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An Alternative 6 MHz Audio Sub-carrier

By Brian V Davies   GW4KAZ

 

For a number of years, I have built ATV transmitters for 23 & 3 cms, however I have always had problems in building the 6 MHz sound subcarrier circuits. This article conveys my thoughts and the utilization of my ideas, and is written in this vein.

 

“Eureka” Moment

In search of a suitable solution to this problem I had a “Eureka” moment and thought that the sound subcarrier systems which were used in UHF modulators in VCRs, satellite receivers, etc might be suitable. I decided to look at the possibility of utilising these modulators.

 

In my workshop I had some faulty Pace analogue satellite receivers. I noted that there were two types of UHF modulator units used. Fig. 1 shows one type that I refer to in this article.

Having removed the units from the receiver PCB, I removed the outer casing to gain access to the circuit within. The units employed an IC as a video and sound modulator. I was not interested in the video side, so concentrated on the sound circuit.

 

First what was required was to find the IC’s identification number, I had to remove the IC in one unit to find the number as the IC was an in-line configuration, so care was required in removing this IC and replacing it back onto the board. This was an LA7053. Fig. 2 shows a typical circuit diagram taken from the data sheet. I will concentrate on this IC.

(I have also used a CXA1433M); this IC has an onboard UHF oscillator, which simply needs to be disabled by removal of the oscillator components).

 

The LA7053 IC

The modulator unit has 2 sections, one contains the subcarrier circuit, the other the through antenna system. This second part can be removed, a simple job done by means of a junior hacksaw and cutting carefully along the internal screen on the antenna side of the unit, then cleaning along the edge of the PCB so that there are no shorts circuits on the PCB edge. (Fig. 3)

 

The LA7053 is an interesting and simple IC, as it has an onboard video test signal, plus some video processing. However, I have not taken advantage of these facilities at this time. Other’s ICs are similar and with careful identification of the input and output circuits from the data sheets, I am sure they can also be utilised.

 

The audio input (pin 4) and output (pin 3) on the IC can be identified, also the supply +ve (pin 7) and 0V (pin 5) on the PCB (Fig. 2).

Usually the sound subcarrier is mixed with the video on the output stages; therefore disconnection of this video circuit is required by removal of components to isolate this circuit.

 

Sub-carrier Amplifier

I found that the 6 MHz sound subcarrier output (pin 3) from the IC did not have enough drive to modulate the transmitter to the required level of -18 dbc.

To amplify this signal, I decided to use a MMIC, which was a simple answer to the problem. I inserted a 100ohm variable resistor in series with the output of the MMIC so that the amplitude of the subcarrier could be adjusted to the required level.

Different MMICs require suitable supply current; in my case I used a surplus MMIC from an old blue cap LNB marked as an a08 or an MSA0886.  I just cut the PCB of the blue-cap LNB around the MMIC added 10nF capacitors as input and output connections, plus a suitable dropper resistor to the required value. (Fig. 4 & 5)

Suitable circuit designs can be found in RSGB’s RadCom February 1998 and in the latest RSGB Radiocommunictaions Handbook. Fig. 6 shows the output from the IC before amplification.

 

Microphone Amplifier

I found that direct connection of the mic to the IC was not sufficient to drive the 6 MHz sub-carrier to the required levels.

To rectify this I built an audio amplifier utilising a 741 IC, this gave the audio gain required together with the facility to vary the audio levels as required. Suitable circuits were published in CQTV 194, Circuit Notebook by John GW3JGA.

The result of the completed project is shown on my spectrum analyser (Fig. 7), with excellent results and audio quality. Fig. 8 shows the completed 100mW 23 cms TX.

Please experiment with these ideas. As far as I am concerned, they were a simple answer to a problem. Good Luck!

 

Data sheets attached :

LA7053

CXA 1433M

MSA0884 (AO8 MIMIC)

 

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