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By Charles Kosina If you have multiple test instruments and one very accurate frequency reference, you need a way to feed that reference signal to each test instrument without attenuating or degrading the signal. That’s precisely what this device does. It has one input and six outputs, and while it’s designed with a 10MHz reference in mind, it can handle other frequencies too. Frequency Reference Signal Distributor T his design was prompted by a ham radio friend who has a GPS-disciplined 10MHz frequency reference and needs to feed its output to several different pieces of equipment. This means that not only are they operating with maximum accuracy (those with internal references aren’t always spot-on), but they are also in lockstep. siliconchip.com.au A typical 10MHz reference signal generator has only the one output, and this cannot easily be fed to more than one device. You can’t just use a Y-cable since it will then have a 25Ω (or lower) load rather than a 50Ω load, which would certainly reduce the signal level and might also overload the generator and cause other problems. You really want a +10dBm (0.7V RMS) reference signal when terminatAustralia’s electronics magazine ed 50Ω at the reference input of each instrument. I decided on a design that will provide six such outputs. In principle, it is elementary. It comprises just six high-bandwidth op-amps feeding the outputs through broadband HF transformers, giving six fully isolated and buffered outputs. Circuit design Fig.1 shows the circuit design. April 2020  77 pot connects to a +3.5V half supply DC bias source via a 39Ω resistor. The bottom of the resistor is bypassed to ground, so the input impedance is 139Ω (100Ω+39Ω). This is a little higher than the 50Ω or The incoming reference signal is fed via BNC connector CON1 and pin header CON2 onto the board. It is then AC-coupled to VR1, a 100Ω trimpot which is used to adjust the output level. The bottom end of the D1 1N4004 CON3 CON10 +12V A REG1 7805 IN K +7V OUT GND 0V 75Ω that most generators are designed to drive, but the VSWR on the short run of coax from the generator will not be significant, so this should not cause any problems. If anything, this means that the Distributor gets a signal with a 470 1.2k 10 F +3.5V 10 F 180 1.2k 2.7k A  100nF IC1–IC6: MAX4450 3 POWER LED1 4 K 5 IC1 1 +7V 100nF OUTPUT 1 (BNC) 51 T1 CON4 2 180 560 +7V 100nF INPUT (BNC) TP CON1 CON2 100nF 100nF 3 VR1 100 4 5 IC2 1 OUTPUT 2 (BNC) 100nF 51 T2 CON5 2 180 39 +3.5V 560 100nF 100nF 100nF 3 ALTERNATIVE TO USING POTENTIOMETER 68 4 5 IC3 560 A 3 K 4 1 2 3 K A T3 +7V GND IN GND 5 IC4 1 OUTPUT 4 (BNC) 100nF 51 T4 OUT 180 560 2020 CON7 +7V 100nF 100nF SC  CON6 2 7805 LED 4 51 100nF 100nF 5 100nF 100nF +3.5V MAX4450 1 OUTPUT 3 (BNC) 2 180 39 1N4004 +7V 100nF SIGNAL DISTRIBUTOR 3 4 5 IC5 1 OUTPUT 5 (BNC) 100nF 51 T5 CON8 2 180 560 +7V 100nF Fig.1: the circuit of the Signal Distributor is relatively simple. The incoming signal is AC-coupled to trimpot VR1 for level adjustment, then fed to six four-times op amp gain stages based on IC1-IC6. These each drive 1:1 RF transformers via 51Ω Ω resistors, which in turn drive the fully isolated outputs. REG1 provides a 7V supply for the op amps. A half-supply rail to bias the signal fed to the op amps is present at the junction of two 1.2kΩ Ω resistors in series across the 7V supply. 78 Silicon Chip 100nF 3 4 5 IC6 1 OUTPUT 6 (BNC) 100nF 51 T6 CON9 2 180 560 100nF Australia’s electronics magazine siliconchip.com.au slightly higher amplitude, so less gain is required to achieve +10dBm. The +3.5V half supply rail is simply derived from the regulated 7V supply rail via a 1.2kΩ/1.2kΩ resistive divider. The 100nF bypass capacitor to ground attenuates any supply noise which makes its way through the regulator and this divider, so it doesn’t affect the signal. The signal is then fed to the six op amp non-inverting inputs (pins 3 of IC1-IC6), which are all connected in parallel. For the op amps, I decided to use MAX4450s which each have a gain bandwidth of 210MHz. So for a 10MHz signal, the open-loop gain is about 21 times. They