A Compact Fixed Frequency Receiver for 144.270...
Transcript of A Compact Fixed Frequency Receiver for 144.270...
A Compact Fixed Frequency Receiver for 144.270 MHz Envelope Modulated SignalsRick Campbell KK7B30 Mar 2020
The receiver board and audio amplifier board fit in a Hammond 1590B diecast box, with internal LiPo batteries. This rev3 version has enough AGC range that the top controls are only volume, on-off, and the PTT button.
There is room inside the box for the compact low power envelope modulated transmitter described in the companion article.
The same circuit board may be used for 144 MHz and 50 MHz receivers by changing the crystal and the inductor and capacitor values on the front-end board.
Sensitivity is 0.1uV for recoverable A2 tones. 0.3uV for clear AM voice. IF bandwidth is 8 kHz. Image rejection is >40dB, using the I Q mixer and 455 kHz IF. Supply is 68mA 9v including audio output amplifier. LO leakage at the RF input port is -61 dBm.
Fig 1 144 MHz receiver
Fig 2 144 MHz AM Receiver rev 3 circuit board
Except for the through-hole components in the 7th Overtone oscillator circuit, the Coilcraft midi-spring and slug-tuned inductors, the bifilar coil for the 144 MHz quadrature hybrid, the 1:1:1 transformer hidden under the ferrite bead, and the two T25-2 toroids, most of the components in the RF front-end of the receiver are surface mount. The surface mount components were chosen from parts on hand. The mix of 1208, 0804, and 0603 sized surface mount components is not intentional, and serves no purpose other than to quickly build a prototype circuit without waiting for a component order.
I and Q interconnects between the RF and IF circuit boards are twisted pair, with 43 mix ferrite bead common mode chokes to isolate the VHF RF and LO from the 455 kHz IF board. Connection of the dc power supply line and ground are as shown, and the white wire +dc connection in the photo works well on the bench.
Fig 4 Back side of the circuit board showing many surface mount components in the RF front-end, and a few in the 455 kHz IF strip. Note the cut line between the upper RF and lower IF sections. These are totally independent.
Detail back side of circuit board photograph
ab
a b
I Q
22
150
0.5p
10n
10n
51
10p
68p
82n
18p
47p
95nH
70nH
10p
22p
22p
10p
47p
820p
1.2u
3.3k
+
22p
22p
47p
820p
1.2u
3.3k
56nH
49.9
5k
95nH
70nH
22p
10n
10n
100
10k
1k
51
+
47p
47p
220p
6.8n
51
I
Q
Fig
5 1
44 M
Hz
Env
elop
e M
odul
atio
n R
ecei
ver S
chem
atic
R C
ampb
ell 1
4 M
ar 2
0
Not
es: 5
6nH
bifi
lar i
nduc
tor w
ound
ar
ound
TLD
tuni
ng to
ol.
Cro
ss-c
onne
ct th
e la
bele
d I a
nd Q
po
rts to
reje
ct th
e 14
3.34
0 M
Hz
imag
e.
Sup
ply
volta
ge fr
om 9
v to
15v
.S
ensi
tivity
opt
imum
at 1
3.8v
Tune
the
IQ p
hase
trim
for r
ejec
tion
of
the
143.
340
MH
z im
age.
Am
plitu
de
trim
not
nee
ded
for 1
4 M
arch
bui
ld.
It w
as n
eede
d fo
r the
20
Mar
ch b
uild
.
ab
a b
I Q
22
150
0.5p
10n
10n
51
10p
68p
82n
18p
47p
95nH
70nH
10p
22p
22p
10p
47p
820p
1.2u
3.3k
+
22p
22p
47p
820p
1.2u
3.3k
56nH
49.9
5k
95nH
70nH
22p
10n
10n
100
10k
1k
51
+
47p
47p
220p
6.8n
51
I
Q
1N4148
Vcc
4.7p
22
150
220n
220n
220n
220n
220n
220n
220n
220n
10n10n
1k
1k
1k
10k
27k
56k
7.5k
51
+
+10u
10u
2.5k
100k
100k
+
J310
144.
270
MH
z in
Coi
lcra
ft ye
llow
cor
e 15
0 se
ries
with
shi
eld
Coi
lcra
ft or
ange
cor
e 15
0 se
ries
with
shi
eld
with
sh
ield
Coi
lcra
ft or
ange
co
re 1
50 s
erie
s
surfa
ce m
t 1:1
:1H
SM
S d
iode
qu
ad ri
ng
optio
nal a
mpl
itude
tri
m fr
om I
or Q
to
grou
nd a
s ne
eded
455
kHz
quad
ratu
re
hybr
id o
n bo
ttom
ha
lf bo
ard
at in
put
to IF
am
plifi
er
18t b
ifila
r FT
37-6
1ph
ase
trim
455
kHz
IF o
ut18
t T25
-2
Coi
lcra
ft m
idi-
sprin
g se
ries
5t b
ifila
r clo
se
wou
nd a
roun
d 0.
162”
form
Xic
on y
ello
w c
ore
600:
60k
Xic
on w
hite
cor
e 30
k:50
0X
icon
bla
ck c
ore
20k:
6k
AG
C fe
edba
ck
volu
me
cont
rol
outp
ut to
R2p
ro
AFa
mp
fig 9
.74
EM
RFD
2N57
70
2N39
04
2N39
04
MM
BF3
10
Qua
rtSLa
b 14
3.80
0 M
Hz
7th
OT
crys
tal
455
kHz
IF c
ans
tune
d to
470
kH
z
Imag
e R
ejec
t mix
er is
from
“I Q
M
ixer
with
Sin
gle
SP
DT
Sw
itch”
by
Cam
pbel
l, IE
EE
refe
renc
e
Fig 6 For a sense of scale, here is the rev3 receiver board next to the new VHF 0dBm AM transmitter. The transmitter board has an electret microphone input in the lower left corner and an antenna connection and receive antenna connection on the upper left corner. The 7th overtone crystal hasn’t yet been installed on the receiver board in this photo.
This pair, 0dBm carrier and +6dBm PEP output at 100% modulaion, and receiver sensitivity of 0.3uV and 8 kHz bandwidth, is useful for medium range line-of-sight paths and has successfully communicated over a mile with a few obstructions. The 0dBm output power is ideal to drive a Mitsubishi Linear Amplifier Module such as the RA07M1317MSA or newer versions, to a carrier output power of 2w and nearly 8w PEP at 100% modulation.
The AGC range and audio fidelity of the detector are appropriate for a larger case and much bigger speaker. This receiver sounds very good to critical ears with high fidelity headphones, and the companion uT1am transmitter is even better.