Wednesday, November 6, 2019

Characterizing the Mini-Circuits ZFSC-4-3, ZFDC-20-3, ZFSC-4-1-BNC+ and ZFSC-2-1+ well below their designed frequency range

Figure 1:
The collection of devices to be tested - plus a few 50 ohm terminators.
Click on the image for a larger version.
Rummaging through a box of RF stuff I ran across several multi-port devices made by Mini-
Circuits Labs that I'd picked up over the years - typically at amateur radio swap meets.

The "official" specs of these devices are easy to find (at least for the newer "plus" versions) but what if, like me, one was interested in using them at frequencies below their official design specs - such as the lower amateur bands, including 80, 160, 630 and 2200 meters?  How much "extra" design margin was built into these device?

Wielding my DG8SAQ Vector Network Analyzer, I decided to find out.  For these measurements I limited the measurement range to between 10 kHz and 60 MHz.  I also built several homebrew versions to see if I could, for little cost, come up with suitable versions of my own - and these will be described in a follow-up article.

Comment:
All of the devices described here could also be used to combine signals from multiple sources.  Unless the signals being combined are "phase coherent" (e.g. from the same signal source) the insertion loss will be the same as that in splitter operation.  They will be referred to only as "splitters" in this article to minimize clutter.

Figure 2:
 Insertion loss of the ZFSC-4-3 from 10 kHz to 60 MHz.
Even though the "official" low-frequency specification is 10 MHz,
it should be quite usable on 160 meters (down to at least 1.8 MHz).
Click on the image for a larger version.
ZFSC-4-3 four-way splitter:

This device, equipped with BNC connectors, has an "official" frequency range of 10-300 MHz (the currently-offered "plus" version has the same ratings), splits the signal 4 ways with a theoretical insertion loss of 6 dB, but practically speaking, the actual loss is rated as being closer to 6.4 dB over the lower end of the design range.  Although this device - and others below - are billed as a splitters, they may be used to combine disparate signals from multiple sources onto a single line with the same amount of insertion loss.

Figure 2 shows the measured insertion loss over the range of 10 kHz to 60 MHz.

Figure 3:
 Isolation of the ZFSC-4-3 from 10 kHz to 60 MHz between ports 1 and 2.
Click on the image for a larger version.
This shows us that down to about 1.8 MHz (160 meters) that the insertion loss (blue trace) is only slightly (0.15dB) higher than the rated specs - and the Smith chart (red trace) is also reasonably well-behaved.  The next marker to the left (#3) is placed at 100 kHz and we see that the insertion loss is closer to 9 dB and that the impedance has dropped to around 12 ohms - getting worse at 20 and 10 kHz where the losses are 20dB or more and the measured impedance is only a few ohms.

What this tells us is that this device is likely to be useful down to about 500 kHz, below which point the insertion loss and impedance mismatch start to become significant - likely due to the fact that the intrinsic impedance of the ferrite devices within the splitter has dropped too low at these frequencies to remain "transparent".

Figure 4:
 Isolation of the ZFSC-4-3 from 10 kHz to 60 MHz between ports 1 and 3.
Click on the image for a larger version.
Figure 3 shows the port-to-port isolation between ports 1 and 2 and the scene is similar:  The insertion loss curve is pretty flat to about 500 kHz where it starts to vary and much below 100 kHz, the isolation seems to increase, but this correlates to the insertion losses.

Figure 4 shows the port-to-port isolation between ports 1 and 3.  This is different from that in Figure 3 because a 4-way splitter actually consists of three two-way splitters:  One to split two ways, and this path is then split two more ways with ports 1 and 2 on one splitter and 3 and 4 on another - and cross-coupling to the "other" splitter is apparently not as good at frequencies below the design.

It is worth noting that all of the above measurements are contingent on all ports "seeing" a 50 ohm source and load - either from the instrument itself doing the port-to-port measurements, or by terminating the "unused" ports (e.g. those not involved in the measurements) with known-good 50 ohm loads.  It is likely that real-world devices (antennas, receivers, amplifiers, filters) connected to any splitter will not have as good a return loss (effectively, VSWR) as a load and this will affect the isolation and apparent insertion loss.

Despite what the "official" ratings say, this device would be suitable down to at least 160 meters (1.8-2.0) MHz and likely usable through the entire AM broadcast band and, possibly, the 630 meter band.
Figure 5:

Insertion loss of the ZFSC-4-1 splitter between 10 kHz and 60 MHz.
Click on the image for a larger version.

ZFSC-4-1-BNC+ four-way splitter:

This device, also equipped with BNC connectors, has an "official" frequency range of 1-1000 MHz, splits the signal 4 ways with a theoretical insertion loss of 6 dB, but practically speaking, the actual loss is rated as being closer to 6.4 dB over the lower end of the range.

Figure 6:
 Typical port-to-port isolation of the ZFSC-4-3 from 10 kHz to 60 MHz.
Click on the image for a larger version.
As Figure 5 shows, this device does a much better job at the low end of things than the ZFSC-4-3:  At 100 kHz, the insertion loss is only slightly (0.2dB) higher than at 1.8 MHz and Marker #3 at this frequency on the Smith chart shows a reasonable (approx. 1.5:1) VSWR.  By the time one gets to 20 and 10 kHz, the VSWR and insertion loss have risen - but not as bad as that of the ZFSC-4-3.

