Sunday, February 27, 2022

High power Tayloe (a.k.a. Wheatstone) absorptive bridge for VSWR indication and rig protection.

Figure 1:  The completed absorptive VSWR bridge.
Last year, I was "car camping" with a bunch of friends - all of which happened to be amateur radio operators.  Being in the middle of nowhere where mobile phone coverage was not even available, we couldn't resist putting together a "portable" 100 watt HF station.  While the usual antenna tuner+VSWR meter would work fine, I decided to build a different piece of equipment that would facilitate matching the antenna and protecting the radio - but more on this in a moment.

A bit about the Wheatstone bridge:

The Wheatsone bridge is one of the oldest-known types of electrical circuits, first having been originated around 1833 - but popularized about a decade later by Mr. Wheatstone itself.  Used for detecting electrical balance between the halves of the circuit, it is useful for indirectly measuring all three components represented by Ohm's law - resistance, current and voltage.

Figure 2:  Wheatstone bridge (Wikipedia)
It makes sense, then, that an adaptation of this circuit - its use popularized by Dan Tayloe (N7VE) - can be used for detecting when an antenna is matched to its load.  To be fair, this circuit has been used many decades for RF measurement in instrumentation - and variations of it are represented in telephony - but  some of its properties that are not directly related to its use for measurement that make it doubly useful - more on that shortly.

Figure 2 shows the classic implementation of a Wheatstone bridge.  In this circuit, balance of the two legs (R1/R2 and R3/Rx) results in zero voltage across the center, represented by "Vg" which can only occur when the ratio between R1 and R2 is the same as the ratio between R3 and Rx.  For operation, that actual values of these resistors is not particularly important as long as the ratios are preserved.

If you think of this is a pair of voltage dividers (R1/R2 and R3/Rx) its operation makes sense - particularly  if you consider the simplest case where all four values are equal.  In this case, the voltage between the negative lead (point "C") and point "D" and points "C" and "B" will be half that of the battery voltage - which means the voltage between points "D" and "B" will be zero since they must be at the same voltage.

Putting it in an RF circuit:

Useful at DC, there's no reason why it couldn't be used at AC - or RF - as well.  What, for example, would happen if we made R1, R2, and R3 the same value (let's say, 50 ohms), instead of using a battery, substituted a transmitter - and for the "unknown" value (Rx) connected our antenna?

Figure 3:  The bridge, used in an antenna circuit.

This describes a typical RF bridge - known when placed between the transmitter and antenna as the "Tayloe" bridge, the simplified diagram of which being represented in Figure 3.

Clearly, if we used, as a stand-in for our antenna, a 50 ohm load, the RF Sensor will detect nothing at all as the bridge would be balanced, so it would make sense that a perfectly-matched 50 ohm antenna would be indistinguishable from a 50 ohm load.  If the "antenna" were open or shorted, voltage would appear across the RF sensor and be detected - so you would be correct in presuming that this circuit could be used to tell when the antenna itself is matched.  Further extending this idea, if your "Unknown antenna" were to include an antenna tuner, looking for the output of the RF sensor to go to zero would indicate that the antenna itself was properly matched.

At this point it's worth noting that this simple circuit cannot directly indicate the magnitude of mismatch (e.g. VSWR - but it can tell you when the antenna is matched:  It is possible to do this with additional circuitry (as is done with many antenna analyzers) but for this simplest case, all we really care about is finding when our antenna is matched.  (A somewhat similar circuit to that depicted in Figure 3 has been at the heart of many antenna analyzers for decades.)

Antenna match indication and radio protection:

An examination of the circuit of Figure 3 also reveals another interesting property of this circuit used in this manner:  The transmitter itself can never see an infinite VSWR.  For example, if the antenna is very low resistance, we will present about 33 ohms to the transmitter (e.g. the two 50 ohm resistors on the left side will be in parallel with the 50 ohm resistor on the right side) - which represents a VSWR of about 1.5:1.  If you were to forget to connect an antenna at all, we end up with only the two resistors on the left being in series (100 ohms) so our worst-case VSWR would, in theory, be 2:1.

In context, any modern, well-designed transmitter will be able to tolerate even a 2.5:1 VSWR (probably higher) so this means that no matter what happens on the "antenna" side, the rig will never see a really high VSWR.

If modern rigs are supposed to have built-in VSWR protection, why does this matter?

