A Return Loss Bridge, that also works as a milli-voltmeter.
The bridge detector is an AD8307 log detector chip made by Analog Devices. The data sheets state
that 3 volts is sufficient, but the one I have needs 4.5 volts to read correctly. The 50uA meter is
shunted by a 1k resistor to make it 200uA full scale. Current consumption is 8 mA
J2
T EST
B1
4. 5v
On button
VCC
R7
C1
1nF
6
5
O U T IN T
7
4
1nF
3
C2
OF S ENB
8
R4
100 R
1
R2
100 R
IN M IN P
J1
R F INPUT
C3
100 nF
U1
AD83 07
U3A
VCC
M1
50u A
R5
100 0R
C4
100 uF 5Vv Tan t
J3
R EFE R ENCE
1
2
3
14
15
C5
VCC
10k
S1
GNDVPS
R3
100 R
2
R1
100 R
74HC 123A
A
B
C LR
C ext
R ext / Cex t
Q
Q
"on" t im e
100 nF
U3B
9
10
11
6
7
13
4
C6
R9
1M0
74HC 123A
A
B
C LR
C ext
R ext / Cex t
Q
Q
5
12
R 10
150 R
10u F
R6
1M0
"off" t i m e
D1
L ED
R8
1M0
C7
VK3ZAV - RETURN LOSS BRIDGE
On timer
VCC
LED flasher
100 nF
VCC
.
I find that flat batteries are a constant nuisance, because its too easy to leave it switched on, so I've
included a timer that gives about 10 minutes operation after pressing the start button.
Full scale is +10dbm, and minimum reading is -80dbm. As the monostable (timer) chip 74HC123
is a dual unit, the second half is used to flash an LED to show its on. The timer, right hand half of
the circuit, can be omitted to save about $2. I have a 61 x 37 mm sample board from Olimex, cost
about $10.
Return loss relates to SWR by the following :
SWR
Return Loss
1.01
46
1.1
26.5db
1.2
21
1.3
18
1.5
14
2.0
9.5
.
To use as a milli-voltmeter, short-circiut J1 and feed the signal into J2 or J3. The input is
terminated in 50 Ω.
To measure SWR by Return loss, a signal source of between -60dbm and +10 dbm is needed,you
can make your own, say using a crystal test oscillator, or use a signal generator. Feed the signal into
the centre BNC jack J1, and then plug a termination into one of J2 & J3 and the unknown into the
other. To check bridge balance, plug in two terminations. You can use 50 or 75 ohms.
Frequencies up to 500Mhz are ok with the AD8307, but an alternate chip would work to 2,500Mhz.
To use only as a mV meter, then leave off J1, J3, R2, R4, and bridge J1 & J3.
The advantage of this is you can check small receiver modules, amplifiers, mixers, filters, and
aerials of all sorts. The disadvantage is you can't feed a transmitter into it.
Some idea of the cost $AUS, PCB $10, AD8307 $12, BNC connectors $3ea, 74HC123 $1.5,
resistors & capacitors $0.10 ea, also meter, battery carrier, case, batteries, push button.