N2PK Vector Network AnalyzerAn inexpensive Vector Network Analyzer of a Lab quality
VNA v4.x dual detector board construction hints and tips
This page has some info specific to the Dual detector VNA v4.1, v4.2, and v4.3 boards, as well as links to the VNA schematics and Bill of Materials. I will be updating it as new information becomes available, but on this page it will be presented in no particular order.
Changes to the schematics - There was little change compare to the original dual detector VNA schematics from Paul N2PK, and it was done to the LO DDS second output (pin 21 of U120), which received its own anti-aliasing filter. The filter circuit is exactly the same as for the original filter connected to pin 20. The elements of the additional filter were assigned the "a" index (i.e. L182 of the first filter becomes L182a in the second one). This results in J120 and J180 having the LO DDS filtered signals of same amplitude and opposite phase. These signals then reach the detectors LO inputs via the onboard traces or the SMA connectors and coaxial jumpers. The filter circuit details are shown below. Other DDS pin connections are not shown and did not change.
Update May 2008: A new anti-aliasing filter topology was introduced - please make sure you read the "New anti-aliasing filter design" section below on this page.
Changes to the components part number - The fast detector additional passive components, introduced by the author, in this layout have 0805 footprint . Because of that theirs part numbers are different from those of indicated in the above document. This will be reflected in the Bill of Materials.
Components placement - download the Assembly Drawings in PDF format. Download high-resolution pictures (courtesy of Paul, N2PK) of an unpopulated v4.1 board top and bottom. The high-res picture of the top of a v4.1populated board is also available. Because of high density of the components per square inch, it was extremely hard to arrange the components' designators in a reasonable manner. There are restrictions at the manufacturing side on the font size and line width. Also the component print layer must not overlay the SMD pads, or you will have problems soldering. Therefore, when working around the crowded spots, treat the designators only as a rough reference to the component's location on the board. Be careful and refer to the schematics to trace the actual component's place. This is true for the detectors in particular. As an advice, I can recommend you to print out the assembly drawings, and do your homework before you start soldering. Go through the tough spots, refer to the schematics, and draw arrow lines from the designators to their actual components. This will save you time when soldering. One of the builders has done a good job and created a drawing for the bottom of the v4.1 PCB with markings between the silk print and the PCB pads. Download it to make your life easier.
The assembly drawings package above also includes a GIF image of the Reflection Bridge PCB to assist you in soldering the bridge components to the right place, since the bridge PCB does not have the silk component print.
The Bill of Materials (BOM) - The Parts List in MS Excel format with the DigiKey part numbers. The file has two BOMs, one for v4.1, another for v4.2 board. Simply go to DigiKey and build your order by cut and paste. Note that some parts, such as SMD resistors and/or capacitors may only be available in packages of 10. Therefore, to have your single 0.1% precision resistor, you'd need to pay for 10. Let me warn you - do not substitute the precision resistors part numbers for anything else, or you will be looking for troubles. Note that some parts such as Minicircuits transformers and Valpey-Fisher master oscillator are not available at DigiKey. The former is obtainable from Minicircuits directly, the latter - through Yahoo N2PK message board periodic group orders.
One of the techniques for the manual SMD assembly is to start at the center of the board, then populate it evenly from the center towards the board edges in circular manner. I think that could reduce thermal stress on the PCB and, as a result, reduce the board warpage. I personally did not try this techniques, but you can give it a try.
When assembling the reflection bridge (and VNA board), trim the transformers leads as in the picture below, where shown original and modified parts.
Here is what you get once you complete the assembly of the bridge PCB. A beautiful piece of your instrument hardware.
A close-up of the soldering area (picture below) showing the proper location of the parts on the PCB. Note orientation of the transformer. For some weird reason Minicircuit marks with a dot not the first pin, but the last one (pin 6). They also win my black rating being the worst company after Valpey-Fisher people in communication with the customers ( Valpey-Fisher is the company manufacturing the master oscillators for the VNA).
If you are planning to install the bridge board in the main VNA enclosure as oppose to the separate one, the edge SMA connectors can be also used, as in the picture below.
I do not know what I was thinking of when I soldered these SMAs. Do not make this mistake. The SMA connectors and the SIP connectors are supposed to be mounted on the bottom side of the board. This is to reduce influence of stray capacitance introduced by the wires and pieces of coax on the elements located on the top side of the board. Additionally, the SIP connectors won't fit if tried to be mounted on the top side of the PCB.
To solder the SMA connectors, use a bigger soldering iron. Since the SMA legs get soldered to the ground plane, you'd need big thermal mass to heat them relatively quickly, or there is risk the ground plane copper will bubble from overheating. I use a 100W soldering iron, and I heat the legs first, then I touch the area around the legs. The solder melts nicely after that and fills the solder pads.
Launching your v4.1 board
Once you assemble your board, and before you apply the power, check the power lines with an ohmmeter. First, check for a no-short condition at the PCB test points marked +5A, +5D, +5D2, -11V, -5V, +5V, +2.5V. There was no room to place +2.5V test point for the second detector, so you have to probe the right end of C461 capacitor. Next, probe +5V and +12V inputs at J100, J300, J220, J420 for no shorts to the ground.
With the power supply still turned off, connect the power receptacles to the above board connectors. Trace with the ohmmeter that +5V and +12V lines from the power supply go to the right pins on the PCB, and there is no +5 and +12V lines shorts to the ground. Now disconnect all receptacles, leave only J100 connected. Apply +5V to it. Control the voltage readings with a voltmeter. Check +5A and +5D testpoints for +5V. This will supply power the DDS part of the board, and the first detector parallel port buffer IC. Turn the power supply off and connect J300. Apply +5V again and check +5D2 test point in the second detector area above the second detector parallel port buffer IC. Once completed, your +5V bus is OK.
