Sunday, March 31, 2024

Using an external clock with the RX-888 (Mk2)

The RX-888 (Mk2) and external clocking

Figure 1:
The RX-888 with external clock input (right)
The enable/disable switch is barely
visible behind the USB connector.
Click on the image for a larger version.
Note: I have posted blog two previous entries related to the RX-888 (Mk2) that you may find relevant:

Adding an external clock connection

While the internal 27 MHz TCXO in the RX-888 (Mk2) is pretty good, there may be instances where one wishes better accuracy and stability.  Fortunately, the RX-888 (Mk2) has provisions for doing so in the form of a jumper to disable the internal clock (when the jumper is removed) and a small connector (a tiny U.Fl) on board to accept that clock.

Unfortunately, it is up to the user to add the cable to feed an external clock - but short 4-6" (10-15cm) cables already fitted with a U.Fl male and SMA chassis-mount female connector are easily obtained from the likes of Amazon, EvilBay and others - just be sure that you do NOT get a "Reverse" (RP) SMA by mistake!

This leaves the jumper.  While many people simply remove the jumper and mount the external clock connector between the HF and VHF inputs - or sometimes to the right of the USB connector knowing - from then on - their RX-888 will be unusable unless there is an external clock input - I prefer to make use of the ability of the internal clock to be switched - using (ahem) a switch allowing for testing/use of the RX-888 in a "stand alone" configuration - but this is up to you.

If one is careful, it's possible to mount the external clock SMA connector and switch on the same panel as the USB connector, orienting so that its handle is toward the "Clock In" connector to indicate that an external clock is to be used - but labels or markings are always nice, too!

If one takes the route of mounting the external clock input between the HF and UHF inputs, the switch could be placed to the right of the USB connector - or, if as in the case of one of my RX-888s where I put a heat sink on the FX3 chip and there wasn't room there - I found a very small toggle switch that just fit between the case screw and left side of the USB connector and tip of this switch may be spotted just behind the USB connector in Figure 1, above.

IMPORTANT:  As the external clock input is simply wired in parallel with the internal 27 MHz clock.  What this means is that with the internal clock enabled, it will be present on the external clock input.  Similarly, if you supply a 27 MHz external clock without disabling the internal one, the two will "fight" each other and you'll get "garbage" results.

What type of signal to use as an external clock

  • The best external clock source is a 27 MHz sine wave of between 1.25 and 3.3 volts peak-to-peak.
  • A series coupling capacitance of between 100pF and 1000pF (470pF typ.) should be present on the "center pin" between the RX-888 to eliminate a DC path to ground on the signal line.

While a capacitively-coupled 27 MHz sine wave is recommended for reasons that will be mentioned later, a lot of devices offer square wave outputs - and getting these to work reliably requires at least a little bit of attention.

Using the Leo Bodnar Precision GPS Clock to drive an RX-888:

Because the RX-888 natively requires a 27 MHz clock this means that if you already have a 10 MHz standard (GPS, Rubidium, etc.) kicking around, you will not be able to use it directly.  While it's not too difficult to synthesize 27 MHz from 10 MHz (a number of Si5351-based devices can do this) it's most common for users of the RX-888 to use a device such as that sold by Leo Bodnar, which can be programmed for almost any frequency (from audio through UHF) with good precision and accuracy.

You can look at these products here:  (I have no stake in Bodnar, but I have used them and I and others have had good success.)

The most commonly-used device is the Bodnar "Mini" - which has one output - and this single output is often "daisy-chained" between RX-888s.  There is also the functionally similar LB-1420 with a single output and the "Precision GPS Reference Clock" which has two signal outputs - but there is very limited ability to set the "second" output to a specific frequency and it's mostly useful for outputting the same frequency on the two ports - or outputting a 1PPS signals on the "unused" port.

As the RX-888 (Mk2) external clock input is directly coupled to its Si5351 clock synthesizer, we have to act as if we are driving that chip directly.  While not directly specified in the Si5351 data sheets (at least the ones that I have found) testing done my myself indicates that a capacitively-coupled sine of about 750 millivolts peak-peak will trigger the '5351 reliably:  A bit of looking in online forums reveals the consensus that a 1 volt peak-peak sine wave is suggested so I would be comfortable with the suggestion of this amplitude being used a a guideline.

