In late 2009 my friend Bryan, W7CBM, came to me with a project that he had in mind: "Can we lock my Icom IC-910H to my 'Z-box'?" - in other words, could the 10 MHz output from his Z-3801 GPS Disciplined Oscillator - known to be accurate to better than one part in 100 million - be used to lock his tri-band (2 meters, 70cm and 23cm) all-mode radio to frequency?
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| Figure 1: The front panel of the modified Icom IC-910H. Click on the image for a larger version. |
During the initial discussion he'd brought with him an article where Rex, VK7MO, had done a similar thing (see the article on theVK3HZ site from the web archive - link) using an external box - but he wanted it to be contained entirely within the radio.
In looking at the requirements and designing the circuit in my head, I decided that we could make it simpler, smaller and easier to use - and with these ideas in mind, I wrote down the specifications for a 30.2 MHz fundamental-mode crystal and he sent an order off to International Crystal.
About 2 months later - in early 2010 - we got back together in my ham shack, crystal in hand, and it was then that I decided that I'd better get around to designing the circuit, so I scribbled the vestiges of a schematic onto a piece of paper and built several circuits that would fit into the aluminum box that Bryan had milled out. At the end of about 3 hours we had a circuit that would faithfully lock the 30.2 MHz crystal oscillator to a 10 MHz external source. This circuit was fairly small and consisting of two boards: The amplifier/counter/PLL section wired on prototype board while the VCXO itself was constructed "dead bug" on to a piece of copper-clad PC board material as seen in Figure 3.
"Patience is a virtue - but this is ridiculous!"
And that was where it stopped. In a case of "out of sight, out of mind" or "other fish to fry" - or any number of other excuses - the partly-completed lock unit stayed on a shelf in Bryan's ham shack for a decade - in plain sight. When I'd go over to his shack, I'd see it as a reminder of a project yet to be completed, but it had become a fixture and was often overlooked.
Until recently.
As it happened, we both had more time available with the onset of winter and we carved out Wednesday evenings to get together to work on various projects and this, being the most senior and nearest completion, came to the top of the pile. Over the course of a couple evenings we worked on the project, having to pause occasionally to get a part, modify some aspect the circuit's implementation, do some physical machine work, or because we ran out of time.
How it works:
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| Figure 2: The schematic of the lock unit for the IC-910H. This schematic is a reverse-engineered version of the (now lost) originals and is likely to be mostly correct. Click on the image for a larger version. |
The schematic diagram is depicted in Figure 2, below.
The VCXO:
The heart of the unit is Y201, a 30.2 MHz fundamental mode crystal in a Colpitts oscillator. Using D201, a varactor diode (approx. 5-20pF) its frequency is made variable, the center of the electronic tuning range being adjusted by trimmer capacitor C201. The output of the oscillator is buffered by emitter-follower Q202 to isolate the oscillator from the load.
The 30.2 MHz output goes two places: To Q103, the 30.2 MHz amplifier, and also to a low-pass filter consisting of C209, L201 and C209 which is then output to the IC-901H's synthesizer.
The 10 MHz chain:
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| Figure 3: The lock unit under test prior to installation in the case. Unfortunately, this is the only picture that I got of the oscillator portion. The small PCB is the RF sense circuit, built using SMD components by Bryan. Click on the image for a larger version. |
The 10 MHz input - which can come from a GPS Disciplined Oscillator (GPSDO), a 10 MHz oven-controlled oscillator (OCXO) or a Rubidium source - is input to and amplified by Q101 to a logic level and buffered by U1a, one section of a 74HC86 quad XOR gate.
The output of U1a is also applied to a 74HC40103 which is wired as a divide-by-50 counter to yield a 200 kHz output - and this is applied to U2a, a 74HC7474 divide-by-two counter to yield a 100 kHz square wave.