One of the first places that the implementation of the "Tayloe" bridge was popularized was in the QRP (low power) community where transmitters have traditionally been very simple and lightweight - but that also means that they may lack any sophisticated protection circuit.  Building a simple circuit like this into a small antenna tuner handily solves three problems:  Tuning the antenna, being able to tell when the antenna is matched, and protecting the transmitter from high VSWR during the tuning process.

Even in a more modern radio with SWR protection there is good reason to do this.  While one is supposed to turn down the transmitter's power when tuning an antenna, if you have an external, wide-range tuner and are quickly setting things up in the field, it would be easy to forget to do so.  The way that most modern trasmitter's SWR protection circuits work is by detecting the reflected power, and when it exceeds a certain value, it reduced the output power - but this measurement is not instantaneous:  By the time you detect excess reflected power, the transmitter has already been exposed - if only for a fraction of a second - to a high VSWR, and it may be that that brief instant was enough to damage an output transistor.

In the "old" days of manual antenna tuners with variable capacitors and roller inductors, this may have not been as big a deal:  In this case, the VSWR seen by the transmitter might not be able to change too quickly (assuming that the inductor and capacitors didn't have intermittent connections) but consider a modern, automatic antenna tuner full of relays:  Each time the internal tuner configuration is changed to determine the match, these "hot-switched" relays will inevitably "glitch" the VSWR seen by the radio, and with modern tuners, this can occur many times a second - far faster than the internal VSWR protection can occur meaning that it can go from being low, with the transmitter at high power, to suddenly high VSWR before the power can be reduced, something that is potentially damaging to a radio's final amplifier.  While this may seem to be an unlikely situation, it's one that I have personally experienced in a moment of carelessness - and it put an abrupt end to the remote operation using that radio - but fortunately, another rig was at hand.

A high-power Tayloe bridge:

It can be argued that these days, the world is lousy with Tayloe bridges as they are seemingly found everywhere - particularly in the QRP world, but there are fewer of them that are intended to be used with a typical 100 watt mobile radio - but one such example may be seen below:


Figure 4:  As-built high-power Tayloe bridge

Figure 4 shows a variation of the circuit in Figure 2, but it includes two other features:  An RF detector, in the form of an LED (with RF rectifier) and a "bypass" switch, so that it would not need to be manually removed from the coax cable connection from the radio.

In this case, the 50 ohm resistors are thick-film, 50 watt units (about $3 each) which means that they are capable of handling the full power of the radio for at least a brief period.  Suitable resistors may be found at the usual suppliers (Digi-Key, Mouser Electronics) and the devices that I used were Johanson P/N RHXH2Q050R0F4 (A link to the Mouser Electronics page is here) - but there is nothing special about these particular devices:  Any 50-100 watt, TO-220 package, 50 ohm thick-film resistor with a tolerance of 5% or better could have been used, provided that its tab is insulated from the internal resistor itself (most are). 

How it works:

Knowing the general theory behind the Wheatstone bridge, the main point of interest is the indicator, which is, in this case, an LED circuit placed across the middle of the bridge in lieu of the meter shown in  Figure 1.  Because RF is present across these two points - and because neither side of this indicator is ground-referenced, this circuit must "float" with respect to ground.

If we presume that there will be 25 volts across the circuit - which would be in the ballpark of 25 watts into a 2:1 VSWR - we see that the current through 2k could not exceed 25 mA - a reasonable current to light an LED.  To rectify it, a 1N4148 diode - which is both cheap and suitably fast to rectify RF (a garden-variety 1N4000 series diodes is not recommended) along with a capacitor across the LED.  An extra 2k LED is present to reduce the magnitude of the reverse voltage across the diode:  Probably not necessary, bit I used it, anyway.  QRP versions of this circuit often include a transformer to step up the low RF voltage to a level that is high enough to reliably drive the LED, but with 5-10 watts, minimum, this is simply not an issue.

While there are many examples of this sort of circuit - all of them with DPDT switches to bypass the circuit - every one that I saw wired the switch in such a way that if one were to be inadvertently transmitting while the switch was operated, there would be a brief instant when the transmitter was disconnected (presuming that the switch itself is a typical "break-before-make") that could expose the transmitter to a brief high VSWR transient.  In Figure 3, this switch is wired differently:

  • When in "Bypass" mode, the "top" 50 ohm resistor is shorted out and the "ground" side of the circuit is lifted.
  • When in "Measure" mode, the switch across the "top" 50 ohm resistor is un-bridged and the bottom side of the circuit is grounded.