Testing with a square wave - such as that produced by the Leo Bodnar GPS reference revealed that the drive level was far more finicky - and this has to do with the fact that a "square" wave with a reasonably fast rise time does NOT remain a square wave for very long as it quickly turns into something rather spiky and distorted as depicted in the image below:

Figure 2:
A typical square wave output from a Bodnar GPS reference at the end of about 3 feet
(1 meter) of unterminated cable.  Ringing is evident!

This 27 MHz signal shows clear evidence of ringing:  This was measured right at the RX-888 with the signal passing through around 3 feet (1 meter) of 50 ohm coaxial cable.  As the '888 does not offer resistive termination, it presents a simple capacitance at the end of the cable and this tends to distort harmonic-rich waveforms like a square wave.

With multiple "spikes" that can occur on such waveforms due to distortion, it's possible - even likely - that certain combinations can result in multiple triggering peaks of the waveform.  In an extreme case, such distortion can cause the Si5351 to be triggered at twice the actual clock rate - but rather the result may be instability resulting in the RX-888 clocking which can be manifest as anything from no signals being "present" to those that are being off-frequency, varying, or just "noisy" - and this errant behavior may vary with temperature and slight changes in operating voltage.

It's important to realize that like the RX-888, the Bodnar is ALSO DC-coupled which explains why the above waveform in Figure 2 largely rests above the center line (zero volts) with the exception of some "ringing" which extends negative and is likely being clamped somewhat by the '888's internal diodes.

With a 3.3 volt waveform emanating from the Bodnar, we can reasonably expect that - if the signal isn't too "ringy" that a signal exceeding about 1 volt positive just once per cycle is likely to trigger the 888's Si-5351 correctly.

IMPORTANT:  If you try to directly drive an RX-888 with the output of a Bodnar, it will probably NOT work reliably!  I have observed this with my own Bodnar/RX-888s and many others have reported the same issue.

Remembering that the external clock input of the '888 goes directly to very sensitive logic devices, a simple resistive attenuator pad will do double duty:

  • Rather than a very high impedance circuit that has a low resistance path from the outside world to a sensitive logic gate, resistance to ground offers a degree of protection by offering a relatively low resistance to ground and the series resistance provides at least some limit to input currents.
  • While theoretically OK, the output of the Bodnar will not reliably drive the input of the Si5351 in the RX-888 directly, but being reduced to half or third of its original output seems to be pretty reliable and is less likely to cause clipping of diodes on the input circuit which can exacerbate ringing and other types of waveform distortion.

A 6 to 12 dB resistive pad - either 50 or 75 ohms - is a reasonable choice offering a bit of voltage reduction - but staying well above the 1 volt usability threshold - and such a pad, even if it is not connected to a 50 ohm load, will provide a bit of resistive termination, likely reducing the tenacity of reflections.  While a resistive pad does not offer DC decoupling between the center pin of the '888's external clock input, it works with the Bodnar as that device sources a square wave referenced to zero volts so the pad simply acts as a voltage divider for that square wave.

Testing has shown that the '888 seems a bit more forgiving of signal drive levels if there is a DC blocking capacitor on its signal input - something that could be provided by placing a "DC block" device (available in SMA, BNC or F-type connectors) between the '888 and the external clock source.

Caveats and warnings - and why the '888 is so finicky about its external clock

The external clock input of the RX-888 - as described in better detail in the next section of this blog post - is connected DIRECTLY to inputs within the '888 and as such, it has a few undesirable properties:

