Mini Construction Project: CRYSTAL-LOCKED 5MHZ RECEIVER FOR VNG

Here's the design for a low cost dedicated receiver for the 5MHz transmissions of Australian Time and Frequency station VNG. It is based on the direct-conversion principle, which allows the local os­ cillator to be used as a secondary frequency transfer standard. by DREW DIAMOND, VK3XU Radio station VNG at Llandilo, NSW, ble, it will continue to supply our plifier is rather prone to pickup of its broadcasts on 2.5, 5.0, 8.638, 12.984 and 5.000MHz local reference. own LO, which may cause blocking of 16.000MHz to provide a valuable time the RF amplifier. and frequency standard service., These The circuit In this application the oscillator is a signals may be tuned-in on an ordinary The circuit is based closely on the discrete shielded assembly with its shortwave or HF receiver. DC91 80-metre receiver of a few years own set of components. The variable To check and adjust the time-base ago (Ref.I). That set uses an MFE131 capacitor in series with the crystal is (TB) of an electronic counter for in­ dual-gate FET for the RF amplifier. adjusted so that oscillation is obtained stance, we could tune to the 5.000MHz However, these devices have become on exactly 5.000MHz. signal (assuming good propagation for scarce, so the popular BF981 has been The input signal is 'directly converted' the time of day) then couple a signal substituted here. About 15dB of gain is to audio frequency, which appears at from the counter into the receiver. If provided. The signal is then applied to pins 4 and 5 of the '602 product detector. the counter TB is harmonically related the input port of the NE602AN balanced A ubiquitous '741 op-amp and '386 to 5MHz then a 'beat' should be ob­ mixer chip, and LO at 5MHz is injected power amplifier raise the detected signal served on the receiver. The counter may into the oscillator port at pin 6. to loudspeaker level. Low frequency now be adjusted for 'zero-beat' with the Normally, for a simple receiver, we audio response is such that the burble standard signal. would use the internal transistor pro­ near zero beat may be heard through the This method will give pretty good ac­ vided in the '602 for the LO. However, speaker, and sub-audible frequencies ob­ curacy, to within a few hertz - which is because the LO and input frequency are served as a 'winking' of the LED con­ probably sufficient for most ordinary the same, a DC receiver with an RF am- nected at the output. When the LO is at practical purposes. My term 'pretty good accuracy' refers to the method just de­ scribed, not that of the VNG carrier fre­ qu9ncy, which is specified as one part in 10 as received. Here are details of a simple dedicated 5MHz receiver designed to receive the VNG signals, which potentially offers significantly improved frequency accu­ racy over ordinary methods. Being a di­ rect conversion job, the local crystal oscillator (LO) runs on exactly 5MHz, so the LO itself may also be used as the base for a workable secondary frequency transfer standard. When the signal is suf­ fi c i en tl y strong, time and other an­ nouncements will be heard. The LO is adjustable to within one or two hertz of 5.000MHz using a LED indicator connected at the audio out­ put, which will vary in intensity if not at zero .beat. Should the station fade out and be­ come inaudible, provided that the crystal As you can see from this photo inside the author's receiver, he built it using has been maintaining zero beat and the single sided unetched PCB, with small pieces used to mount each IC. The ambient temperature remains fairly sta- oscillator is built In a shield box made from unetched PCB laminate. 48 ELECTRONICS Australia, July 1995