The RF sense circuit:
A sample of the 10 MHz signal from U1a is also applied to the input of Q301, which amplifies it: This RF gets rectified to DC by D301 and D302 and its presence turns on Q301 which pulls R303 to ground and turns off Q301 which is connected to U301 - a 5 volt regulator that is connected to the +5 volt lead of the original TCXO in the IC-910H: In this way, the internal oscillator in the IC-910H is enabled when there isno 10 MHz signal, but disabled when it is connected.
Also connected to the emitter of Q301 is PNP power switch Q203 which, when R208 is pulled to ground when Q301 turns off, applies power to U201 - a 9 volt regulator - to power up the 30.2 MHz oscillator when the external 10 MHz source is applied, preventing both oscillators from being turned on at the same time.The Harmonic mixer:
A sample of the 30.2 MHz signal applied to Q103 is amplified and applied to U1b, another XOR gate buffer, which is then applied, along with the 10 MHz from U1a, into U1d - yet another XOR gate. This gate acts as a harmonic mixer: By virtue of the multiplying action of the XOR gate, the 3rd harmonic of the 10 MHz input mixes with the 30.2 MHz input and at the output of this gate is a small amount of the difference frequency - 200 kHz - which easily is filtered by L101 and C103 and amplified by Q102.
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| Figure 4: The unit in place - final test. SMA connectors are used for 10 MHz input and 30.2 MHz output and feedthrough capacitors are used for the 13.8 volt DC input and the switched 5 volts for the TCXO. Click on the image for a larger version. |
The use of a harmonic mixer is a very old, simple technique and it has an advantage over a more "conventional" digital divider network - albeit more "analog".
A more "conventional" way of doing this might be to divide both the 30.2 MHz and 10 MHz signals down to a common sub-multiple - say, 200 kHz - but to do so would require both a divide-by-50 (to take the 10 MHz down to 200 kHz) and a divide-by-151 (to take the 30.2 MHz down to 200 kHz).
This method works, but adds the a bit of hardware (an additional divider) and, more importantly, these divider steps and subsequent comparisons reduce the PLL loop gain. By contrast, directly using the 3rd harmonic of the 10 MHz reference to mix with the 30.2 MHz, the 200 kHz difference (ultimately 100 kHz - see below) may be used directly - and loop gain preserved, potentially improving PLL performance and simplifying the design.
The comparison with the reference frequency:
The 200 kHz "difference" signal from the harmonic mixer, filter and amplifier is applied to U2d to yield to yield a 100 kHz square wave. The 100 kHz square wave from the divided-down 10 MHz reference signal and that from the 100 kHz "difference" signal are applied to U1c, an XOR gate, which is used as a phase detector. As the phases of the 100 kHz from the reference signal and that of the difference signal "slide" past each other, the voltage - smoothed by R107 and C107 - will vary from 0 to 5 volts. If C201 in the 30.2 MHz crystal oscillator is adjusted so that 2.5 volts applied to the "VCXO Tune" line, this will cause the crystal oscillator to lock to the reference when the two signals are 90 degrees apart, being steered into back onto frequency if they start to drift apart.
I chose to use an XOR gate as a phase detector over a conventional phase/frequency detector because other than the desired DC component, the lowest-frequency component from its output cannot be lower than the comparison frequency - 100 kHz, in this case. In comparison, many of the flip-flop phase/frequency detectors tend to output "occasional" pulses at very low frequency when at/near lock, which are nearly impossible to filter out. There is a minor penalty, though: An XOR gate phase detector requires use of 50% duty cycle square waves to work most efficiently, so each of its inputs is divided-by-two by a single 74HC74 dual flip-flop.