Figure 5:  Inside the bridge.
Wired this way, there is no possible condition during the operation of the switch where the transmitter will be exposed to an extraordinarily high VSWR - except, of course, if the antenna itself is has an extreme mismatch - which would happen no matter what if you were to switch to "bypass" mode.

An as-built example:

I built my circuit into a small die-cast aluminum box as shown in Figure 5.  Inside the box, the 50 ohm resistors are bolted to the box itself using countersunk screws and heat-sink paste for thermal transfer.  To accommodate the small size of the box, single-hole UHF connectors were used and the circuit itself was point-to-point wired within the box itself.

Figure 6:  The "switch" side of the bridge.
For the "bypass" switch (see Figure 6) I rescued a 120/240 volt DPDT switch from an old PC power supply, choosing it because it has a flat profile with a recessed handle with a slot:  By filing a bevel around the square hole (which, itself was produced using the "drill-then-file" method) one may use a fingernail to switch the position.  I chose the "flush handle" type of switch to reduce the probability of it accidentally being switched, but also to prevent the switch itself from being broken when it inevitably ends at the bottom of a box of other gear.
 
On the other side of the box (Figure 7) the LED is nearly flush-mounted, secured initially with cyanoacrylate (e.g. "Super") glue - but later bolstered with some epoxy on the inside of the box.
 
It's worth noting that even though the resistors are rated for 50 watts, it's unlikely that even this much power will be output by the radio will approach that in the worst-case condition - but even if it does, the circuit is perfectly capable of handling 100 watts for a few seconds.  The die-cast box itself, being quite small, has rather limited power dissipation on its own (10-15 watts continuous, at most) but it is perfectly capable of withstanding an "oops" or two if one forgets to turn down the power when tuning and dumps full power into it.  It will, of course, not withstand 100 watts for very long - but you'll probably smell it before anything is too-badly damaged!
 
Operation:

As on might posit from the description, the operation of this bridge is as follows:

  • Place this device between the radio and the external tuner.
  • Turn the power of the radio down to 10-15 watts and select FM mode.
  • Disable the radio's built-in tuner, if it has one.
  • If using a manual tuner, do an initial "rough" tuning to peak the receive noise, if possible.
  • Switch the unit to "Bridge" (e.g. "Measure") mode.
  • Key the transmitter.
  • If you are using an automatic tuner, start its auto-tune cycle.  There should be enough power coming through the bridge for it to operate (most will work reliably down to at about 5 watts - which means that you'll need the 10-15 watts from the radio for this.) 
  • If you are using a manual tuner, look at both its SWR meter (if it has one) and the LED brightness and adjust for minimum brightness/reflected power.  A perfect match will result in the LED being completely extinguished.
  • After tuning is complete, switch to "Bypass" mode.
 * * *
This page stolen from ka7oei.blogspot.com

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Saturday, January 22, 2022

Testing the FlyDog SDR (KiwiSDR "clone")

As noted in a previous entry of this blog where I discussed the "Raspberry Kiwi" SDR - another clone of the KiwiSDR - there is also the "FlyDog" receiver - yet another clone - that has made the rounds.  As with the Raspberry Kiwi, it would seem that the sources of this hardware are starting to dry up, but it's still worth taking a look at it.

I had temporary loan of a FlyDog SDR to do an evaluation, comparing it with the KiwiSDR - and here are results of those tests - and other observations.

Figure 1:
The Flydog SDR.  On the left are the two "HF" ports and
the port for the GPS antenna.  Note the "bodge" wires
going through the shielded area in the upper left.
The dark squares in the center and to its right are the A/D
converter and the FPGA.  The piece of aluminum attached
to the oscillator is visible below the A/D converter.
Click on the image for a larger version.

How is this different from the Raspberry Kiwi?

Because of its common lineage, the FlyDog SDR is very similar to the Raspberry Kiwi SDR - including the use of the same Linear Technologies 16 bit A/D converter - and unlike the Raspberry SDR that I reviewed before, it seems to report a serial number, albeit in a far different range (in the 8000s) than the "real" KiwiSDRs which seem to be numbered, perhaps, into the 4000s.

The most obvious difference between the FlyDog and the original KiwiSDR (and the Raspberry Kiwi) is the addition of of a second HF port - which means that there is one for "up to 30 MHz" and another that is used for "up to 50 MHz" - and therein lies a serious problem, discussed below.