  • There is a DC connection between the external clock, the oscillator output and the input to the 888's internal Si5351 synthesizer.  This exposes the clock input directly to extremely static and voltage-sensitive inputs.
    • Because of this, it's very easy to damage the RX-888 when using and external clock, particularly if there are voltage potentials between different pieces of equipment.
  • There is diode clamping between ground and the 3.3 volt input.  In the '888, this is primarily a BAT99 dual diode, but it also includes the protection diodes of the other devices in the circuit - namely the output of the onboard 27 MHz oscillator and the input of the Si5351 itself.  At first this might seem like a good thing - and it sort of is - but this means that any signal input to the RX-888 should be capacitively coupled - or directly to a 0-3.3 volt signal.  This is one aspect of the '888 that was definitely not well considered or implemented.
    • What this means is that if you try to drive the RX-888's clock input with a source that is DC "grounded" - which includes devices that are transformer-coupled (e.g. a splitter to send the clock to multiple units) that the voltage output will be bipolar.
    • For example: 
      • If you were try to use a T1-1 isolation transformer to break a ground loop between the external clock input and the Bodnar - as well as other devices - the signal input may be 3.3 volts - but bipolar - that is, it will go above and below "ground" by about 1.65 volts - but since there is diode clamping, the negative-going signal will distort the waveform.
      • The result of this can either be finessing required to find the precise drive level to make it work at all or - sometimes - you will find the signals at the wrong frequencies (sometimes at about half the expected frequencies) if the badly-distorted waveform triggers the input of the Si5351 synthesizer in the '888 twice on every clock cycle.
All of these factors often confound users of the RX-888 (Mk2) trying to feed an external clock - and things get more complicated if multiple devices are use.  For example:
  • As with any sensitive piece of RF equipment, having multiple, disparate connections between pieces of equipment will usually end up with circulating currents - and since every conductor has resistance, this can cause noises to appear in the RF input.  A few examples:
    • The RX-888 - or any SDR - will have multiple connections to it - typically the antenna and power input.  In the case of the RX-888 and many other SDRs, this means an antenna and USB connection.
      • Isolating the RF signal lines from longitudinal currents (e.g. common mode) is a useful tool.
        • Often, this can take the form of small coaxial cable (RG-142 or RG-174) wound with 8-12 turns on an FT-140 or FT-240 core of 31 or 43 material (the former being better for lower frequencies).  This is useful for HF (160-10 meters) but it loses efficacy below this and is not helpful if your interest extends into the AM broadcast bands and lower frequencies (e.g. longwave - including LF and VLF which includes the 2200 and 630 meter amateur bands.)
        • Another tool can be an "voltage balun" - essentially an isolation transformer with no DC connection at all.  Often, these are built around the Mini-Circuits T1-1.  These lose their efficacy below a MHz or so so they may have excessive attenuation on LF and VLF frequencies.  At higher frequencies (above 10 MHz) their common-mode rejection also starts to drop meaning that in a very noisy environment, signals can "leak in" at high HF from the surrounding equipment - something that needs to be checked if you try it.
    • Power supplies and computers (via a USB cable) are notoriously noisy, so you WILL get circulating currents flowing between the devices.  Having a choking USB cable (e.g. 6-12 turns on an FT-140 or FT-240 core of 31 or 43 material) can help significantly, as can doing similar on a DC supply line and also choosing a "known RF-quiet" power supply.
    • Adding a "third" connection to the receiver - such as the external clock, in case of the RX-888 (Mk2) - can further complicate issues as it adds yet another  avenue of common-mode currents and noise.
      • This connection, too, should be appropriately isolated - but doing so is complicated by the way the external clock input is implemented.
      • The fact that the external clock device is connected to a potentially-noisy power supply and  a GPS antenna - which may or may not have its own grounding (which can further introduce circulating currents) is yet another thing about which you should be wary!
One issue that also arises is that output of devices like the Bodnar are square wave.  This, by itself, isn't a problem - and a direct connection between the Bodnar and '888  - since they both have 3.3 volt signal levels - works OK, at least with very short cables when using a 6-12 dB pad.
Conveying this square wave signal - particularly over greater distances and considering that the clock input to the RX-888 is high-impedance with a bit of capacitance means that long runs (anywhere near 1/4 wave at the clock frequency or longer) can result in reflections due to unterminated cables.  What one can do is put a 50-75 ohm termination at the far end of the cable. This, however, does not help with the issue of DC/galvanic isolation between individual receivers.
Testing the stability of your external clock mechanism:
As properties of solid-state devices change over temperature - and signal levels may vary depending on what other devices are connected to your clock source - it would be a very good idea to varying the clock signal to determine if you have enough margin to allow it to work if levels change, or if you are on the "ragged edge".

Reducing the signal level is the most obvious test:  The use of a step attenuator - or use a variety of fixed attenuator pads (be sure that they pass DC) and reducing the level by between 1 and 15 dB - and then observing when clocking becomes unreliable:  This will give you a good idea as to the margin between what you are feeding to the '888 and when it will quite - and it may prompt you to reduce your signal level slightly.

