Frequency Reference
Signal Distributor
If you have
multiple test
instruments and
one very accurate
frequency reference,
you need a way to feed that
reference signal to each test
By Charles Kosina
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.
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.
A typical 10MHz reference signal
generator has only a single 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
34
the generator and potentially cause
other problems.
You really want a +10dBm (0.7V
RMS) reference signal terminated
with 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. 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 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
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
slightly higher amplitude, so less gain
is required to achieve +10dBm.
Practical Electronics | April | 2021
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 IC1IC6), which are all connected in parallel.
D1 1N4004
CON3
CON10
+12V
A
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.
They are configured as non-inverting
amplifiers and the 560Ω/180Ω feedback
resistors give a gain of about four.
The bottom end of each feedback
divider connects to ground via a 100nF
capacitor. The feedback network cannot
be connected directly to ground due
REG1 7805
+7V
OUT
IN
K
GND
0V
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.
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
39
K
4
5
K
A
51
T3
CON6
+7V
GND
IN
GND
5
IC4
1
OUTPUT 4
(BNC)
100nF
51
T4
CON7
2
7805
LED
4
100nF
100nF
3
MAX4450
1
560
100nF
A
3
IC3
OUTPUT 3
(BNC)
100nF
+3.5V
1 2
5
2
180
1N4004
+7V
100nF
OUT
180
560
+7V
100nF
100nF
SC SIGNAL
Signal
Distributor
DISTRIBUTOR
3
2020
Reproduced by arrangement with
SILICON CHIP magazine 2021.
www.siliconchip.com.au
4
5
IC5
1
OUTPUT 5
(BNC)
100nF
51
T5
CON8
2
180
560
+7V
100nF
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 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 resistors in series across the 7V supply.
Practical Electronics | April | 2021
3
4
5
IC6
1
OUTPUT 6
(BNC)
100nF
51
T6
CON9
2
180
560
100nF
35
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.
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.
PCB design
A good ground plane is essential for
stability. Most components are surfacemount types, allowing most of the underside of the board to be a solid ground
plane. The resistors and capacitors are
metric sizes: 2012 (2.0 × 1.2mm/imperial 0805) and 3216 (3.2 × 1.6mm/imperial 1206), 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.
Performance
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
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.
36
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.
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
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 soldered,
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
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.
Ensure 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.
Practical Electronics | April | 2021
the metal surrounds to the top of
the box.
2 1
10 F
If you measured from the top
GND
180
REG 1
of
the bump on the RCA socket,
CSE200103
7805
470
add
5.5mm to this measurement,
10 MHz DISTRIBUTOR
10 F
otherwise, add 5mm. Then meas560
560
560
560
560
ure that far down from the top of
560
100nF
100nF
100nF
100nF
100nF
100nF
the case on the outside, directly
1 IC1
1 IC2
1 IC3
1 IC4
1 IC5
1 IC6
opposite one of the connectors,
51
51
51
51
51
51
and mark the case there.
For example, if you measured
T4
T6
T3
T5
T2
T1
23mm on the inside, from the top
of the bump, mark the outside
28.5mm from the top. Then punch
CON8
CON9
CON5
CON4
CON6
CON7
A
K
that location using a hammer and
OUTPUT 5
OUTPUT 6
OUTPUT 2
OUTPUT 1
OUTPUT 3
OUTPUT 4
LED1
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
3/4-inch (19mm) apart, so drill
Fig.4: use this PCB overlay diagram and the photo below as a guide during
five more pilot holes at the same
assembly. Most of the components are SMDs, with the op amps being in small
level, each spaced 19mm apart,
5-pin SOT-23 packages and the RF transformers in larger six-pin plastic
corresponding to the locations of
packages. The only components which could be fitted with the wrong orientation
the other BNC sockets. Then drill
are diode D1 and LED1.
a 3mm hole 14mm to the right of
the right-most socket for the LED.
Enlarge the other six holes to
12.7mm (0.5-inch) 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.
In terms of board assembly, that just
Through-hole parts
You can now mount the PCB in
Solder diode D1 in the usual manner, leaves LED1. We’ll solder it in verti- the case using four machine screws
ensuring it is oriented as shown in cally now, but it can be bent over later through the base and into the tapped
Fig.4. Then bend the leads of REG1 to protrude through a front panel hole spacers, and refit the BNC socket
down so that they fit through their pads next to the BNC connectors. Its anode washers and nuts. Stick the rubber
with the tab hole lining up with the PCB (longer) goes to the pad closest to the feet onto the bottom of the case, in
mounting hole. Attach it using an M3 2.7kΩ SMD resistor. The flat side of the the corners.
screw and nut, and do it up tight before lens indicates the cathode, opposite
Measure the distance from the two
the anode. Solder it with the base of chassis-mount connectors to their
soldering and trimming the leads.
Follow with headers CON2 and its lens 10mm above the top of the PCB corresponding headers on the board,
CON3, oriented as shown, then trim- and trim the leads.
then cut a generous length of shielded
pot VR1. Orient VR1 with its adjustcable to suit both. Strip back the outer
ment screw on the side facing away Case preparation
sheath at each end of both cables, then
from CON2. Then mount the six BNC Fit the four tapped spacers to the separate out the shield wires and twist
sockets. They are quite bulky, so make corner mounting holes with short
them together.
sure they are sitting completely flat machine screws and place the board
Attach the polarised header plug
on the PCB before soldering the two in the case. Slide it so that the BNC pins to the inner conductor and shield
signal pins and the two larger mount- sockets are touching the side, and at one end of each (we recommend you
measure the distance from the top of crimp and solder), then push them into
ing posts in place.
100nF
TP
VR1
100nF 100
39
100nF
180
2.7k
100nF
100nF
180
100nF
100nF
180
100nF
100nF
180
100nF
180
100nF
100nF
100nF
100nF
180
1.2k
CON2
10MHz IN + –
1.2k
CON3
+ – 12V IN
4004
D1
Practical Electronics | April | 2021
37
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
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
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,
Jaycar 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
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
Exciting Freelance Design
Engineer Opportunity
1 500mm 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 39Ω
1 100Ω multi-turn vertical trimpot (VR1)
[eg, Jaycar Cat RT4640]
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).
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Practical Electronics | April | 2021
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04/02/2021 12:00
