Double mixer frequency up-conversion

Dear freinds

In some TV modulators the Modulated IF signal (38.9 MHZ) is first up converted over the UHF band (by mixing with a 915 MHz local) and after some filtering it is down converted to the desired UHF channel (in the 470 MHz to 860 MHz range) by passing the second mixed signal through a filter. I would like to know what is the benefits of this method in comparison with the single mixer up converters.

Thanks for the answers.

When choosing the intermediate frequency for a superheterodyne radio receiver there is a trade-off to be made between the advantages of using a low frequency IF or a high frequency one:

High frequency IF: The use of a high frequency IF means that the difference between the wanted frequency and the unwanted image is much greater and it is easier to achieve high levels of performance because the front end filtering is able to provide high levels of rejection.
Low frequency IF: The advantage of choosing a lower frequency IF is that the filters that provide the adjacent channel rejection are lower in frequency. The use of a low frequency IF enables the performance to be high, while keeping the cost low.

Accordingly there are two conflicting requirements which cannot be easily satisfied using a single intermediate frequency. The solution is to use a double conversion superheterodyne topology to provide a means of satisfying both requirements

the most important reason is the fact that the TV signal is VSB, the lower sideband is suppressed below Fv-1.25 MHz by at least 40 dB. So the fiddling about could be to stop the original lower sideband (missing) becoming the upper side band on transmission, i.e. the one that goes out and having image side bands that are in-band.
Frank

Thanks for your answer. But what do u mean with "to stop the original lower sideband (missing) becoming the upper side band on transmission". I really appreciate it if you explain more.

---------- Post added at 13:54 ---------- Previous post was at 13:52 ----------


Thanks but please know that this technique is beeing used in the transmitter side.

It can be used for the receiver side to....
It is the Super heterodyne Double conversion receiver....

Analogue TV Modulators create DSB AM signals and then this signal is filtered by a SAW filter to obtain a VSB signal.TVIF SAW filters have very sharp bandpas filters...
Then if you convert this signal over 1GHz, filtering will be easier at that frequency band to sperate lower and higher sidebands and after that wanted sideband is transferred onto
desired TV channel while other sideband is filtered easily...