Figure 6 shows the typical port-to-port isolation (ports 1 and 2 in this case) showing that down around 100 kHz, the isolation has dropped to about 20dB - still reasonable, and comparable to the isolation to be expected at the high end (published specs, near 1 GHz) of the design frequency range.

Clearly, if one has the amateur 2200 and 630 meter bands in mind - or one is splitting signals above about 100 kHz to feed several receivers - this is a much better choice than the ZFSC-4-3.
 
Figure 7:
 Insertion loss of the ZFSC-2-1+ two-way splitter between 10 kHz
and 60 MHz.
Click on the image for a larger version.


ZFSC-2-1+ two-way splitter:

This device, equipped with BNC connectors, has an "official" frequency range of 5-500 MHz (the "plus" version has the same ratings), splits the signal 2 ways with a theoretical insertion loss of 3 dB, but practically speaking, the actual loss is rated as being closer to 3.3 dB over the lower end of the range.

Figure 8:
Port-to-port isolation of the ZFSC-2-1+ two-way splitter between 10 kHz
and 60 MHz.
Click on the image for a larger version.
Figure 7 shows the measured insertion loss and surprisingly, it looks quite good down to 100 kHz - probably due, in part, to the fact that unlike the four-way splitters, there is likely only a single ferrite device contained within to incur losses at the low end where it "runs out" of inductance on the transformer.  Down at 20 kHz the loss has gone up by about 2dB and the impedance is in the area of 20 ohms, but this device may still be fairly usable in some applications.

Figure 8 shows the port-to-port isolation and this remains above 20dB down to about 250 kHz, quickly dropping to about 14dB at 100 kHz.

What this tells us is that this device is still likely to be usable down to 100 kHz if one is able to tolerate a couple of extra dB of loss and only mediocre isolation.

Figure 9:
Insertion loss of the ZFDC-20-3 20 dB coupler from 10 kHz
to 60 MHz.  Because the "Couple" port is pulling a slight amount of
energy from the through line, a small amount of insertion loss
is to be expected.
Click on the image for a larger version.
ZFDC-20-3 20dB directional coupler:

This device is not a splitter, but rather a device designed to directionally "siphon" a small amount of signal from the "through" line - but do this only do this in one direction.  This device is typically used to sample (with 20dB of attenuation) a signal on a given line, or if turned around to couple in the opposite direction it can insert a signal on this same line.  A common application of this device is to measure return loss (or VSWRm using a pair of these devices), allow non-intrusive monitoring of signals on a cable and it can be used to insert a signal on that same line - say for receiver sensitivity testing - on a cable that cannot be interrupted.  Unlike a splitter, connecting/disconnecting a device on the "Couple" port will have a very small effect on the through-signal.

Figure 10:
Forward coupling loss of the ZFDC-20-3 20 dB coupler from 10 kHz
to 60 MHz.
Click on the image for a larger version.


The "official" specs of the ZFDC-20-3 indicate a frequency range of 200 kHz to 250 MHz, but one can see in Figure 9 that the insertion loss is well below 1 dB down to around 20 kHz - although the VSWR at this frequency climbs to nearly 3:1:  At 50 kHz, the insertion loss is still only about 0.25dB and the VSWR is about 1.5:1 - still within the usable range for applications that can tolerate a small amount of degradation.

On the sample port we can see on Figure 10 that the coupling is ruler-flat down to at least 100 kHz and still staying within 1dB of the nominal value down to 10 kHz - but one should keep in mind the fact that the insertion loss and the varying impedance will likely affect the through-line's signals below around 50 kHz.
Figure 11:
Reverse coupling loss of the ZFDC-20-3 20 dB coupler from 10 kHz
to 60 MHz.  Because the coupling is 20dB in the forward direction,
the attenuation values depicted in the above graph should be reduced
by that amount.  The "bump" at the extreme low end is an artifact
of the configuration of the test instrument.
Click on the image for a larger version.

Figure 11 shows the "reverse" coupling loss (e.g. "directionality").  Ideally, no signal should be detectable when the "load" is a perfect, non-reflective 50 ohms but due to imperfections in the load, device, cabling and measurement will reduce this.

This shows that the absolute directionality+coupling exceeds about 60dB (about 40dB of directivity compared to the "forward" coupling) at all frequencies below 60 MHz down to about 20 kHz:  Values below about 70dB (the "floor" between 20 kHz and 10 MHz) are representative of the limits of the test instrument and its configuration so they may actually be greater than this.  Below about 15 kHz, the "bump" is mostly due to measurement artifacts - but this still indicates that the relative directionality is at least 30dB.

These measurements indicate that this device is usable down to 50 kHz with only minor degradation, and would probably work down to 25 kHz in applications where one can tolerate a bit of extra insertion and return loss.

Final comment about the Mini-Circuits devices:

In reviewing the above tests, it would appear that these Mini-Circuits four-way splitters and the directional coupler are generally useful down to about 1/10th of their "official" low frequency rating and that down to 1/5th of their low-frequency rating, they more or less meet their "official" specs.


Follow-up article to come:

I have built several homebrew versions of 2 and 4 way splitters to see if I could, for little cost, come up with suitable versions of my own - and these will be described in a follow-up article.

* * *

Stolen from ka7oei.blogspot.com


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