Using HDSDR under Windows

Determining when the clocking signal into the '888 becomes unreliable is a bit trickier in some cases.  By far the easiest is to use a program like HDSDR with the "SDDC" ExtIO driver on a fairly fast Windows computer with USB3 ports:  A higher-end Intel i5 or medium-high end Intel i7 will suffice.  Connecting the '888 to an external antenna and tuning in a reliable signal (like a shortwave broadcaster or a time station like WWV/H or CHU - or tuning it your own signal generator) while watching the waterfall will tell you immediately when the external clocking fails.

If you are using Linux with ka9q-radio, you can use the "Monitor" program to tune a signal with the audio being sent to the default audio device - but doing this is beyond the scope of the document.  If you are using a Mac, I don't have a suggestion unless someone speaks up.

Transformer-based signal isolation NOT recommended for the '888's clock input - sort of...

It is important for any receiver to minimize the amount of current circulating through the "ground" connections.  Such currents in an analog receiver can induce hum in unbalanced audio lines and if the receiver is actually a transceiver, those same signal paths can induce RF into seemingly unrelated equipment in the ham shack.

Sometimes overlooked is the fact that these same currents can induce RF currents on the cables interconnecting equipment and it is likely that these will find their way into the receiver's front end and degrade performance by raising the noise floor.  This is especially true when a computer-connect software-defined radio - like the RX-888 - is involved as we now have a connection (via the USB cable) to a device that is likely to be "noisy" at RF - namely the computer - but this also means that noise can come from other devices to which this computer is connected directly or indirectly, namely its power supply, other peripherals, its power supply - and noisy devices on the AC mains into which this power supply is plugged.

Current "balun"

For receiver RF connections one way to deal with this is to use a common-mode RF choke which is typically a dozen or so turns of coaxial cable wound on a T-140 or T-240 toroid - usually with 31 or 43 type material.  This will break up common-mode currents on the cable - at least at HF - and can reduce such issues and this works for both the signal (antenna) and external clocking lines.

At DC and mains frequencies such chokes offer little/no efficacy and at low frequencies (below a MHz or so) these chokes lose their effective series resistance owing to limited inductance.  What this means is that if you have strong circulating currents (e.g. current flowing between your antenna "ground" and house mains "ground") they will have little effect.

Voltage "balun"

A possible alternative is to use a transformer to couple between RF sources:  A reliable, low-cost, commonly-available device for this is the Mini-Circuits Labs T1-1 which provides complete galvanic isolation between the source and load with a reasonable degree of longitudinal isolation.

While the T1-1 works well for the RF input, it will not work so well for the RX-888's external clock input by itself and the reason for this is that the output from a transformer winding is, by definition, bipolar about the zero volt point.  In the case of an external clock signal of, say, 1 volt peak-peak, each half would be above and below zero volts and with a direct DC connection to the Si5351's input it is unlikely to properly drive/trigger it.

If the signal is of higher amplitude - such as our 3.3 volt square wave - half of this "ugly" waveform will lie below ground potential and that below the 0.6 volt diode conduction voltage will be clamped, potentially distorting the waveform even more.

If a transformer-based method of isolation is used it is strongly suggested that a capacitor be placed in series with the '888's signal input to allow the waveform and voltage to float above ground and avoid negative clamping.  As mentioned earlier, a "DC Block" device could be used if you choose not to build your own device.

Example homebrew devices:

Here are a few (relatively) simple devices that one could build on a piece of scrap PC board - or you could go through the effort of designing and building a board with these features.

Figure 3, below, shows a simple resistive coupler incorporating the features suggested above:
Figure 3: 
A simple 10-ish dB resistive pad with DC blocking to keep the external clock input of the RX-888 "happy" and to prevent clipping of negative-going voltage by built-in protection diodes.  The "small" capacitor value also minimized the amount of stored charge dumped into the '888 due handling/shorting of the input cable.

