This is a brief explanation of the circuit shown in the GIF. Two components of the circuit that are not shown are the receiver amplifier and the VCXO. Both of these circuits are very straightforward. The receiver/amp was built using feroxcube pot cores selected accordingly. The active devices are dual gate MOSFETs. The fun part of the VCXO was the crystal. I had an old WW2 package at 5.8Mhz and took the crystal to 6Mhz by grinding it on a piece of glass with kitchen cleanser.
I hope the processor is what most of you are interested with for now. Keep in mind, this was a quick construction and some of the practices used may leave much to be desired. The devices used are: 7492, 4518, 4016, and (2) 074 quad FET op-amps. The GIF is rather large so I will just link it here:
[The Receiving Processor GIF]
- How it works
The receiver/amp was only three stages so the signal level was rather low. The output impedance of this amp was on the order of five K ohms so the first stage of the processor has about 13dbv of gain. The signal is rectified and the voltage is reversed to drive one of the gates on each stage of the receiver/amp. This AGC is set to provide about 2 volts of signal as I recall. It will actually be set to obtain a good detected AM output.
The signal is gated by pin 13 of the 4518 to create a phase detector. Before lock, the 1 meg resistor is shunted by the next gate so the loop filter is driven strongly. This loop will lock very quickly, on the order of a few seconds. But a loop on such a short leash will conduct a lot of the noise received to it's output. So when lock is detected, the 1 meg resistor is placed in the loop to create a very long time constant. Now the loop will float gently along with the input signal and remain relatively clean of phase noise. You have to switch the loop filter because it would never make the initial lock with such a long time constant. It will sometimes take a couple of tries for the loop to lock and track.
Pin 14 of the 4518 drives a gate that is 90 degrees to the loop gate. This creates a synchronous AM detector. This detector is marginally engineered. It works well enough to see the time code and allow for lock detection. When I built this, I was after the frequency standard and the time code was not of a lot of concern to me. The lock detector is an integrator driving a Schmidt trigger.
I hope this helps those who are interested in receiving the signal from WWVB. Like I said, I will document this project better in the future. With the price of GPS hardware dropping, you may want to consider it as an alternative to you time and frequency needs.
Thanks for reading and write if you'd like, Dan. firstname.lastname@example.org
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Page created: 23 nov 97   rev: 6 Jul 01