23-10-2016, 02:43 PM
Here's an idea I had a while ago but never got round to writing up.
Occasionally there is a need to operate modulators on two adjacent channels but, unless those channels are B1 and B2, it is out of the question because of the DSB nature of commonly available sources, whether they be separate modulators or an Aurora.
In practice, only the signal on the lower of the two channels needs to be VSB although, if my idea works, there is no reason why all 13 VSB channels couldn't be generated simultaneously!
The problem, of course, is the need to remove most of the upper sideband and any image of the sound carrier. A SAW filter would be nice but I doubt that anybody ever produced one for system A!
That doesn't mean that a SAW filter cannot be used for the purpose, though.
The idea is to adjust the parameters of a DSB IF modulator such that an 8MHz SAW DTT IF filter can be used to remove the unwanted sideband. In the case of the IF, of course, this will be the lower one.
I came across a datasheet for a suitable SAW 2 or 3 years ago but, although I spent several hours yesterday, I can't find out where I got it from!
To start, you will need the IF SAW filter from an old DTT box - it is a 5-pin in-line device 14mm x 2.5mm, so should be easy to spot.
Here is the response with the System A IF overlaid:
I experimented with the carrier frequency and hence the vestigial sideband position slightly. Initially I set the -750kHz point to be 3dB down but was concerned that the adjacent sound carrier might coincide with the sharp cut of position of the filter, resulting in an indeterminate amount of rejection. I have now made -500kHz the -3dB point which means that -750kHz is ~ 7 to 8 dB down on peak carrier. Unwanted signal falling on the adjacent sound carrier frequency is now guaranteed to be better than 40dB down and should drop neatly into the -50dB notch!
Does anybody know roll off data for a broadcast VSB filter for comparison?
Having generated the VSB IF signal, all that is now needed is to upconvert it to the required VHF channel.
Anybody fancy taking this on?
Occasionally there is a need to operate modulators on two adjacent channels but, unless those channels are B1 and B2, it is out of the question because of the DSB nature of commonly available sources, whether they be separate modulators or an Aurora.
In practice, only the signal on the lower of the two channels needs to be VSB although, if my idea works, there is no reason why all 13 VSB channels couldn't be generated simultaneously!
The problem, of course, is the need to remove most of the upper sideband and any image of the sound carrier. A SAW filter would be nice but I doubt that anybody ever produced one for system A!
That doesn't mean that a SAW filter cannot be used for the purpose, though.
The idea is to adjust the parameters of a DSB IF modulator such that an 8MHz SAW DTT IF filter can be used to remove the unwanted sideband. In the case of the IF, of course, this will be the lower one.
I came across a datasheet for a suitable SAW 2 or 3 years ago but, although I spent several hours yesterday, I can't find out where I got it from!
To start, you will need the IF SAW filter from an old DTT box - it is a 5-pin in-line device 14mm x 2.5mm, so should be easy to spot.
Here is the response with the System A IF overlaid:
I experimented with the carrier frequency and hence the vestigial sideband position slightly. Initially I set the -750kHz point to be 3dB down but was concerned that the adjacent sound carrier might coincide with the sharp cut of position of the filter, resulting in an indeterminate amount of rejection. I have now made -500kHz the -3dB point which means that -750kHz is ~ 7 to 8 dB down on peak carrier. Unwanted signal falling on the adjacent sound carrier frequency is now guaranteed to be better than 40dB down and should drop neatly into the -50dB notch!
Does anybody know roll off data for a broadcast VSB filter for comparison?
Having generated the VSB IF signal, all that is now needed is to upconvert it to the required VHF channel.
Anybody fancy taking this on?