are configured as non-inverting amplifiers and the 560Ω/180Ω feedback resistors give a gain of about four times. The bottom end of each feedback divider connects to ground via a 100nF capacitor. The feedback network cannot be connected directly to ground due to the +3.5V DC signal bias, and also cannot connect to the +3.5V reference since it is unbuffered and thus has a high source impedance (600Ω). Each op amp has a 100nF supply bypass capacitor for stability. Their outputs are capacitively coupled to six Coilcraft 1:1 broadband transformers, T1-T6. A 51Ω series resistor sets the source impedance for the transformer drive close to the required 50Ω. The six BNC output connectors are isolated from ground; they are grounded by the instrument being fed, eliminating the possibility of any Earth loops. The transformers have a 50Ω output impedance, suiting virtually all device reference inputs. IC1-IC6 have a supply voltage range of 4.5-11V; I am using 7V as this gives enough headroom for the required output voltage swing. This is supplied by REG1, a 5V fixed regulator which has its output voltage raised to 7V by a 470Ω/180Ω voltage divider between its output and GND pins and circuit ground. The 7V rail also supplies around 2mA to power indicator LED1 via a 2.7kΩ currentlimiting resistor. REG1’s output is filtered by a 10µF capacitor, and its input is similarly bypassed. It is supplied with around 12V DC via header CON3 and reverse polarity protection diode D1. CON3 can be wired to a chassis-mounted DC barrel socket. siliconchip.com.au Fig.2: the scope grab of the signal from one of the unit’s outputs shows an amplitude of 2.18V peak-to-peak, which is just over +10dBm. And as you can see, the frequency is reading exactly 10.00MHz. Fig.3: the scope was also used to produce this spectrum analysis of the output waveforms, which demonstrates that harmonic distortion is low, with the first three harmonics all well below -40dB. Note that the circuit shows that you can replace trimpot VR1 with a 68Ω SMD resistor if you don’t need to be able to set the gain exactly. We won’t go into any more details about this option (and that part is not in the parts list), so if you want to build it that way, check out our board photos as that is how the prototype was built. x 1.6mm/imperial 1206) sizes which are quite easy to solder. The MAX4450 op amps are tiny chips as they only come in SOT-23-5 packages, so they require special care in assembly, but those with SMD assembly experience should be able to manage them with no real difficulties. PCB design The signal from the GPS-disciplined oscillator is a clean sinewave of 2.9V peak-to-peak (about 1V RMS or +13dBm). Its second harmonic is at -40dB, the third harmonic at -50dB and it has no significant higher harmonics. The outputs from the Distributor into 50Ω loads are similar, with the A good ground plane is essential for stability. Most components are surface-mount types, allowing most of the underside of the board to be a solid ground plane. The resistors and capacitors are metric 2012 (2.0 x 1.2mm/imperial 0805) and 3216 (3.2 Australia’s electronics magazine Performance April 2020  79 REG 1 7805 1 IC2 51 IC3 51 1 IC4 51 GND 1.2k 100nF 560 560 100nF 100nF 100nF 1 180 180 560 100nF 100nF 180 180 560 100nF 1 IC5 51 100nF 51 10 MHz DISTRIBUTOR 100nF IC1 39 CSE200103 100nF 1 100nF 1 IC6 51 T1 T2 T3 T4 T5 T6 CON4 OUTPUT 1 CON5 OUTPUT 2 CON6 OUTPUT 3 CON7 OUTPUT 4 CON8 OUTPUT 5 CON9 OUTPUT 6 2.7k 100nF 560 100nF 100nF 100nF 560 10 F 100nF 100nF 180 470 1 VR1 100nF 100 100nF 2 10 F 180 100nF TP 1.2k CON2 10MHz IN + – 180 CON3 + – 12V IN 4004 D1 A K LED1 Fig.4: use this PCB overlay diagram and the photo below as a guide during assembly. Most of the components are SMDs, with the op amps being in small 5-pin SOT-23 packages and the RF transformers in larger six-pin plastic packages. The only components which could be fitted with the wrong orientation are diode D1 and LED1. dered, check that there are no bridges. If there are, apply some flux paste and use solder wick to soak up the excess solder. That should leave just enough solder to form good joints which are