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| Figure 5: Power connection to the TCXO - L511 was removed. Click on the image for a larger version. |
Interfacing to the IC-910:
Switching the internal oscillator:
Bryan's IC-910 has the standard TCXO - X512 (the "CR-452") rather than the "High Stability" option (the "CR-293"). Either unit operates at 30.2 MHz, but there is a difference: The standard TCXO operates from 5 volts while the high stability unit operates directly from the 13.8 volt supply. Because the internal oscillator must be disabled when an another source is applied, one will need to do one of two things, depending on how the radio is configured:
- Because this radio had the standard TCXO (CR-452 a.k.a. X512), inductor L511 was removed to make the power externally switchable and L510 and C501 (the "L510" on Figure 2, above) was connected to power X512. It is this voltage that is switched by Q303 and regulated by U301 to provide switchable 5 volts.
- If the "High Stability" option ("CR-293")had been present (as described in the VK7MO case) we would have interrupted the 13.8 volt supply at C511/C512 and switched it using Q303 directly rather than regulated to 5 volts by U301. Comment: It is unknown how much current the high stability oscillator consumes so a slight modification of the Q302 circuit may be required.
Another difference between the way the two oscillators are interfaced appears to have something to do with the output level. The standard TCXO outputs an RF signal of about 1.2 volts peak-to-peak while it can be seen from the IC-910H service manual that R515 is in series with the output of the high stability oscillator - presumably to reduce its level.
Injecting the locked 30.2 MHz signal:
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| Figure 6: Connection of the 30.2 MHz to the PLL board showing D501 and L502. Click on the image for a larger version. |
If the external reference is not activated, Q203 - the VCXO power switch - is off and no voltage is applied to diode D501 via R207 and it remains "off", effectively isolating the original oscillator from the powered-down VCXO: By placing the diode at the end of the coax, farthest from the external reference, there is minimal effect on the signal by that coax to the IC-910H's internal oscillator when the external reference is not being used.
Mechanical installation within the IC-910H:
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| Figure 7: The back panel of the modified IC-910H. The added BNC connector is in the lower-left corner. Click on the image for a larger version. |
Spectral purity:
There was a small of concern that the spectral purity of the transceiver with the new reference oscillator would be worse than the original as I'd made no attempt to construct very low noise oscillator (e.g. a lightly-loaded Butler or similar) so I compared the spectrum with both the internal oscillator and the "new", externally-locked oscillator on the various bands - particularly on 23cm.
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| Figure 8: Transmitter spectrum +/-500 kHz of a CW carrier on 23cm. Click on the image for a larger version. |
On 2 meters and 70cm, very weak (-70dBc) spurs at +/- 200 kHz were noted, barely above the broadband noise floor of the transmitter itself - but these were pretty much absent on 23cm as can be seen in Figure 8. If these had been of concern, it would have been easy to further-improve the loop filter - which is currently a very simple R/C design as evidenced by Figure 2.Another possible concern was closer-in phase noise: Would various noise sources of the new circuits (VCXO phase noise, 1/F noise from regulators, loop noise, etc.) cause notable degradation?
Figure 9 gives a clue: For this test, trace "A" is the original TCXO and trace "B"(the slightly fainter one corresponding with the peak on the right) was produced using the new, externally-locked reference. As can be seen, the "close-in" phase noise performance of this radio isn't super great, anyway, but the two "noise humps" on either side of the carrier appear to be identical.
This trace also shows a slight difference in frequency, with the original TCXO (the peak on the left) being slightly low in frequency compared to the GPS-referenced, externally locked version.
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| Figure 9: A comparison of the close-in phase noise using the original TCXO (left) and the new, externally locked oscillator (right). Click on the image for a larger version. |
Even thought it has been a long time in the making, this project is complete - and working as well as we hoped that it would. I'm gratified that a mere decade ago, the circuit that I scribbled onto a piece of paper - and then built one evening works just as it was expected, with no significant modification!
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P.S. Alas, if you wanted to order a crystal, yourself, International Crystal Mfg. is no more, but custom crystals are still available via Quartslab - link, among a few other places.
(You would have to check with them to see if they will make a 30.2 MHz fundamental crystal, though - either that or modify the oscillator to use a 3rd overtone crystal...)
This page stolen from ka7oei.blogspot.com.
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