70 MHz FM TV Generator
Frequency Modulated television offers a better way of transmitting sound and vision in the SHF bands. Linearity difficuties are avoided, so that full class-C ratings can be obtained from the transmitter output stages.
Several transmitter types are possible:
- Oscillator-varactor tripler (see Figure 1a).
- Oscillator-amplifier (Figure 1b).
- Oscillator-transverter (Figure 1c).
The last method is attractive if a transverter is already available, and our FM-TV generator is based on this concept. The transverter IF could be changed to suit the FM TV generator output frequency by altering the local oscillator frequency. Alternatively, for transverters with a 2 m IF, the 70 MHz generator output can be doubled to 140 MHz. However, this may cause intolerable QRM to your 2 m reception.
The FM TV generator uses an NE564 PLL demodulator IC in an unconventional mode. Only the VCO section is used; the RF input and demodulator output are ignored. The trimmer connected to pins 12 and 13 controls the VCO centre frequency. This can range between 30 and 70 MHz, which allows a fair degree of flexibility for transverter IF.
Pins 4 and 5 are the modulation inputs, with pin 5 being the noninverting input (increasing the voltage on this pin increases the VCO frequency), and pin 4 being the inverting input. If your transverter inverts the the sense of sidebands, ie, the LO frequency is greater than the output frequency, the video modulation should be injected into pin 4 instead of pin 5. The final relationship between the video signal and the transverter output should be positive or noninverting. The New Zealand standard is positive modulation, although an FM TV receiver should be switchable between positive and negative modulation for maximum flexibility, eg, when used with a Gunnplexer.
Because there are two modulation inputs to the NE564, the one not used for the video signal can be used to inject the FM sound subcarrier, without the need for a trap filter. The sound signal is passed through a CCIR pre-emphasis network before reaching the modulator. This gives a noise advantage analogous to that obtained when pre-emphasis is used for FM audio transmission.
The circuit used with the Wellington VHF Group's 70 cm VSB transmitter (Break-In, March 1985) could be used as the FM sound subcarrier generator.
From time to time faulty NE564 ICs may be encountered, where the demodulator output has failed (they seem susceptable to this). Such ICs are useless for reception, but are usable in this generator circuit.
ZL1ABS, Mike Sheffield and ZL1TBG, Ralph Sanson.