Sideband rejection by single ceramic filter?

[neazoi]

Ok my idea goes like this:

A superhet receiver that does use a simple ceramic filter in the 455KHz IF, but instead of selecting the IF (plus USB, minus LSB), to select the IF+30KHz with a single ceramic filter. This will select 2 sidebands that are +30KHz and -30KHz away from 455KHz instead of close to it.
One of these sidebands (say LSB) can be easily filtered then by this single ceramic filter. After filtering and detection, this will leave you with a +30KHz (plus 3KHZ for the audio range) USB detected audio. Now what you want is to shift this 30KHz (plus 3KHZ for the audio range) audio down to 0-3KHz. At that point you re-introduce a sideband, but this is now at audio. And you can make a steep audio filter relatively easily.

The reverse procedure can be used for transmission.

How do you find my idea?

 

[FvM]

You are talking about simple superhet, not quadrature mixer? Then LSB and USB will always touch at 0 kHz signal frequency. To select a single side band by filter, you need a super-steep crystal filter. Shifting the IF doesn't change anything.

 

[neazoi]

You are talking about simple superhet, not quadrature mixer? Then LSB and USB will always touch at 0 kHz signal frequency. To select a single side band by filter, you need a super-steep crystal filter. Shifting the IF doesn't change anything.

Yes I am talking about simple superhet, not quadrature. It is better to explain my thought with an example.
Assuming a tone on 10.003MHz mixed with 9MHz LO.
sidebands output= 1.003 (USB) and 0.997 (LSB)

But, if I use a ceramic filter at 1.030MHz instead of a crystal filter at 1.003MHz then I will reject the LSB (0.997) as well as the USB (1.003) and instead of detected audio, I get a 30KHz second IF signal. The signal BW will be larger, due to the ceramic filter used, but small enough to reject the close to carrier sidebands produced in the previous step (both!).

Humans cannot hear to 30KHz, so next step is to downconvert this 30KHz into audible range.
With a LO of say 27KHz, the outputs will be 3KHz and 57KHz.
The 57KHz can now be easily removed with a 3KHz audio LPF.

I have not checked about inversion of the audio spectrum yet due to the mixing actions, but at any case re-inversion can be done easily on the audio domain.

 

[FvM]

Consider the spectrum of a DSB signal, could be also a SSB signal with the other sideband containing unwanted noise. (With thanks to Wikipedia)

By superhet mixing, you can shift the pattern up and down in frequency, but not change the distance of carrier and sidebands. Now you want to suppress a sideband by filter. You still have both sidebands close to each other, respectively need a steep filter.

 

[neazoi]

See my crude illustration. The first case is the case you mention. The second case it the one I mention.
Of course the whole sideband exists after mixing (down to the carrier frequency), but we select a higher frequency within the USB, which is 30KHz in this particular example.
The IF offset of the filter, 30KHz in this example, depends of how well a ceramic filter can cut off the lower products (carrier and LSB). The idea is the same otherwise.

 

[FvM]

Some points should have been clarified before. What's the signal you want to receive? If it's SSB, what kind of detector are you using, how do you regenerate the carrier?

If you receive a SSB signal, you want to suppress the other sideband because it contains noise respectivelly other radio stations. The signal in second picture doesn't actually exist. What you want to suppress is the range next to the signal of interest. Your filter doesn't achieve this.

 

[neazoi]

Some points should have been clarified before. What's the signal you want to receive? If it's SSB, what kind of detector are you using, how do you regenerate the carrier?

If you receive a SSB signal, you want to suppress the other sideband because it contains noise respectivelly other radio stations. The signal in second picture doesn't actually exist. What you want to suppress is the range next to the signal of interest. Your filter doesn't achieve this.

Indeed this method does not add selectivity. It just suppresses the sideband products of the mixer.
I have done something similar (not exactly as described here) in the FRG-7 receiver. I tuned the ceramic filter a bit offset to reject the opposite sideband. Of course, I could then hear hightr audio frequencies only out of the receiver.

 

[FvM]

You are still not clear about the actual input signal. If the input signal is pure SSB, there are no sideband products added by the mixer.

 

[neazoi]

You are still not clear about the actual input signal. If the input signal is pure SSB, there are no sideband products added by the mixer.

They are! You just receive noise in the other sideband.

 

[FvM]

The same noise that you get with your "offset" IF.