Tuesday, May 14, 2013

A 10MHz TXCO master oscillator

The heart of any transverter system is the local oscillator. In practice, a transverter is as good as its local oscillator is. For my transverter project I choose a Toyocom TCO-291L2 oscillator I rescued from a UMTS board I have from surplus.

After spending a lot of evenings looking for its datasheet, I could not find it. I found the datasheet from other TCO-291 oscillators, but nothing from the TCO-291L2. Maybe it is a custom device, o maybe it is discontinued.

The first thing to do was to check the oscillator operation. I used the pinout from the TCO-291 oscillators I found, and fortunately it was the same. I applied 5 volts and the oscillator started to oscillate nicely. The frequency of oscillation was a little bit under 9999 kHz. Applying voltage to the control pin the frequency of oscillation can be tuned aprox. between 9999 and 10002 KHz, with 10000 kHz output with a voltage control around 1.75 volts.

TCO-291 series pinout. Bottom view.

The output signal was great, but when loaded with a 50 ohms load, it fades a lot, delivering less than -10dBm over the 50 ohms resistor. The output seems to be TTL or CMOS level, so a direct 50 ohms load is quite a lot for obtaining a useful level. The only solution I found is to amplify the signal, so I built this circuit:

The TXCO is supplied using a LM317LZ regulator. I choose a LM317 because they have much better thermal stability than the 78xx series. The voltage at the control pin is adjusted using a 10k multiturn variable resistor. With the aid of the 150k and 82k resistors the voltage at the control pin can be adjusted between 1.7 and 1.9 volts, so it can easily adjusted to get a 10000000 Hz output.

The LM317 regulator gets its voltage from a 7808 regulator. This double regulation must eliminate frequency variations because of supply voltage variations. These 8 volts are used to supply an ERA-2 MMIC, used to amplify the TCXO signal. The output from the ERA2 amplifier is around +10 dBm over a 50 ohms load, more than enough to drive any other circuit.

The oscillator. Inside view

The circuit is build on a small PCB and this one is inside a small aluminum box. The total current drawn is about 70mA. The oscillator needs around 20 minutes to get stable, or at least as stable as I can measure. Taking into account that 1 Hz drift at the TCXO will produce nearly 1 kHz drift at 10 GHz it seems the oscillator will suffice for SSB / CW, but I will not be sure until the transverter is finished and tested.

The oscillator. Outside view

1 comment:

  1. For me this is not L2 but D2 engraved on the top, and if so it determines the type of package.