bandwidth is so low that it sounds awful.
Click to expand...
We can very well stop listening to aired broadcasts and depend on Internet radio
Though many time you opined against AM Receivers, it is used across the world as it can cover loner distance unlike FM.
perhaps it is not that Space Varmint doent have FM receiver, so he was discussing issues of AM receptin.
let us discuss the means of improving on the problems faced like superhet tone .

today there are god receivers with DSP and synchronous detection , and i feel it is time that you may try to receive , decode and listen to DRM broadcasts (Digital Radio Mondiale)
I am sure you would appreciate the judicious use of OFDM technology right across Long, Medium and Shortwaves.
these signals are right on air across and covering your area even.
 
The beat tones you hear on AM radio are generally caused by other stations, typically distant, mixing with your intended station. Intermod can also be an additional cause when receiver is overloaded with lots of strong signals causing cross modulation. Short of minimizing receive signal strength to just what your local station needs, there are not much you can easily do to eliminate them.

Direct conversion can be used for AM but controlling the freq of L.O. and DC offset correction is complicated. Just about all cellphones these days are using direct conversion receivers but they have digital signal processors to handle the AFC on L.O. and demodulator algorythms to eliminate freq offset and D.C. offsets. They handle MSK FM (GSM), and multi-level QAM digital (CDMA) signalling. Many modern car radios are direct conversion.
 
In North America AM radio stations are 10kHz apart. Most AM radios have an output that is nearly nothing at 10kHz so there is poor intelligence and no 10kHz beat tone. In between the SNAPS, CRACKLES and POPS of interference picked up by an AM radio is other stations being heard at the same time.
FM radios ignore the noises and have hi-fi frequency response.
 
Sounds like I need to be looking into sychronis detection receivers. But no, Guru, some people like AM. I for one but I'm sort of an odd ball. I actually enjoy the static or I can enjoy just listening to a faint carrier using superhet, and observe the fading of the signal and sort of relate it to ionospheric conditions. Yeah, I do not believe that FM is is very reliable at HF frequencies.
 
I listen to AM weather and news sometimes. It is full of static and has no high audio frequencies that are normal to hear (unless you are deaf).
I listen to FM music almost all the time, sometimes CDs and MP3s.
One FM station I like is about 60 miles away and there are mountains in between sometimes. But my home hi-fi tuner and car radio are very sensitive so the sound is perfect (wideband and no static) nearly all the time.
My cheap Sony Walkman FM radio and my cheap clock radio have difficulty picking up the FM radio station that is 60 miles away. The extremely cheap "radio" from The Dollar Store (using a TDA7088 IC) does not pick it up.
 
Well there is another thing very important about AM on the HF bands. See, they are gradually shifting us over to an internet 2. Many fine fine programs that are on the internet audio streams will no longer be available I'm sure. Plus you know I'm a ham operator so of course I love HF.
 
I agree that the whole issue is not related end user .
we get satisfaction of effective Dxing.
i definitely suggest to create a small interface card for dual conversion
in US you get the type of crystals and filters yo meet you need for dual conversion, assuming it would solve the issues with image signals.
i forgot to inform, in case the BFO is ON in your receiver, i am sure you get the tone. we may better power it off when we dont need.
later you can resort to Synchronous detection. please see my pm
 

OK I will see what you mean. Thank you.

*edit

WOW! Very impressive Sarma! Very very nice work. Not something one could read and understand in an afternoon for sure...LOL. I must read this over though. I am sure it will be very insightful. Also might I say it is beautifully laid out the way you have it documented and so forth. Nice work!
 
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