Interestingly, the FlyDog SDR has some "bodge" wires connecting the EEPROM's leads to the bus - and, unfortunately, these wires, connected to the digital bus, appear to run right through the HF input section, under the shield!  Interestingly, these wires might escape initial notice because they were handily covered with "inspection" stickers. (Yes, there were more than two covering each other - which was suspicious in its own right!  To be fair, there's no obvious digital "noise" as a result of the unfortunate routing of these bodge wires.) 

Why does it exist?

One would be within reason to ask why the FlyDog exists in the first place - but this isn't quite clear.  I'm guessing that part of this was the challenge/desire to offer a device for a the more common, less-expensive and arguably more capable Raspberry Pi (particularly the Pi 4) - but this is only a guess.

Another reason would have been to improve the performance of the receiver over the KiwiSDR by using a 16 bit A/D converter - running at a higher sampling rate - to both improve dynamic range and frequency coverage - this, offering usable performance up through the 6 meter amateur band.  Unfortunately, the Flydog does neither of these very well - the dynamic range problem being the same as the Raspberry Kiwi in the linked article compounded by the amplitude response variances and frequency stability issues discussed later on.

Observations:

Getting immediately to one of the aspects of this receiver I'll discuss the two HF ports - and their implementation can be stated in two words:  Badly implemented.

When I first saw the FlyDog online with its two HF ports, I wondered "I wonder how they selected between the two ports - with a small relay, PIN diodes, or some sort of analog MUX switch, via hardware?" - but the answer is neither:  The two ports are simply "banged" together at a common point.

When I heard this, I was surprised - not because of its simplicity, but because it's such a terrible idea.  As a few moments with a circuit simulator would show you, simply paralleling two L/C networks that cover overlapping frequency ranges does not result in a combined network sharing the features/properties of the two, but a terrible, interacting mess with wildly varying impedances and the potential for wild variations of insertion loss.

The result of this is that the 30 MHz input is, for all practical purposes, unusable.  Additionally, if one checks the band-pass response of the receiver using a calibrated signal generator against the S-meter reading, you will soon realize that the resulting frequency response across the HF spectrum is anything but flat.

For example, one will see a "dip" in response (e.g. excess loss) around 10 MHz on the order of 20 dB if one puts a signal into the 50 MHz port, effectively making it unusable for the 30 meter amateur band and the 31 meter shortwave broadcast band.  Again, there is nothing specifically wrong with the low-pass filter networks themselves - just the way that they were implemented:  You can have only one such network connected to the receiver's preamplifier input at a time without some serious interaction!

Work-around:

Having established that out-of-the-box that the FlyDog has some serious issues when used as intended on HF, one might be wondering what can be done about it - and there are two things that may be done immediately:

  • Do microsurgery and disconnect one of the HF input ports.  If you have the skills to do so, the shield over the HF filter may be unsoldered/removed and the circuit reverse-engineered enough to determine which component(s) belong to the 30 MHz and 50 MHz signal paths - and then remove those component(s).  Clearly, this isn't for everyone!
  • Terminate the unused port.  A less-effective - but likely workable alternative - would be to attach a 50 ohm load to the unused port.  On-bench testing indicated that this seemed to work best when the 50 MHz port was used for signal input and the 30 MHz port was connected to a 50 ohm load:  The frequency of the most offensive "null" at about 10 MHz shifted down by a bit more than 1 MHz and reduced in depth, allowing still-usable response (down by only a few dB) at 10 MHz, and generally flattening response across the HF spectrum:  Still not perfect, but likely to be adequate for most users.  (In testing, the 30 MHz port was also shorted, but with poorer results than when terminated.) 

In almost every case, the performance (e.g. sensitivity) was better on the 50 MHz port than the 30 MHz port, so I'm at a loss to find a "use case" where its use might be better - except for a situation where its lower performance was outweighed by its lower FM broadcast band rejection - more on that later.

The other issue - which is shared with the RaspberryKiwi SDR - is that the low-pass filter (on the 50 MHz port) is insufficient to prevent the incursion of aliases of even moderately strong FM broadcast signals which appear across the HF spectrum as broad (hundreds of kHz wide) swaths of noise with a hint of distorted speech or music.  This is easily solved with an FM broadcast band filter (NooElec and RTL-SDR blog sell suitable devices) - and it is likely to be a necessity.

