N2PK Vector Network AnalyzerAn inexpensive Vector Network Analyzer of a Lab quality
Recommendations and tips on building the VNA TransverterThis page has some useful information as how to build the VNA Transverter for both UHF and VHF options. The Transverter was designed with SMT technology in mind. Some of the components, especially in the BPF section, can be hard to mount if a regular soldering iron is used. It is strongly recommended using an SMT hot air soldering station and SMT soldering paste for this build. Type, make or model of a hot air station is not important. As long as it produces hot air, that is all what is needed.
1. Solder the voltage regulators and their assossoated parts.
2. Prepare the shield for the BPF. Use a tin, copper or brass sheet to make a 110x7mm strip. Bend the strip to form the shape shown in the picture and cut off the excess.
|Each time you make a bend, try the shied on the PCB to maintain tolerances and make necessary corrections for the next bend. When done, check if the shield bottom sits flat on the PCB, and if it does not, try to correct by light twisting. Then place a sheet of sandpaper on a flat surface and level out the bottom of the shield.|
|Moving the coils away from the ground plane reduces coupling loss and inreases Q. For same reason, when soldering a coil, try to center it in the shield section so that it equally spaced from the shield walls.|
5. Flip the board and solder the bottom PCB parts except the BPF trimmer capacitors and output filter parts between J2 and U5. Solder a temporary jumper from J2 to the right pad of L3, connecting J2 to the U5 mixer output. Use a solid wire for the jumper. I used a lead cut off from an axial 0.125W resistor.
The PCB layout can accomodate both 70cm and 2m BPF. But there is a difference in topology. When building the 2m BPF, refer to the picture below. Note that there is no some inter-stage capacitors to the ground for the 2m BPF and a couple of pads are jumpered. Follow the proposed component layout below to solder the 2m BPF components. Also, there are 3pF capacitors in parallel to each filter stage not shown in the picture - they are soldered on the other side of the PCB, where the coils are.
6. Solder the BPF Trimmer capacitors. Check with the white silk outlines to position the capacitors properly, or you may end up with the rotor and stator swapped. The rotor has to be soldered to the grounded side.
ATTENTION: Use minimum amount of flux when soldering the trimming caps. Do not use SMT soldering paste on them. Excessive flux or flux from SMT paste will alsmost certainly glue the caps' rotors to stators. Then when you try to trim a cap, the slotted piece will break and come off the rotor, and the cap will need to be replaced. After you solder the trimmer caps, use flux cleaner or alcohol to remove all flux residue from them.
7. Solder the RF connectors if you use them. Before soldering try the connectors on the PCB and cut the excess of the legs and center pin off. Leave just enough length to be soldered. At frequencies that high any fraction of a millimeter will radiate contributing to stray coupling.
8. Solder a jumper between two pads located to the left from the J5 center pin.
9. Solder U4 and U5 mixers. At last, solder the Oscillator. Shorten the mixers and LO pins as you did to the RF connectors. Ideally, only small blobs of solder should be left after soldering. No excess should protrude above the PCB plane.
Because on the first step we will loop the Transverter's up- and downconverters with a coaxial jumper, the BPF can be adjusted on the low side using an HF (60...100MHz upper limit) spectrum analyzer with a tracking generator, sweep generator and oscilloscope, DDS wobbler, HF network analyzer, or the N2PK VNA itself.
If the N2PK VNA is used, set it to the fastest sampling rate and use a software that will do repetitive scans such as Paul N2PK's software for DOS, or Dave G8KBB's myVNA for Windows. Still, the best method is a spectrum analyzer with a tracking generator. It has a minimal delay between scans and a convenient detector screen.
If myVNA program is used, switch the program to the fastest ADC speed and calibrate the VNA in Transmission mode from 10MHz to 60MHz and 200 points. After calibration, set myVNA to 50 points for rough tuning. When the BPF gets closer to the proper shape, you can increase number of points to 100...200 for fine tuning.
1. Connect a 50 Ohm load to J4. Connect a coaxial jumper between J3 and J5.
2. Power the Transverter up and check +5 and +6 voltage levels on the test points in the middle of the PCB bottom (the trimmer caps side). Allow to warm up for 20 minutes.
3. Connect the signal source (0...+5dBm) to J1, and the detector to J2. Set the appropriate frequency sweep range. The 70cm Transverter has a 400MHz LO, so the sweeper frequency could be set from 20 to 60MHz. After mixing with the LO the BPF input will see the lower sideband 380...340MHz and upper sideband 420...460MHz.
4. Adjust the BPF trimming capacitors so that the detector connected to J2 shows the BPF tuned to 30...50MHz. That means on the high side of the Transverter you tuned it to 430...450MHz.
5. Disconnect the signal source and detector. Power the Transveretr off. This completes the BPF adjustment procedure.
|Scans of the 70cm BPF using myVNA and a spectrum analyzer. Frequency scale is 10...60MHz. When using myVNA, set the markers to the BPF center (40MHz) and cutoff frequencies 30 and 50MHz, then ajust the BPF for the left and right markers showing -3dB difference with the center marker. Try to produce a curve with flat top the best you can.|
For the best performance all critical sections of the Transverter have to be sheilded. The board does not suffer much from coupling when in the open on the bench, but when it goes into the enclosure box, level of coupling increases. Shielding helps keeping it low.
1. Make a lid for the BPF shield and drill a couple of small holes in the lid to allow for air circulation. Solder the lid to the BPF shield.
2. Flip the board. Make a small shielding box to enclose the BPF trimming capacitors. Drill four holes for the adjustment tool before soldering to the PCB.
3. Make a divider wall between the up- and downconveters. Scrap the solder mask off in a few spots along the PCB to solder this wall.
4. Make an additional shield to isolate U8 output and J5 from the rest of the circuit. The center pin of J5 works as a tiny antenna even when trimmed as was recommended in the Assembling section, so it needs to be isolated from the downconverter path.
5. Power the Transverter up and re-check the BPF adjustment. There is a good chance the BPF got detuned a little because of placing the top lid and trimming caps shields. Re-adjust if needed.
6. Power it off and disconnect everything. Remove the jumper between J2 and U5.
7. Solder the downconverter output lowpass filter C1...C5, L1..L3.
8. Mount the board in the enclosure and complete necessary interface and power wiring. To reduce coupling, any wiring should be placed on the voltage regulators side of the board. Keep the bottom side free of anything under it.