not bridged. Next, solder all the SMD resistors and capacitors, referring to Fig.4 to see which goes where. Their orientation is not important; simply tack down one side, check that the part is flat on the PCB and not too crooked, then once you are sure the first joint has solidified, solder the other side. Make sure in each case that the solder adheres to both the part and the PCB pad. The last set of surface-mounting parts are transformers T1-T6. These are not entirely symmetrical, as they have a centre-tap on one side only, but we don’t connect to that tap. So it doesn’t matter which way you fit them, although we suggest you match the orientation shown in our photos to guarantee you get the stated performance. Use the same technique as with the smaller SMDs, tacking one pin and then checking the remaining pin locations are square over their pads before soldering them. Through-hole parts harmonics down by more than 40dB. Fig.2 shows the shape of the output waveform on my scope, while Fig.3 is a spectrum analysis of this waveform. The vertical scale is 10dB/div, which makes the second harmonic -44dB, the third harmonic -46.5dB and the fourth -46dB. Construction The Signal Distributor is built on a PCB coded CSE200103 which measures 125.5 x 60mm. Refer to Fig.4, the PCB overlay diagram, which indicates 80 Silicon Chip which parts go where. Start with IC1-IC6. These are the only ones with small pins close together. As they have two pins on one side and three on the other, their orientations should be obvious. Tack them down by one of the two pins which are more widely spaced, then check the part is sitting flat on the board and that all the pins are over their pads before soldering the other four. If necessary, re-melt the first joint and nudge the part. Once all the pins have been solAustralia’s electronics magazine Solder diode D1 in the usual manner, ensuring it is orientated as shown in Fig.4. Then bend the leads of REG1 down so that they fit through their pads with the tab hole lining up with the PCB mounting hole. Attach it using an M3 screw and nut, and do it up tight before soldering and trimming the leads. Follow with headers CON2 and CON3, orientated as shown, then trimpot VR1. Orientate VR1 with its adjustment screw on the side facing away from CON2. Then mount the six BNC sockets. They are quite bulky, so make sure they are sitting completely flat on the PCB before soldering the two signal pins and the two larger mounting posts in place. In terms of board assembly, that just leaves LED1. We’ll solder it in vertically now, but it can be bent over later to protrude through a front panel hole next to the BNC connectors. Its anode (longer) goes to the pad closest to the 2.7kΩ SMD resistor. The flat side of siliconchip.com.au the lens indicates the cathode, opposite the anode. Solder it with the base of its lens 10mm above the top of the PCB and trim the leads. Case preparation Fit the four tapped spacers to the corner mounting holes using short machine screws and place the board in the case. Slide it so that the BNC sockets are touching the side, and measure the distance from the top of the metal surrounds to the top of the box. If you measured from the top of the bump on the RCA socket, add 5.5mm to this measurement, otherwise, add 5mm. Then measure that far down from the top of the case on the outside, directly opposite one of the connectors, and mark the case there. For example, if you measured 23mm on the inside, from the top of the bump, mark the outside 28.5mm from the top. Then punch that location using a hammer and nail, and drill a pilot hole there (or use a centre punch, if you have one). You should find that this hole corresponds with the centre of the BNC socket. The connectors are mounted 3/4in (19mm) apart, so drill five more pilot holes at the same level each spaced 19mm apart, corresponding to the locations of the other BNC sockets. Then drill a 3mm hole 14mm to the right of the right-most socket for the LED. Enlarge the other six holes to 12.7mm (0.5in) diameter, then check that the BNC socket surrounds all fit. Once they do, remove the nuts and washers from the