Turn the power off before moving to the Detector 1 and 2 +12V connectors. Connect J220, double check again for no short of +12V to the ground, and apply +12V. Check -11V test point, it will not necessarily show exactly -11V, but rather the negated value of your +12V supply reduced by 0.4V. With exact +12V input for example the testpoint will show -11.6V. Now check -5V and +5V testpoints located to the left of T210. Once OK-ed that, move to the bottom of the PCB and check +2.5V reference voltage test point located to the left of J211. This completes verification of the First detector supply voltages.
Turn the power supply off. Connect J420 and repeat the no-short check on +12V line. Apply the power and check -11V test point in the Detector 2 area. Same rule applies to its value as for the Detector 1. Check -5V and +5V testpoints to the left of T410. Move to the bottom of the PCB to U450 and T411, and probe the right end of C461 capacitor for +2.5V of the reference voltage produced by U450. There is no formal test point pad on the board for this one since there was no space on the PCB available for it. This completes the Second detector voltages check.
Now, when your board has all voltages generated correctly, you can connect the first detector's signal connector J160 to the parallel port (or USB convertor, if you have one) connector, and try one of the available VNA programs to talk to the VNA. Make sure you can do this before you try the second detector. Once your Detector 1 supplies the data to the VNA program, connect J360, run the program again and make sure it sees the Detector 2.
To verify the DDS functionality, hook up a counter or a scope to J170 (RF DDS), J120 or J180 (LO DDS). Run the software and watch the signal changed once you vary it from the software. You may also want to check the master oscillator buffer IC at J150, which outputs the signal at the master oscillator frequency. Refer to the VNA schematics for the values of the output signals.
Before starting probing the VNA functionality in either reflection or transmission modes with one of the available programs, connect the LO DDS outputs to the detectors inputs with the coaxial SMA male-male jumpers from J120 to J410, and from J180 to J210. Alternatively, you can make use of the on-board microstrip lines by soldering the corresponding jumpers.
Board revision v4.2 details
In March 2008 the PCB was upgraded to v4.2 revision number. The main reason for upgrading the board version was adding a land pattern for the alternative Master Oscillator (Connor-Winfield CWX823), available from DigiKey. Introducing this oscillator eliminated the need in group buys that were conducted for the Valpey Fisher MO, and long waiting times. The schematics change was this:
The board layout was modified in a way it could take either Valpey Fisher or Connor- Winfield MO, which gives much flexibility. Below is a picture of the MO area on the v4.2 PCB, showing the Connor Winfield oscillator and its assossiated components mounted.
Refer to this picture when mounting the Connor-Winfield oscillator. This is a 3.3V part, so it needs a 3.3V voltage regulator U180 and some blocking capacitors. Resistors R151 and R153 provide bias voltage for proper leveling of the output clock signal. All necessary part numbers are specified in the v4.2 BOM. Note that as was stated above, the Valpey Fisher oscillator also can be used with v4.2 PCB, and simply mounts over the extra land pads. Note the notch in the white silk otline of the VP part just below the U180 designator. This notch shows the proper position of the MO if the VP oscillator is used. In this case none of the parts shown with the red designators should be soldered - refer to the v4.2 BOM for details.
Other changes in the layout include removal of the MO buffer AD8041 IC and its assossiated components (and one SMA connector), connecting the grounds in the second Detector ADC area, removal of the on-board traces between the LO DDS and Detectors, and adding ground strips around the perimeters of the Detectors. These strips are to facilitate soldering of the grounding shields to improve Detectors isolation.
New anti-aliasing filter design
A new filter design for the RF and LO DDS was proposed by Paul N2PK. There are changes in both topology and component values. The new filter has an extended cut-off frequency just above 60MHz, better return loss, and better DDS images supression. Schematic below shows the RF DDS filter section. Introduced are two new 12pF capacitors C176, C177 in the RF DDS section, C186, C187, C186a, C187a in the LO DDS sections. The inductors values changed from 220nH to 150nH, and all existing capacitor values changed as well. The LO DDS filters have exactly same topology and component values (but different designators) as the one shown below.
The new filter becomes more advantageous on v4.2 board when used with the Connor Winfield 156.25MHz master oscillator introduced in the previous section, because of extended upper frequency bound of the DDS chips. However, it can also be used with any other board version to improve matching and images filtering. The Bill of Materials has now been updated to reflect this change.
Implementing the new filter on v4.1 and v4.2 boards: The two added capacitors have to be soldered before assossiated inductors. The capacitors are of 1206 type and fit the inductor's PCB pads. Then solder the inductors on top of the capacitors. Again, this is only for the two added capacitors. All other components are placed and soldered as usual. Note that if you already have the old filters soldered, all of the components have to be removed and replaced with the new values. Refer to the BOM for the new part numbers.
Board revision v4.3 details
Beginning mid-June 2008 a new board v4.3 is shipped. This board layout incorporates all changes necessary for the new anti-aliasing filter. All capacitors mentioned above have now their own landing pads. The updated BOM has now all capacitors of 0805 type.
Board revision v5.0 details
Beginning end of June 2009 a new board v5.0 is shipped. This board has an on-board USB interface and can only work with N2PK VNA software that supports USB interface. For details on the new board v5.0 click here.
Please feel free to direct your questions or comments to the email address below.