Video Transmitter Spectrum

[adnan012]

hi,

I am designing a video transmitter using MC1374 modulator ic. The output spectrum contain sidebands with carrier. Is there any chip available to generate only single side band with suppressed carrier? Is it possible to use FM modulation for video instead of AM modulation. My receiver Dany USB Tv tuner. It receives video at 55MHz .

 

[BigBoss]

There are 3 ways to obtain Single Side Band spectrum..

1-Using a sharp filter around the main carrier ( for VSB )
2-Modulate CVBS signal+Sound at 38.9MHz ( main carrier) and then passing this low frequency modulated ( 38.9-33.4MHz for European standard ) signal through a proper SAW filter and upconvert to GHz region and then filtering wanted ( because there will be 2 SSB signal far away and a LO signal ) one and downconvert to wanted carrier signal.( I have applied this technique years ago and it's really difficult but very efficient )
3-Discriminate CVBS signal+Sound into I and Q components ( there are few analog and digital techniques ) and use I/Q modulation to obtain SSB sepctrum.( Nowadays, it's very straightforward with digital techniques)

 

[BigBoss]

A simplified SSB system and its far and near spectrum.I presumed that the DSB modulated around 38.9MHz signal is already filtered by SAW filter and wanted carrier is at 21st channel of UHF regarding to European CCIR standard.So, Fv=471.25MHz, Fs=476.75MHz.

 

[D.A.(Tony)Stewart]

LM1889n also supports reduced carrier modulation where slight overmodulation inverts the carrier phase.

- while the TV receiver suppresses the LSB there is no need to filter the transmitter, because for low power, we don't care about wasting 50%.

if you wanted to make a simple SSB filter for Channel 2 or 3 or below for Ch 4 , it might look like this.

- note that this has two bandpass to spread the bandwidth with deep skirts and one lower SB is a notch filter. But in reality for local use filtering when no high power broadcast is using this channel, EMI is no issue, but be aware.

 

[adnan012]

Thanks for reply.

I never have success in designing band pass filter even at low frequencies like 50MHz.
For filters i use SMD 0805 package inductor

https://www.digikey.com/product-detail/en/744765118A/732-1729-2-ND/1994120

 

[FvM]

You should be able to make acceptable filters with these wirewound inductors. I used similar types for UHF LC filters. I presume that you are putting realistic component parameters into the filter calculation, particularly inductor Q respectively ESR.

Referring to another question, I'm not aware of video transmission using FM. Your tuner won't be able to receive it.

Simple video modulators, e.g. built-in to video recorders have neither single side band nor supressed carrier.

 

[betwixt]

55MHz seems low for a video transmitter, are you sure that's correct. It is more like the IF (if the receiver isn't completely digital) than a normal transmission frequency.

You can use FM video and it is widely used in point-to-point link applications but you would need to occupy typically 12MHz of bandwidth for good quality and that would be a problem at such low carrier frequency. Most FM video links run at > 1GHZ.

Brian.

 

[FvM]

55 MHz is VHF band I, channel 2 https://en.wikipedia.org/wiki/Television_channel_frequencies

You can use FM video and it is widely used in point-to-point link applications

Yes, but you need a different receiver.

 

[D.A.(Tony)Stewart]

Often no BPF is used for short link transmitters as the sidebands are too weak to cause interference. No need to suppress carrier just modulate to max. and transmit DSB.

 

[Borber]

If neighbor channel lower than your transmit channel (55.625MHz is 3rd channel according to PAL B) is free than you do not need VSB filter on transmitter. Modulation can be AM DSB but never SSB. Carrier is full present on transmission and receiver standard IF filter has at 38.9MHz 6dB attenuation. Not needed part of lower side band will not pass receiver IF filter.