|a 10.00000000 Mhz timebase locked to NBS station WWV|
|by ralph klimek VK3ZZC March 24 2016|
this article is copyleft, contents free to be used for all honorable purposes
Presented here is a remarkable design for a homodyne receiver that self locks and demodulates the time and frequency transmissions from my favourite radio station WWV. The purpose of this project was to replicate the experience of WA5BDU K7Q0 VE3DNL (see below) whose simple 5 transistor homodyne receiver is claimed to lock on to WWV, allthough the primary purpose of that design was to be a simple receiver for 30 Meter ham band. In the mainland USA, one would expect to receive an overwhelmingly strong local signal from WWV. On the other side of the planet here on the bottom end of Oz, the signal is good, mostly good in fact but hardly enough strength to be considered usable as a timebase. There is severe interference from digital appliance garbage . A homodyne receiver will spontaneously lock on to the strongest carrier within its lock range and passband. If it locks onto WWV , the self oscillation frequency will be a faithfully replica of WWV's atomic clock allowing for the ionospheric phase distortion , then I should expect 1 part in 10^9 from my receiver in this part of the world. My modifications to WA5BDU simple design include a 10Mhz LSB crystal ladder filter to ensure that the only signal around will be either WWV WWVH or the local Chinese governments standard signal.
With the signal for WWVH measured at my location from the big antenna at an average of -80dBm, this receiver achieves a homodyne lock and a rock solid lock at -70 dBm. This gives me access to a NBS accurate timebase for about 7 hours in a day, if the lords of the ionosphere permit.
The action of the crystal ladder filter is quite amazing. The main antenna picks up nearly -30dBm over a spectral range 3Mhz to 20Mhz of radiated garbage from my neighbors plasma screen TVs. This cacophony of garbage provides for spurious mixing products in my main shortwave receiver a TS780 that render WWV allmost inaudible. When the filter is applied, even allowing for the 10dB insertion loss of my poorly designed ladder filter, WWV become plainly audible.
The little homodyne receiver can pick up WWV via the ladder filter when it is inaudible via a "proper" sw radio. Thats the miracle of regenerative detectors.
My ladder filter leaves much to be desired. Rigorous design requires one to measure each crystal's dynamic equivalent motional reactances and design the ladder reactances to suit. I was able to purchase a bag of 10Mhz computer crystals for a few dollars, so I just grabbed those that resonated closest in frequency. I used mica compression trimmers instead of "designing" the ladder reactances. I just twiddled the trimmers until I could hear WWV. Nasty, nasty nasty ! The penalty I pay for this is high insertion loss and WWV sounds pretty horrible but most importantly it lets the precious carrier signal through and little else which is that which is actually required. The audio signal, horribly distorted by the uncertain group delay of my primitive filter , merely confirms the correct reception and gives good audio indication of a true lock.
I have included an audio power amplifier and little loudspeaker in this radio which was a mistake. The reason that if the volume is too loud it modulates the free run oscillator which results in audio feedback. It also thus compromises the phase stability of the timebase. It works ok with an external speaker.
And now, still to do. This requires a post filter amplifier to give extra headroom so that lock is not lost during QSB fades.
An external frequency control is actually required as the free run frequency has to be within about 15Khz of 10Mhz or lock cannot be aquired with my average -80dBm of WWV. WWV is also available at Melbourne on 5Mhz nearly every evening and sometimes on 15Mhz if the ionosphere permits. I have never heard WWV on 20Mhz and only once on 25Mhz on their new test transmission there. This radio is so simple that I should make one for each of these channels given that cheap computer crystals for ladder filters are so readily available. I have now incorporated a 10dB post filter amplifier that compensates for its high insertion loss.
Should the free run oscillator use a generic 10Mhz crystal rather than a LC resonator ? That is a interesting question.
I think the answer is yes. Proving that lock is achieved would be much harder.
How do I know when lock is achieved ? Monitor the free run oscillator with a counter. It will be at least a few kilocycles off frequency. When lock is acquired it will jump immediately to 10.0000000Mhz or to the audio carrier standard tone sidebands which is not usefull. How to detect this ? that is the purpose of the audio amplifier. When it is correctly locked you can hear the deep 100 cycle subcarrier on which is modulated the BCD timecodes and the voice announcements are correctly demodulated and "sounds right" .
