Simple WWV receiver for 10 MHz calibration signal

"Zero beating" to the WWV carrier for calibration of oscillators is easier said then done. To truely "zero beat", you must do exactly that, produce a 0 Hz beat note. This is difficult to do, since most receivers do not have an audio responce down to zero Hz! The next best thing to do is to misstune the receive by say 1000 Hz and match your oscillator to that tone. However, you must be very careful with the signal levels from your oscillator into the receiver as to not overload it and to be able to hear both your oscillator and WWV at the same time.

A better approch is to build a simple Direct Conversion receiver with a tunable crystal oscillator for the LO, then look at the tones sent by WWV on a PC running a PSK program such as DigiPan. Now you can adjust the 10 MHz oscillator so that the tones are at the right frequency as measured on the frequency ruler used in the PSK program. Once this is done, you have about as accurate a 10 MHz signal source as your likely to get with out going to percison lab equipment for calibrating a frequency counter or what not.

If you use a run of the mill crystal and trimmer cap to set the frequency, it will not stay calibrated for long, especially if the temperature changes. Using a TCVXO is better for long term stability. Diz at www.kitsandparts.com sells an inexpensive 10 MHz TCVXO kit which can be used as the LO and once tweaked to WWV, will stay pretty darn close. In the case of using one of these as the frequency source, it can be connected to pin 6 of the SA612 and the other oscillator parts deleted. The signal from the TCVXO will have to be attenuated some and that can be done with a capacitor divider between the output of the TCVXO and the input to the SA612. The SA612 wants to see about a 600 mV p-p signal on pin 6.

WWV time tones:

WWV broadcasts several different tones bursts and a 100 Hz subcarrier with BCD time information on it.

5 ms bursts of a 1000 Hz tone marks each second "tick"

A 500 Hz tone marks the start of even minutes and a 600 Hz tone the start of odd minutes.

A 440 Hz tone is sent once an hour during minute 2, except in the first UTC hour. This tone isn't very useful for our needs, since it occurs so infrequently. In additon, some minutes do not include any tones.

Since the 100 Hz BCD time carrier is sent continuesly, this is the easiest to see and can be used to calibrate your oscillator initally.

The receiver circuit:

 

T1 and T2 are standard 10.7 MHz IF transformers with the internal cap left intact, so are not shown on the schematic. These will easialy retune down to 10 MHz. Q1 buffers the output of the SA612 oscillator so that a counter connected will not load down the oscillator or shift it's frequency. C6 which is used to tweak the oscillator freqeuncy should be of high quality, ideally an air variable. The supply voltage should be regulated.