Other differences:

  • Lower gain on the FlyDog SDR:  Another difference between the FlyDog and KiwiSDR is the RF preamplifier.  On the KiwiSDR and Raspberry Kiwi, a 20 dB gain amplifier (the LTC6401-20) is used, but a 14 dB gain amplifier (LTC6400-14) is used instead - a gain reduction of about 6 dB, or one S-unit - and the effects of this are evident in the performance as described below.  Was this intentional, a mistake, or was it because the 14 dB version was cheaper/more available?
From a purely practical stand point, this isn't a huge deal as gain may be added externally - and it's generally better to have a too-little gain in a system and add it externally rather than to try to figure out how to reduce gain without impacting noise performance.  As it is the gain of the receiver is insufficient to hear the noise floor of an antenna in a "rural quiet" station on 20 meters and above (when the bands are closed) without amplification.  This also means that it is simply deaf on 10 and 6 meters, requiring additional filtering and amplification if one wishes to use it there for weak signal work.  The KiwiSDR and Raspberry SDRs have a similar issue, of course, but the 6 dB gain deficit of this receiver exacerbates the problem.
  • "X1.5/X1.0" jumper:  There is, on the silkscreen, indication of a jumper that implies the changing of the gain from "1.5" to "1.0" when J1 is bridged.  I didn't reverse-engineer the trace, but it appears to adjust the gain setting of the LNA of the A/D converter - and sure enough, when jumpered, the gain drops by about 4 dB - precisely what a "1.5x" factor would indicate.  Despite the gain reduction, the absolute receiver sensitivity was unchanged, implying that the system's noise floor is set either by the LNA itself (the LTC6400-14) or noise internal to the the A/D converter.  If there's any beneficial effect at all I would expect it to occur during high signal conditions, in which case the "1.0" setting might make it slightly more susceptible to overload.
  •  "Dith/NA" jumper:  Also on the board is a jumper with this nomenclature marked J2 - and this (apparently) disables the A/D converter's built-in "dither" function - one designed to reduce spurious/quantization effects of low-level signals on the A/D converter, which defaults to "on" with the jumper removed as shipped.   Although extensive testing wasn't done, there was no obvious difference with this jumper bridged or not - but then, I didn't expect there to be on a receiver where the noise limit is likely imposed by the LNA rather than the A/D converter itself.
  • Deaf GPS receiver:  I don't know if it's common to these units, but I found the Flydog being tested to be very insensitivity to GPS signals as compared to other devices (including Kiwi and Raspberry SDRs) that I have around, requiring the addition of gain (about 15dB) to the signal path to get it to lock reliably.  This has apparently been observed with other FlyDog units and it is suspected that a harmonic of a clock signal on the receive board may land close enough to the GPS frequency to effectively jam it - but this is only a guess.

Clock (in)stability:

The Flydog SDR uses a 125 MHz oscillator to clock the receiver (A/D converter) - but there is a problem reported by some users:  It's a terrible oscillator - and it's bad enough that it is UNSUITABLE for almost any digital modes - particularly WSPR, FT-8, and FT-4 - to name but a few unless the unit is in still air and in an enclosure that is very temperature stable.

Figure 2:
Stability of the "stock" oscillator in the Flydog at 125 MHz in "still" air, on the workbench.  The
amount of drift - which is proportional to the receive frequency - makes it marginally usable for
digital modes and is too fast/extreme to be GPS-corrected.
Click on the image for a larger version.

Figure 2, above, is an audio plot from a receiver (a Yaesu FT-817) loosely coupled and tuned to the 125 MHz oscillator on the Flydog's receive board:  Due to the loose coupling (electrical and acoustic), other signals/noises are present in the plot that are not actually from the Flydog.  The horizontal scale near the top has 10 Hz minor divisions and the red has marks along the left side of the waterfall represent 10 seconds.

From this plot we can see over the course of about half a minute the Flydog's main receiver clock moved well over 50 Hz, representing 5 Hz at 12.5 MHz or 1 Hz at 2.5 MHz.  With this type of instability, it is probably unusable for WSPR on any band above 160 meters much of the time - and it is likely only marginally usable on that band as WSPR can tolerate only a slight amount of drift, and that's only if its change occurs in about the same time as the 2 minute WSPR cycle.  The drift depicted above would cause a change of 1 Hz or more on bands 20 meters and above within the period of just a few WSPR - or FT8 - symbols, rendering it uncopiable.

"The Flydog has GPS frequency correction - won't this work?"

Unfortunately not - this drift is way too fast for that to possibly work as the GPS frequency correction works over periods of seconds. 

What to do?