Why did not I just build a GPS disciplined oscillator ? Because that is actually hard to do. GPS receivers only output a 1 pulse per second signal that has an average 50nS jitter because it is generated by software running the GPS algorithms. On long average the 1 pps signal is a good replica of the atomic clocks in the sky, but it is impossible to phase lock to this slow clock. You can only measure and compare averages and thus GPS disciplined oscillators have an aquisition time of the order of 10-20 minutes. The homodyne radio , if it locks correctly , aquires lock in milliseconds. (but still subject to ionospheric random phase shifts).
Why did I embark on this little project. Recently I have aquired a HP model 105 quartz oscillator which I have restored to life. I would like to have frequency references accurate enough to attempt QRSS of which one part in 10^9 accuracy in your timebases seems to be required. I have a number of good frequency counters that do not concur in the seventh or eighth digit and so a NBS locked timebase seems to be the only way. I need to calibrate my HP model 105 quartz oscillator and simple zero beating can only get me within about 50Hz of the truth due to poor audio low frequency repsonse. I will be using the method detailled in the various HP service manuals that describe counting the rate at which WWV sine waves wonder across a CRO sceen that is synced to your local timebase. You can get on part in 10^10 achievable without elaborate test gear or atomic clocks.
Here is what I based my WWV receiver on . Attribution and thanks due to WA5BDU K7Q0 and VE3DNL
|above... this is my implementation of the 10Mhz homodyne timebase receiver|
|theory of operation|
The homodyne principle mixes the incoming RF signal with a replica of itself. This is the basis of many QRP direct conversion receivers . The replica comes from an injection locked oscillator. There exists only a very poor theoretical framework for the operation of injection locked oscillators and my musings with SPICE have not succeeded in modeling the behavior of regenerative circuits like Q multipliers, injection locked oscillators or regenerative detectors. An oscillator can also be thought of as an amplifier with self positive feedback, but moreso. Positive feedback can be modeled and in the absence of some kind of energy restraint, signal will build up to infinite amplitude. An oscillator may be also thought of as an amplifier of neat infinite gain....but only at the frequency of oscillation. Now what happens when we apply a weak signal that is infinitesimally different from the natural free run frequency ? Our oscillator will amplify it to the energetic maximum and provide a substantially amplified replica of the input signal. It begins oscillating not at its preferred natural resonance but at the frequency of the source ! Such an "amplifier" has significant limitations. Its gain is unstable, it reacts non deterministically with its immediate envoironment, also amplifies natural noise and imposes phase uncertainty on the little signal that we are attempting to amplify. It also has limited lock range, it may shift modes and lock onto undesired signals present in its passband.
It has been observed by the clock makers of antiquity that two identical clocks when placed side by side will spontaneously phase lock. The phenonemom was called synchrony.
Here the top two transistors form an emitter coupled tuned oscillator. This circuit will happily oscillate in the complete absence of an input signal. The bottom transistor provides a constant current source and this current sloshes back and forth between the transistors at its free run rate mostly determined by the resonant collector circuit. A tiny signal applied to the base of the constant current transistor now provides an infinitesimal variation in collector current that acts to slightly change the phase of the free run oscillator. The circuit acts to track this phase with the input signal, eventually the oscillator creates a good replica of the input signal.
Our WWV signal is also amplitude modulated with announcements and tones and ticks and this SLOW amplitude and phase modulation will be superimposed on the oscillator emitter currents. It will be switched at the oscillation frequency....in effect it will be acting like a switching mixer. The DC component of this mixed signal is integrated ( low pass filtered) by the collector load circuit capacitor and this has now become our demodulated audio signal. It has been DIRECT converted with an RF signal that is a good replica of the signal being demodulated. This is the homodyne principle as distinct from non phase coherent direct conversion ( as is a simple product detector for cw/ssb)
Yet to come, some images of my WWV timebase.
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|Thu Apr 14 18:59:10 EST 2016|