Quattro Oscillator

The heart of every receiver and amplifier is the oscillator. To receive a stable frequency you must use the VXO variable crystal oscillator where the crystal is connected in sequence with the frequency condenser or tuning circuit. For example, a crystal with 7MHz can be tuned in or out by several kHz.

Most tuning can be done by connecting a Super VXO or also the VXO Excellent, developed by Japanese radio amateurs JAOAS and JH1FCZ(1). A double parallel connection of the crystal with the same frequency is sequentially tuned using a tuning circuit. At 7MHz it is possible to mistune it by several kHz.

We should do the connection according to DJ1ZB with a sequential tuning circuit in combination with several inductors which allow for greater tuning even when using a simple crystal. At 7 MHz it is about 20 kHz (2,3).

The oscillator can also be connected with two different crystals where one oscillates at a fixed frequency and the other can be adjusted to a different frequency. The transistor oscillates on both frequencies at the same time and also functions as a mixer. The frequency difference is taken from the lower frequency or the band filter. This connection was introduced by G3ESP and is called their Wilde Range VXO (4).

Some of the crystal oscillators.

At the AMAVET Q-Club in Pribram, during the development of the QRP complex, specific experiments were undertaken with the crystal oscillator. The result of the four crystal oscillator, “Quattro-oscillator” (QO) has now been made public for radio amateurs. The Quattro-oscillator makes it possible to select from a wide tuning band and uses a small voltage. The most considerable quality is the constant tuning of the receiver and broadcast frequency (RIT).

Schematic of the Quattro-oscillator.

The connection includes four large crystals with large electrodes which are arranged in two double-crystal configurations. The double-crystals are two crystals with the same frequency, connected parallel and placed in one case. The frequency of the two double-crystals have a different RIT value.

Parallel double-crystal with the same frequency
before being placed in the shared case.

The double-crystals are switched over by symmetrical single-pole relay coils ending in a double varicap. The relay coils are set according to their value, where the smallest value is on the side of the crystal and the largest on the side of the varicap. The coils are connected in such a way that their magnetic poles are the same. The induction value and the mutual position of the coils is set according to the size of the tuning range. The delicately set constant RIT in the entire tuning range is helped by the capacitor trimmer which is between the case of one double-crystal and the ground. Hand-tuning of the RIT is achieved by using a helical potentiometer with a small axial shift which controls the micro switch. For perfect separation of the oscillator from other levels you can use an optocoupler component.

Quattro-oscillator with an opto-coupler buffer.

The Quattro-oscillator was designed for the 7MHz band, but it can also be used for other frequencies. It can be tuned to 350kHz. For the QRP, the unit was tuned from 7,000 to 7045 kHz, which includes the start of the band with remote control operation, and also European QRP frequency at 7030 kHz and American QRP frequency of 7040 kHz. The tuning voltage is in a range from 2 to 9 V.

Internal connection of the symmetrical tongue relay,
used for switching-over the double-crystal.

Internal connection of the opto-electric switch,
used as a buffer.

A Quattro-oscillator on a PCB.
Coils are experimentally slid into the contact space from the FRB connector. The testing board on the right serves as a storage for the coils. In the background is the QRP set OK1DPX in the making.

The PCB in detail.
Two double-crystals, tongue relay and trimmer for constant RIT. Left are coils, lower the varicaps. The marking 2-7M040000 on a x-tal case means double-crystal 7,040 MHz.

MicroPot, combination of tuning helical potentiometer and micro switch.
The potentiometer has a small axial shift.

The output peak-to-peak voltage of the Quattro oscillator is 4.5 V.

Materials Used:
[1] 7NWVM: Super VXO, http://www.qsl.net/7n3wvm/supervxo.html
[2] DJ1ZB: VXO with large mistuning, OK QRP Info 26/1996, p. 14
[3] OK1DPX: CW QRPP TX, http://www.quido.cz/qrp/tx/txc.htm
[4] G3ESP: Wide-band VXO, OK QRP Info 37/2000, p. 12