Tuesday, September 28, 2010

N2PK VNA Transverter options

The N2PK VNA frequency range can be extended by using transverters. While the N2PK VNA covers the entire 0.05 to 60 MHz, additional frequencies are covered only on specific bands. Paul, N2PK and Ivan, VE3IVM developed two transverters in order to cover the 2m and 70 cm amateur bands.

Both transverters share exactly the same design and PCB with only a few differences in the BOM - the frequency of the Local Oscillator (180 MHz for the 2m version and 400 MHz for 70cm) and the the components of the Band Pass Filter (different set of components for each filter, changing their frequency range).
I started with the 2m transverter kit from Ivan, VE3IVM. It was an easy and pleasant build - Ivan provided a partial kit - PCB, LO, BPF coils, balun transformer for the bridge, both mixers and a power splitter- the rest of the BOM is from Digikey.

The top side of the PCB. The 4 coils of the BPF are shielded, each in its own compartment. It is important that they are centered and lifted from the ground plane in order to increase the Q and minimize losses. The other components are the LO, both mixers, RA SMA connectors and the voltage regulators - +6V and +5V. The most time consuming task here is to make and install the shield for the BPF coils - a certain level of precision is required while fabricating the RF shield as the tolerances are not very large. The top cover is soldered during the finalization stage. I used tin-plated brass sheet to make all RF shields - this material is very easy to form and solder.

Most of the components are installed on the bottom side. Again, a lot of attention is required when building the BPF and the bottom shield for it. After adjusting the BPF, the bottom shield is installed and the filter is checked and re-aligned again as the shield might de-tune it a little. The attenuators are pretty small and should be soldered very carefully. Same thing goes for the 100 Ohm 0604 bridge resistors. The two amplifiers should be soldered carefully to avoid over-heating. Ivan provides specific instructions for installing the variable caps - no flux (or very minimal) should be used and the board must be washed with alcohol or flux remover to avoid mechanical damage to the trimmer capacitors. This picture shows an incomplete and bypassed Low-Pass filter on the low-side output. The jumper is removed and the rest of the components are installed during the finalization stage and after final alignment of the BPF.

This is a plot of the BPF frequency response from the initial alignment of the filter. I was aiming at 143 MHz - 153 MHz range. The BPF filter design allows for almost 10 MHz bandwidth in the 2m band and 20 MHz at 70cm. It can be moved a few MHz up or down from this general range if needed just by adjusting the variable BPF capacitors. During the alignment, it is important to achieve as flat as possible band-pass response and proper shape of the skirts. MyVNA really simplifies the use of transverters and takes care of all calculations - the user must enter the LO frequency and frequency range for the high side and myVNA translates this to the VNA's HF working range, while making a plot in the transverter's frequency range.

This picture shows the 70 cm transverter. The most visible difference between the two transverters is the configuration of the band-pass filter. For instance, on the 2m transverter there are some BPF capacitors mounted on the top side just under each coil - they should be installed before the coils are soldered. These are omitted in the 70cm version.

The BPF frequency response of my 70cm transverter. The insertion loss at 70cm is higher then the one of the 2m transverter at about -13 db. It can be lowered to up to -6 dB if some of the bandwidth is sacrificed but I wanted to cover the whole 430 - 450 MHz range.

The finalized PCB. Shown here is the completed Low-Pass filter and the RF shielding of the BPF. As Ivan, VE3IVM noted in his build instructions, after installing the BPF shields they will slightly de-tune the filter. As expected, I had to re-align it again. A fence-type shield was installed to divide the up-convert and down-convert paths in order to reduce stray coupling and cross-talk.

Almost done. I installed a toroid choke on the power line. Ivan has designed the PCB to fit in a compact extruded aluminum box - Hammond 1455C801. The board fills up the space with very little room to spare - just enough for the RF choke. It was a bit tricky to drill the holes for the SMA connectors in both panels so they match perfectly the RA connectors - the board has a small horizontal play which helps it a little. A small piece of conductive RFI gasket material (neoprene covered with metalized fabric) is pinched between the base of the RA SMA conectors and the front aluminum faceplate in order to improve grounding of the aluminum enclosure.

This is the front panel. The labels on the top row are for Transmission mode measurements and the ones on the bottom - for Reflection. I made the small jumper (needed to configure the transverter for Reflection measurements) out of semi-rigid hand-formable RG-405 coax and used a spacer made from a strip FR4 board to fix the distance between the connectors.. The Hammond box is assembled in a way so the plastic trim for the front and rear panels is on the outside (instead of the "normal" way - between the enclosure and the panel plate). This improves the RF shielding as the end panels are in direct electrical contact with the enclosure - there is no gap.

The rear panel and power cable. The Hirose connector for the power cable is the same type that I am using with my RF-IV test head. The accessories connector on the VNA's back panel provides +9V DC and I can power the RF-IV or the transverter directly from there. Two small semi-rigid coax jumpers with Male-BNC to Male-SMA are used to attach the transverter to the VNA's ports. Each of the transverter's ports has a Male-SMA-to-Female-SMA port saver installed (not shown).