BNC sockets, along with one of the tapped spacers from the PCB. Push the BNC sockets fully through their mounting holes, then mark the location of that one hole in the base of the case. Refit that tapped spacer, remove another one and repeat until you have marked all four holes. Then drill them out to 3mm. Decide where you want to mount the input socket and DC power socket, then punch and drill those locations large enough to fit the connectors. Clean up the case and deburr all the holes. You can now mount the PCB in the case using four machine screws through the base and into the tapped spacers, and refit the BNC socket washers and nuts. Stick the rubber feet onto the bottom of the case, in the corners. siliconchip.com.au Parts list – Signal Distributor 1 double-sided PCB coded CSE200103, 125.5 x 60mm 1 diecast aluminium enclosure with room for the PCB and chassis connectors [eg, Jaycat Cat HB5046, 171 x 121 x 55mm 6 Coilcraft PWB-1-BLC 425MHz transformers, SMD-6 package (T1-T6) [element14] 1 chassis-mount BNC socket (CON1) 2 2-pin polarised headers and matching plugs (CON2,CON3) 6 PCB-mount BNC sockets (CON4-9) 1 chassis-mount DC barrel connector (CON10) 1 12V DC 150mA+ plugpack or other power supply 9 M3 x 6mm panhead machine screws 1 M3 hex nut 4 9mm tapped spacers 1 500mm length of single-core shielded cable 4 stick-on rubber feet Semiconductors 6 MAX4450EXK+T 210MHz op amps, SC-70-5 (IC1-IC6) 1 7805 5V 1A linear regulator, TO-220 (REG1) 1 3mm LED (LED1) Capacitors 2 10µF 16V X5R ceramic, SMD 3216/1206 size 20 100nF 16V X7R ceramic, SMD 2012/0805 size Resistors (all 1% SMD 3216/1206 size) 1 2.7kΩ 2 1.2kΩ 6 560Ω 1 470Ω 7 180Ω 6 51Ω 1 100Ω multi-turn vertical trimpot (VR1) [eg, Jaycar Cat RT4640] Measure the distance from the two chassis-mount connectors to their corresponding headers on the board, then cut a generous length of shielded cable to suit both. Strip back the outer sheath at each end of both cables, then separate out the shield wires and twist them together. Attach the polarised header plug pins to the inner conductor and shield at one end of each (we recommend you crimp and solder), then push them into the plastic plug housings, referring to Fig.4 to see which side the shield braid goes to (marked “–” in both cases). Solder one cable to the chassismounting BNC socket, so that the shield braid goes to the outer tab and the inner wire goes to the middle pin. Similarly, for the DC socket, solder the shield braid to the tab connecting to the outer barrel of the connector when it’s plugged in, and the inner wire to the tab connecting to the tip. Don’t be trapped by the fact that many sockets have a third switched negative tab. It’s initially connected to the outside of the barrel but is disconnected when a plug is inserted. Check for continuity between the tab and the outside of the barrel when the plug is inserted. Plug the polarised headers into the correct sockets and bend LED1’s leads Australia’s electronics magazine 1 39Ω so that the lens pokes through the hole in the front panel without shorting its leads together. Testing You can now apply power via the DC socket and check that LED1 lights up. If it doesn’t, check that you’ve wired up the DC socket to the board correctly, so that there is continuity from the centre pin of the DC socket to the anode of D1 (opposite the striped end). Also check that D1 and LED1 have been fitted with the correct polarity. If it still doesn’t work, your power supply may be a tip-negative type. In that case, you will have to swap the pins going into the plug for CON3. Now feed a signal into the input and use a scope or frequency counter to check that the correct frequency signal appears at each output. Assuming you have a scope or some other means of measuring the output amplitude, adjust VR1 for +10dBm which is around 0.7V RMS or 2V peak-to-peak. You could adjust for a different level if needed. Don’t forget to apply a 50Ω load when making these adjustments. Given that each buffer provides four times gain, it should be possible to get a +10dBm output with an input signal as low as +4dBm (350mV RMS or 1V peak-to-peak). SC April 2020  81