Mini transceiver image rejection question

[neazoi]

Hi, I have found this mini CW transceiver.
The author states that:

"Because the D-C receiver receives both sidebands equally well, there are twice as many signals as you’d expect of a more capable receiver"

Well this is the case with any DC receiver, but in his circuit a crystal is used as a narrow RF band pass filter. So I do not understand why is the image frequency not rejected in his design?

 

[vfone]

I think the statement in the pdf is wrong.
The title of the article mentioned that the receiver is an SSB receiver.
A direct conversion receiver will receive both sidebands ONLY IF both side bands are transmitted (AM case).
But this is not the case of SSB, when only one of the side bands is transmitted. So, even if you want to receive the unwanted sideband you can't, because is not transmitted.

On the other hand, a direct conversion receiver doesn't have an image frequency, because the IF is actually the audio frequency.
The input crystal Y1 is used not for image rejection, but for channel selectivity.

 

[biff44]

ham guys just make me shake my head. if you want to build a image rejection receiver, why not just by a cheap image reject mixer from mini-circuits or some semiconductor house, instead of trying to kluge up some abortion?

 

[neazoi]

I think the statement in the pdf is wrong.
The title of the article mentioned that the receiver is an SSB receiver.
A direct conversion receiver will receive both sidebands ONLY IF both side bands are transmitted (AM case).
But this is not the case of SSB, when only one of the side bands is transmitted. So, even if you want to receive the unwanted sideband you can't, because is not transmitted.

On the other hand, a direct conversion receiver doesn't have an image frequency, because the IF is actually the audio frequency.
The input crystal Y1 is used not for image rejection, but for channel selectivity.

Where did you find the SSB note? The article title is "A SIMPLE SINGLE-BAND CW TRANSCEIVER".
Saying so, in a DC SSB or CW receiver the problem is not the other station that transmits SSB, but the other stations that transmit simultaneously in a crowded band. So another unwanted station may appear in one sideband while you listen to the other (you actually listen to both simultaneously.

A DC receiver (even if it is CW) DOES have an image frequency and the image frequency appears in audio range. In the RF, you just listen to both sidebands close to the carrier.

Now about the specific transceiver, according to this article **broken link removed** in the last filter, the single crystal should give very sharp response and this is could be used as a very selective front end filter that will enable to reveive say 1KHz (maybe much less). This filter should be according tot he page, so selective that will reject the image in the RF front end (single channel narrow reception). So why the author states that it just improves a bit selectivity is questionable to me.

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ham guys just make me shake my head. if you want to build a image rejection receiver, why not just by a cheap image reject mixer from mini-circuits or some semiconductor house, instead of trying to kluge up some abortion?

Some do, but we are here to educate others don't we? To understand how things work and build them yourself has nothing to do with buying a building block (chip) that does the job for you. As you see the whole circuit is very simple, such circuits try to get more from the time and effort (and cost) you spent on them. The single crystal front end idea, if it works to reject image, solves many of the image problems of such simple receivers (although to change channel, another crystal set is required)

 

[G4BCH]

The filter is not sharp enough to reject the image, it is only there to reduce the effects of srong signals on the mixer. See top left of page 4.

 

[neazoi]

The filter is not sharp enough to reject the image, it is only there to reduce the effects of srong signals on the mixer. See top left of page 4.

According to the article I posted, it should be very sharp indeed. It also presents the response of the filter, which is not even suitable for SSB. So one of the two articles should be obviously wrong...