This diagram shows a resistive pad that offers about 10 dB of attenuation - the values being determined assuming a 50 ohm system - but since the '888's input impedance is almost exclusively capacitive (a few 10s of pF) it is operating more as a voltage divider presenting a resistive load that just happens to be around 50 ohms.  The coupling capacitor between the pad and the '888 offers DC blocking to make it more forgiving to varying signal levels.  While the capacitor blocks DC, the signal being input to the Si5351 will find its own level due to the clamping effects of the protection diodes in the '888.

Also shown is the optional inclusion of a 1000pF capacitor that can be inserted at point "X":  This will decouple DC and mains AC currents that might flow between the clock source and the RX-888 itself - but it is low enough impedance that it does not necessarily offer RF decoupling between devices.  With the circuit shown above, however, you can precede it with decoupling device - such as a common-mode choke (e.g. current balun - the type with a dozen or so turns on a toroid) or even a T1-1 transformer.

Figure 4, below, shows another possible approach:
Figure 4: 
This circuit provides both common-mode isolation and a degree of band-pass filtering of the 27 MHz clock signal:  Filtering to a sine-like waveform reduces glitching due to cabling issues (reflections, misterminations) as well as offers a degree of protection to the RX-888's input as the filter will limit the amount of energy that could be imparted.  It also provides a (small) degree of termination (<150 ohms).   The "optional" 1000pF capacitor shunts low level leakage of the 27 MHz signal due to transformer imbalance - but it is suggested that one use a common-mode choke to restore isolation at HF frequencies.

This device is slightly more complicated, but it offers several advantages:

  • "L1" is a trifilar-wound toroidal transformer (that is, its turns consist of three wires gently twisted together before winding on the toroid).  Its intrinsic inductance is around 0.22uH and with the 150pF capacitor seen on the lower half of the diagram, it resonates broadly at 27 MHz - the external clock frequency for the '888.
  • The resistors shown offer a bit of resistive termination to the signal source (a bit below 150 ohms) which can help to reduce reflections on the cable.
  • These series 150 and 100 ohm resistors "decouple" the resonant circuit from the signal path somewhat and the values were chosen to allow sufficient "Q" to offer reasonable filtering of the input signal into a fairly good sine wave.
  • Figure 5: 
    The (nearly) sine wave output from the circuit depicted
    in Figure 4.
    Click on the image for a larger version.
    As this is a transformer-coupled circuit, there is no DC connection at all between the input and output.  Because it is resonant at 27 MHz, it will also offer a degree of rejection of other signals that might be present.  As the resonant circuit is wired to the "RX-888 side" of the circuit, it offers excellent protection to it.
  • As with the previous circuit, an optional 1000pF capacitor is shown as well:  Including this will reduce the common-mode isolation between the input and output but it will suppress a bit of leakage of the 27 MHz clock signal that can occur owing to the fact that the transformer that is L1 is not perfectly balanced.

The disadvantage of this circuit is that it requires the winding of a toroidal transformer and tuning it to 27 MHz - something easily done with a NanoVNA or an oscilloscope and an oscillator.  

Figure 5 shows the resulting waveform that has passed through the circuit depicted in Figure 4:  It is nearly a sine wave and as such, it is much more resistant to causing false triggering on "ringing" edges as compared to a square wave.

Figure 6: 
The prototype transformer/filter circuit depicted in Figure 4
connected at the Bodnar, connected to the '888 with a
short BNC<>SMA jumper.
Click on the image for a larger version.

Figure 6 shows the circuit of Figure 4 in action, connected directly to the Bodnar's output and - via a very short BNC to SMA cable - to the RX-888 sitting atop it.

This prototype unit was built in a piece of copper-clad PC board material.  On the top side, the components were wired with flying leads to the connectors and "dead bug" on the copper itself:  Between the "Bodnar" and the "RX-888" side the copper was cut to provide the two separate signal "grounds" with only the transformer coupling between the two.

At some point, it may be worth designing a small PC board for this, but for the meantime a small number of these prototypes have been built and put into service very successfully.  As suggested earlier, the a step attenuator was inserted between the Bodnar and this circuit and the signal reduced until the '888 no longer reliable locked to the external clock and it was found that there was plenty of margin to assure stable operation under varying conditions.

Lots of other possibilities

Now that you know what the RX-888 "wants", you have a better idea of what you are likely to be able to "safely" use to drive the external clock input of the RX-888.

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