FM television sound DF generated with a PLL - UK Vintage Radio Repair and Restoration Discussion Forum

oldeurope · 13th May 2006, 6:58 pm

Good evening,

I made some thaughts about FM TV sound because I am not satisfied with my variable reactance modulator. Its linearity is better than a varicap modulator but not perfect. It has a frequency drift that is not acceptable, if the thing is used to agline a receiver.
A perfect linearity between deviation and sound signal is only given by using a RC VCO. The best frequency stability by a XTAL controlled VCO.
I did not find anything to guide me building a FM-PLL. Do you know where to find something? All I found was about narrow band FM and VXO devices that are multiplied to the output frequency.
There are two reasons for me to make a quasi parallel sound system.
First the TV set is an intercarrier type thus the phase noise of the vision carrier is compensated.
Second the output channel must be switchable between the OIRT channels R1 and R2.
I decided to use the 74HCT4046 to generate the 6,5MHz sound difference frequency.
The idea is to FM- modulate its VCO directly with the sound signal at pin 9 (VCO in).
By dividing the frequency modulation as much as possible (I used 25KHz) FM is changed to PM at the low frequency. This makes sure that the PLL does not lock out.
I don’t know how it is done in IC modulator applications. Is it done in the same way or is there a better way to gererate FM with a PLL?

Kind regards,
Darius

Attached the schematic of the FM PLL some pics how it looks and the schematic of the old reactance modulator

13th May 2006, 6:58 pm	#1
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	FM television sound DF generated with a PLL Good evening, I made some thaughts about FM TV sound because I am not satisfied with my variable reactance modulator. Its linearity is better than a varicap modulator but not perfect. It has a frequency drift that is not acceptable, if the thing is used to agline a receiver. A perfect linearity between deviation and sound signal is only given by using a RC VCO. The best frequency stability by a XTAL controlled VCO. I did not find anything to guide me building a FM-PLL. Do you know where to find something? All I found was about narrow band FM and VXO devices that are multiplied to the output frequency. There are two reasons for me to make a quasi parallel sound system. First the TV set is an intercarrier type thus the phase noise of the vision carrier is compensated. Second the output channel must be switchable between the OIRT channels R1 and R2. I decided to use the 74HCT4046 to generate the 6,5MHz sound difference frequency. The idea is to FM- modulate its VCO directly with the sound signal at pin 9 (VCO in). By dividing the frequency modulation as much as possible (I used 25KHz) FM is changed to PM at the low frequency. This makes sure that the PLL does not lock out. I don’t know how it is done in IC modulator applications. Is it done in the same way or is there a better way to gererate FM with a PLL? Kind regards, Darius Attached the schematic of the FM PLL some pics how it looks and the schematic of the old reactance modulator Attached Thumbnails Last edited by oldeurope; 13th May 2006 at 7:05 pm.