Friday, September 17, 2010

3.5mm VNA Calibration Standards

Lately, I've been obsessing with calibration standards. I know that it is an overkill to use lab-grade calibration kit rated for up to 26 GHz with a home-brewed 60 MHz VNA, but the commercial calibration kits are well documented and come with cal reference definitions (data about offset delay, stray C for the OPEN, stray L for the SHORT, etc. The data is stored on a disk and normally loaded in the VNA but it will give me chance to properly measure/characterize my DIY calibration kit. This way I can have the proper constants for my DIY standards entered in MyVNA software. It will also help me to match the calibration plane with the port connector mating plane and measure other connectors or adapters.
I picked up a nice calibration kit on eBay for a very reasonable price (these kits are normally VERY, VERY expensive, but I guess I was lucky this time). The kit is equipped with 3.5 mm connector. This type connector was developed by Hewlett-Packard and it is used in lab instruments for microwave measurements. The 3.5mm connector is using air dielectric and it is machined to extremely tight tolerances. It is rugged (compared to the SMA), allowing thousands of connect-disconnect cycles with repeatable results. The beauty is that it mates with standard SMA connectors. On the flip side - it is a precision lab grade connector and one should be extra careful when handling and using - it could be easily damaged when mated with low-quality SMAs or by poor mating technique.

The geometry and construction of the 3.5 mm connector provides mode-free operation up to 34 GHz. My kit is actually made of two different sets - the male cal standards are rated for up to 26 GHz and the female for up to 9 GHz. Such kit requires proper care and storage. The male pin and female receptacle are supported only in the back of the connector (air dielectric) and can be damaged by rough handling/cleaning. Enemy #1 of these connectors are low-quality or already damaged SMAs. When mated with such, a damage is almost certain to occur.

"DO NOT ROTATE"! This is very important rule when the 3.5 mm calibration connector is mated! The cal standard must be held / clamped in a way to prevent any rotation of the whole cal standard body - only the nut should be rotated. This is especially important with the female connector! The pin receptacle of the female connector is EXTREMELY DELICATE!

The female pin receptacle under 10x magnification. Note the 6 contact fingers, lining the inside wall of the pin receptacle. They are barely visible on the picture under 10x magnification and even people with very good eyesight could not always see with a naked eye if there is a damage. When the body of the connector is rotated during mating or any of these contact fingers gets caught by a burr on a low-quality male SMA center pin, the whole calibration standard becomes a very expensive paper-weight. A center pin, protruding too much, pin with out of specs diameter, misaligned pin, improper cleaning attempt are just some of the things that can cause irreversible damage to the female receptacle. The male pin of the 3.5mm is more rugged but still requires a lot of attention when mated or cleaned.

Tuesday, September 14, 2010

VNA Calibration Reference Plane

Having different types of calibration standards helps to cancel out the measurement error introduced by the use of connector adapters. If the DUT has a port type different from the one of the VNA, the use of "between series" connector adapters is inevitable. Using a calibration standard which mates with such adapter will move the calibration reference plane at (or close to) the DUT, "absorbing" the adapter in the calibration process. Speaking of calibration reference plane - the concept is very simple - the reference plane is the plane which divides the VNA-DUT system. Everything "behind" the plane belongs to the instrument and will be taken out of the measurements. Calibration is performed at the plane. Everything at or "in front" of the plane is treated as DUT and it will be measured. Often, calibration device can not be placed exactly at a specific plane inside the VNA's port connector or test fixture due to the standard's physical size and/or need for a connector. There is an electrical length associated with the cal standard (pin / mating plane to actual OSL plane) which will cause a small delay and phase angle error (such standards are called an "offset" type). To account for this phase shift, commercial calibration kits come with a floppy disk containing correction data for the offset among other parameters (for instance - the stray capacitance for an OPEN or the stray inductance for a SHORT). After calibration is performed and the corrections are applied to the calibration model, the reference plane is "moved back" and located at the port's mating plane. For this very reason, when a calibration standard is measured as DUT with already calibrated VNA, it doesn't look "perfect" - the VNA shows the standard's internal phase offset and strays.
An excellent technical article about calibration standard definitions by William Highton (Chemandy Electronics) can be found here.

The main reason for matching the calibration reference plane and the mating plane of the VNA test port connector is to allow industrial VNAs to measure whole assemblies and devices, including their connectors. In the N2PK VNA (at least with the original software) the reference plane is assumed to be where the physical OSL calibration takes place. Instead of adding negative offset to move the reference plane back to the mating surface of the VNA's test port connector, it is accepted that the plane is right at the rear surface of the calibration standard connector (the location of the actual Open-Short-Load). The error introduced by the electrical length of the pin inside the connector is not that great at lower frequencies - for instance, assuming teflon dielectric (VF ~70%) in a standard SMA connector used for a home-brewed calibration standard, an offset of 5 mm will yield about 0.5 degree phase error @60 MHz. Commercial calibration standards use air dielectric, but this puts greater mechanical requirements during the fabrication process. By calibrating the VNA so the reference plane is on the back of the cal connector and fully exposed, it is more convenient to measure a single component but it means that the DUT test fixture should be made of (or at least include) the same type connector as the one used for the home-brewed calibration standards in order to maximize the accuracy. The setup can be as simple as the component (DUT) is just soldered on the back of the "test fixture" connector, while measures are taken to minimize external error (minimal lead length). With SMDs, this approach works well as the device is practically located directly at the "reference plane" when the test fixture connector is from the same batch connectors used to make the OSL standards. Another method for calibration is to use the test fixture connector itself for calibration - for OPEN the DUT is simply omitted, SHORT and LOAD are created on the spot during each calibration step and finally the DUT is soldered (Still, stray C for the OPEN must be known)