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Old 1st Mar 2019, 12:42 am   #1
Semir_DE
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Join Date: Jan 2019
Location: Heusenstamm, (Near Frankfurt am Main), Germany.
Posts: 59
Default Simple System A Band I Modulator - (No Processors)

Dear All,

since I am now the proud owner of a 405 line TV set, a Sony TV9-90UB I decided to build a basic System A modulator that requires no processors or programming. The design uses a simple crystal as a frequency reference and the well known SA/NE612 mixer chip as a modulator. The design consist of the following blocks:

- Sync tip clamp for DC restoration.
- Vision carrier generator and modulator SA612
- Intermediate mixer to generate the audio carrier, this uses another SA612
- Sound modulator with a third SA612
- Low pass filter to eliminate 3rd and higher harmonics from output signal

The design requires only one crystal for the vision carrier. Luckily I have found a supplier that offers 45MHz crystals at €2.50 each, so the design is a channel B1 modulator. Since it is near impossible to find a 41,5MHz crystal for the audio carrier I have decided to generate the audio carrier by mixing the vision carrier with a 3.5MHz signal generated by an LC circuit. The original idea was to use a 3.5MHz crystal but so far I have not been able to source one. Alternatively a ceramic resonator could be used, but I have found using an LC circuit to generate the 3.5MHz offset to be stable enough.

Here is a brief description of the circuit, please refer to the attachments. Transistors Q1 and Q2 form the sync tip clamp together with D1. Residual carrier level can be adjusted by RV1.

IC U2 is the vision carrier oscillator and modulator Fi1 is needed to ensure the crystal Y1 oscillates at its 3rd harmonic and not its base frequency, it should be adjusted for maximum carrier level. Since the SA612 is very sensitive the input video signal level at pin #1 needs to be very small in the order of 100mV this is difficult to achieve by just decreasing the value of R13 as the DC component of the signal would become too low for U2 to operate properly. I overcame this problem by feeding the modulating video signal to both inputs at differing levels via R14. This common mode component reduces the input sensitivity while maintaining a proper DC level at these pins. This trick also allows me to add C11 and C14. In order to achieve a good clean modulation the input pins #1 and #2 need to have a low impedance with respect to ground at the RF carrier frequency. Due to the relatively high frequency components of the video signal present at these pins a simple capacitor of e.g. 1nF to ground can not be used in this position as it would impair the video frequency response. In this circuit C14 needs to be adjusted for flat video frequency response this is achieved when it is set to about 45pF. A 45pF capacitor connected to ground at 45MHz has an impedance of roughly 80Ω, enough to sufficiently attenuate the carrier signal. Without this measure there would be a higher level of the 3rd harmonic in the output of the modulator.

The output signal is transformed to an impedance of around 75Ω by RF transformer T2. This is a simple dual-aperture core with only a few turns as indicated in the diagram.

The audio carrier is generated by mixing the 45MHz vision carrier with a 3.5MHz signal generated by U3. Originally I had planned to use a 3.5MHz crystal but found that an LC circuit works very well in this position. The output at pin #5 of U3 contains both the sum and difference frequencies of 45MHz and 3.5MHz i.e. the desired 41.5 MHz and an unwanted 48.5MHz component. Resonant circuit Fi2/C15 is tuned to the desired 41.5 MHz and delivers the audio carrier to U1 via C12 and RV2. Fi2 needs to be adjusted for maximum level of the 41.5 MHz carrier. RV2 should be set for the desired 4:1 vision/audio signal level ratio.

The audio modulation is done with U1 yet another SA612. R9 unbalances this modulator to achieve a regular AM modulation with carrier. Without this resistor the output would be a dual side band (DSB) signal and would sound very distorted on a TV with normal envelope demodulation. In the audio section RF transformer T1 performs the impedance transformation to 75Ω for the modulated audio signal.


The resistor matrix consisting of R15/16/17 joins the modulated vision and audio signals into a single combined signal. This is then passed through a 5th order 75Mhz low pass filter consisting of L2/3 and C25-27. The output of the circuit is about 90dBµV and can thus feed several TVs via a passive distributor.

This circuit has worked quite well with my TV9-90UB. In the attached image you can see the prototype and the TV displaying a cross hatch from my generator while “Radio Jackie” from London is playing as an audio feed

I am planning to build a Band I modulator using MC145106 PLL chips based on this design. This modulator will cover channels B1-B5. Once I get it finished I will post the design here.

I am aware that there are dedicated modulator chips like the Motorola MC44… one used in the Hedghog and Aurora. My intention was, however, to build a project that does not require a processor or programming skills and is easy to build for someone with basic RF skills. I am planning to design a PCB for this project and also the upcoming PLL based version…

Semir
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Name:	System A modulator block diagram.jpg
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Name:	Prototype.jpg
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Name:	TV9-90UB-w-Modulator.jpg
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Attached Files
File Type: pdf Sys-A_Modulator.pdf (58.9 KB, 257 views)
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