Ever since the long-waves broadcast station Kalundborg changed its frequency from 245 kHz to 243 kHz to comply with the CCIR’s recommendations for a 9 kHz raster in the LW and MW bands, it has become possible to use the carrier in the locked frequency standard described in Ref. 1. Kalundborg is a 300 kW long-wave transmitter in Denmark, with a range of about 500 km (300 miles). This article is, therefore, of particular interest to our Scandinavian readers.
Briefly, what is proposed here and in the next item (`preamplifier for Kalundborg frequency reference’) is to change the divider in the frequency reference such that a carrier input frequency of 243 kHz can be used instead of 77.5 kHz (the transmit frequency of DCF77 in Germany), for which the circuit was originally designed. The changes are outlined in the simplified block diagram, Fig. 1. which is best compared with Fig. 1 in Ref. 1. In practice, the original circuit of the reference is changed to the extent that a new circuit diagram is required—see Fig. 3. The new circuit is much simpler than the original, mainly because some circuit sections could be omitted, including the VLF preamplifier (T1 -T4 in the original design), the 10 MHz ‘locked only’ output (IC7 and T12 in the original design) and the ‘error’ detector (N2, N3, N4 and the beeper in the original design).
In the ‘Kalundborg’ circuit shown here, the 10-MHz signal supplied by X1 and T3 is multiplied by 3 by parallel tuned circuit L4-C39. The 30-MHz signal is subsequently divided by 100 (IC5) and again by 100 (IC6) to obtain the 3 kHz reference for multiplier IC3. The 240 kHz signal used to heterodyne with the 243 kHz carrier is obtained by dividing the 30 MHz signal by 25 (IC5) and then by 5 (74LS90).
The antenna, formed by an inductor wound on a ferrite rod and resonated by capacitors C1, C2, and C3, is connected directly to the balanced inputs of the SO42P mixer (IC1). The error signal at the output of the multiplier (IC3) is filtered and converted into a tuning voltage, which is applied to a dual varicap, D5. The varicap is capable of detuning (to a small extent) the 10 MHz quartz oscillator, and so closes the phase-locked loop (PLL). Provided Kalundborg is received with adequate strength (rotate the ferrite rod), the LED at the output of IC8 lights, and a ‘rock-steady 10 MHz reference signal is available at the output of N2.
Figure 2 shows a detailed diagram of the x3 multiplier. Inductor L4 consists of 4.5 turns of 0.5 mm dia. an enameled copper wire on a former with a ferrite core. The ‘ former has an outside diameter of 6 mm, and L4 is drawn out to a length of about 8 mm. The operation of the multiplier is easily checked with the aid of an oscilloscope and a frequency meter, the signal levels being quite high. The core in L4 is adjusted for the highest 30 MHz level at pin 5 of N4.
- “DCF77 receiver and locked frequency standard”, Elektor Electronics January 1988.