While replacing the 125 MHz clock oscillator with another device (I would suggest a crystal-based oscillator rather than a MEMs-based unit owing to the former's lower jitter) is the best option, one can do a few things "on the cheap" to tame it down a bit.  While on the workbench, I determined that this instability appeared to be (pretty much) entirely temperature-related, so two strategies could be employed:

  • Increase the thermal mass of the oscillator.  With more mass, the frequency drift would be slowed - and if we can slow it down enough, large, fast swings might be slowed enough to allow the GPS frequency correction to compensate.  With a slow enough drift, the WSPR or FT-8 decoders may even be able to cope without GPS correction.
  • Thermally isolate the oscillator.  Because it's soldered to the board, this is slightly difficult so our goal would be to thermally isolate the mass attached to the oscillator.

To add thermal mass I epoxied a small (12x15mm) piece of 1.5mm thick aluminum to the top of the oscillator itself.  The dimensions were chosen to overlap the top of the oscillator while not covering the nearby voltage regulator, FPGA or A/D converter and the thickness happens to be that of a scrap piece of aluminum out of which I cut the piece:  Slightly thicker would be even better - as would it being copper.

The epoxy that I used was "JB Weld" - a metal-filled epoxy with reasonable thermal conductivity, but "normal" clear epoxy would probably have been fine:  Cyanoacrylate ("CA" or "Super" glue) is NOT recommended as it is neither a good void filler or thermal conductor.

Comment:  If one wishes to remove a glued-on piece of metal from the oscillator during experimentation, do not attempt to remove it physically as this would likely tear it from and damaging the circuit board, but slowly heat it with a soldering iron:  The adhesive should give way before the solder melts.

The "thermal isolation" part was easy:  A small piece of foam was cut to cover the piece of aluminum - taking care to avoid covering either the FPGA or the A/D converter, but because it doesn't produce much heat - and is soldered to the board itself - the piece of foam also covered the voltage regulator.

The result of these two actions may be seen in the plot below:

Figure 3:
The stability of the oscillator after the addition of the thermal mass and foam.  Still not great,
but more likely to be usable.
Click on the image for a larger version.
 
Figure 3, above, shows the result, the signal of interest being that around 680-700 Hz and again, the loose coupling resulted in other signals being present besides the 125 MHz clock.
 
Over the same 30 second period the drift was reduced to approximately 10 Hz - but more importantly, the period of the frequency shift was significantly lengthened, making it more likely that drift correction of the onboard GPS frequency stabilization and/or the WSPR/FT8 decoding algorithm would be able to cope.
 
Not mentioned thusfar is that adding a cooling fan may dramatically impact the frequency stability of the Flydog":  I did not put the test unit in an enclosure or test it with a fan blowing across it - with or without the added thermal mass and isolation - so that is territory yet to be explored.
 
Conclusion:
 
Is the Flydog SDR usable?

Out-of-the-box and unmodified:  Only marginally so.  While the issue with frequency stability is unlikely to be noticed unless you are using digital modes, the deep "notch" around 10 MHz and lower sensitivity are likely to be noticed - particularly in a side-by-side comparison with a KiwiSDR.

IF you are willing to do a bit of work (remove the components under the shield connecting the 30 MHz receiver input, modify/replace the 125 MHz oscillator) the Flydog can be a useful device, provided a bit of gain and extra filtering (particularly to remove FM broadcast signals' ingress past the low-pass filter) is appropriately applied.

Finally, it must be noted that the Flydog - like the Raspberry Kiwi (which works fine, out of the box, by the way) is a "clone" of the original KiwiSDR.  Like the Raspberry Kiwi, there are factors related to the support available to it as compared to the KiwiSDR:  The latter is - as of the time of posting - an ongoing, actively-supported project and there are benefits associated with this activity whereas with the clones, you are largely on your own in terms of software and hardware support.

For more information about this aspect, see a previous posting:  Comparing the "KiwiSDR" and "RaspberrySDR" software-defined receiver" - link.
 
Comment:
I have read that the Flydog SDR is no longer being manufactured - but a quick check of various sites will show it (or a clone) still being available.  The Flydog is easily identified by the presence of three SMA connectors (30 MHz, 50 MHz and GPS) while the more-usable Raspberry Kiwi SDR has just two and is a black case with a fan. 
Unless you absolutely must have 6 meter coverage on your Kiwi-type device (doing so effectively would be an article by itself) I would suggest seeking out and obtaining a Raspberry Kiwi - but if you don't care about 6 meters, the original KiwiSDR is definitely the way to go.
 
This page stolen from ka7oei.blogspot.com
 
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