Every now and again on forums such as this there is a request for adjacent channel working. Usually because one set has been acquired that is only capable of receiving on the adjacent channel to that used by the rest of the collection.
Unfortunately, all of the modulators usually available are Double Sideband so that, with the exception of channels 1 & 2, adjacent channel operation is out.
I don't have a fully worked solution but an idea that someone interested in the subject might like to develop into a working model.
Some time ago I stumbled on the response curve of a SAW capable of filtering out a single 8MHz channel and presumably intended for use in digital set top boxes. It wasn't what I was looking for at the time but saved a copy to study later. (I also found the curves for a switchable 7MHz/8MHz SAW, presumably for continental VHF/UHF transmissions).
This is what I found:
It occurred to me that, if a vision carrier were to be placed at exactly the right place in the passband, an IF signal with the lower sideband suppressed could be generated. After up-conversion to the required channel - basically the reverse of what happens in a TV tuner unit - the IF signal will undergo frequency inversion which will place the suppressed sideband above the vision carrier, which is exactly what is required.
Here's the first attempt:
Residual sideband suppression in the adjacent channel which, in signal terms, is the upper adjacent channel, is >40dB down at the adjacent sound carrier frequency and +>45dB down at the vision carrier. The unwanted image of the sound carrier is down by a similar amount.
There is. of course, no attenuation of products which fall in the majority of the vision passband of the lower adjacent channel, again in signal frequency terms, so the figures here will depend on how well the vision modulation signal is filtered above 3MHz.
I've placed the limit of the Lower sideband at the 3db down point but it doesn't reach the -30dB point until 1.75MHz below vision carrier. Is this realistic? What is the practical response of a VSB filter expected to be - it obviously can't be the text book square response!
I've played around with the Vision Carrier frequency to see the effect on the VSB of -250kHz increments - see what you think.
I've added a poll so that you can express your opinion.
Unfortunately, all of the modulators usually available are Double Sideband so that, with the exception of channels 1 & 2, adjacent channel operation is out.
I don't have a fully worked solution but an idea that someone interested in the subject might like to develop into a working model.
Some time ago I stumbled on the response curve of a SAW capable of filtering out a single 8MHz channel and presumably intended for use in digital set top boxes. It wasn't what I was looking for at the time but saved a copy to study later. (I also found the curves for a switchable 7MHz/8MHz SAW, presumably for continental VHF/UHF transmissions).
This is what I found:
It occurred to me that, if a vision carrier were to be placed at exactly the right place in the passband, an IF signal with the lower sideband suppressed could be generated. After up-conversion to the required channel - basically the reverse of what happens in a TV tuner unit - the IF signal will undergo frequency inversion which will place the suppressed sideband above the vision carrier, which is exactly what is required.
Here's the first attempt:
Residual sideband suppression in the adjacent channel which, in signal terms, is the upper adjacent channel, is >40dB down at the adjacent sound carrier frequency and +>45dB down at the vision carrier. The unwanted image of the sound carrier is down by a similar amount.
There is. of course, no attenuation of products which fall in the majority of the vision passband of the lower adjacent channel, again in signal frequency terms, so the figures here will depend on how well the vision modulation signal is filtered above 3MHz.
I've placed the limit of the Lower sideband at the 3db down point but it doesn't reach the -30dB point until 1.75MHz below vision carrier. Is this realistic? What is the practical response of a VSB filter expected to be - it obviously can't be the text book square response!
I've played around with the Vision Carrier frequency to see the effect on the VSB of -250kHz increments - see what you think.
I've added a poll so that you can express your opinion.