A scratch built S meter for FM900
Pascal Nguyen, VK2IHL
- The measurement of RSSI voltage related to the RF input of the FM900 is
quite simple to achieve. The result might have difference values from one site
to another due to test equipment, FM900 recipient and amateur handling.
- Under -120dBm or 0.2uV, noise floor had affected the result .
- Above --60dBm or 200uV, RSSI circuit is saturated. So the consideration of
relative strength is bounding by mentioned limits.
The writer's FM900
with bells & whistles, S6.
| RSSI 100k
Analog VM |
0.40 |
0.45 |
0.62 |
0.95 |
1.28 |
1.47 |
1.68 |
2.04 |
2.27 |
2.33 |
2.45 |
| uV HP608 generator |
0.1 |
0.2 |
0.5 |
1 |
2 |
5 |
10 |
20 |
50 |
100 |
200 |
| RSSI |
0.46 |
0.57 |
0.77 |
1.08 |
1.31 |
1.43 |
1.55 |
1.77 |
2.07 |
2.25 |
2.30 |
2.37 |
2.47 |
| -dBm |
120 |
115 |
110 |
105 |
100 |
95 |
90 |
85 |
80 |
75 |
70 |
65 |
60 |
Metering
Transfer characteristic for FM900
Relative bargraph led display to
V,S,dBm
Schematic Diagram of LM3914 Voltmeter
Here is the full
circuit diagram, for printing.
- LM 3914 and bargraph led display were tested, breadboarded and prototyped
on a strip of veroboard, point to point wiring. The trimpot 5k ,arbitrary
selected for the brightness of led, is adjusted for half scale deflection with
a 1v2 nicad reference.
- + 10v Vcc is supplied via terminal "L".
- Link S point on receiver board to rear speaker connector socket (spare pin
3)
- Link pin 5 of LM 3914 to socket 601 pin 3 via a pi network of 10k and
0.001uF.
- Made a connection with small coaxial cable (audio or RG 172) and 5 pin
line plugs from control head SK 601 socket to transceiver speaker socket.
Despite the relative measurement of signal strength, the meter is
quite satisfied to differentiate a 0.2 and 0.5uV signal.
There is another approach to display signal strength by reprogram EPROM to
display RSSI data from IC402 A/D converter. Hope that tip will bring in another
elegant design of FM900 metering from our members.
LM3914, 18 pin DIP is locally available from DSE Z6295 or JC ZL3914.
Thank you for reading, please give me a signal report on next qso
73 de VK2 IHL
