HF TRANSCEIVER Prof. Yosef PINHASI

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HF TRANSCEIVER
Prof. Yosef PINHASI
The transceiver presented in this article is designed to operate in the radio amateur
bands of the HF frequencies (3-30MHz). It was constructed on four printed circuit
boards:
•
•
•
•
Receiver and audio amplifier
Intermediate frequency (IF) modulator and product detector
Up-converter mixer and radio frequency (RF) power amplifier
Transmit / Receive (T-R) relay and RF low pass filter.
A block diagram of the transceiver is illustrated in Figure 1. In order to avoid mutual
electromagnetic interferences, the receiver and IF sections are separated from the RF
power stage. This requires transferring three signals between the two units; IF
double-side band (DSB) modulated signal, local oscillator and the RF antenna signal.
Much effort has been directed to keep the design simple and compact by utilizing
integrated circuits and direct couplings. Each of the four single-sided PCBs has its
own power supply, including a voltage regulation circuitry. In its present version, the
transceiver is operating in the 20m amateur band, covering 14.000-14.350MHz. The
receiver can detect AM, FM and SSB signals, while the transmitter operates in a DSB
mode.
RECEIVER
SA605
MPSH10
9
FM
7
AM
Quadrature
1
LNA
RF
455KHz
detector
LM380
VFO
AUDIO OUT
AF
3
14
Buffer
2N918
VFO
455KHz IF
CW/SSB
7404
Product detector
FREQUENCY
COUNTER
BFO
Buffer
Buffer
2N2222
BFXO
2N4416
455KHz
2N 918
BC149
2N2222
BFXO
MICROPHONE
LM741
MIC.
PA
IF
455KHz
Driver
Balanced mixer
MRF150
MRF150
2N3866
SA602
DSB
AF
Balanced modulator
MC1496
2N3553
AUXILARY
TONE
TONE
1KHz
2N3904
TRANSMITTER RF
455KHz IF
BALANCED MODULATOR & PRODUCT DETECTOR
Figure 1: Block diagram of the HF transceiver.
I.
THE RECEIVER
Figure 2 is the electronic scheme of the receiving module. The front-end of the
receiver consisted of a 4.7µH inductor in series with a 27pF capacitor, constituting a
resonant circuit at 14.128MHz. The two diodes at the input protect from excessive
signals by clipping spikes and large amplitudes that may damage the receiver frontend. The series resonant filter it is followed by an impedance matching network
transferring the aerial 50Ω impedance into 4.5KΩ input impedance of the SA605 RF
stage. The matching network is shown in Figure 3.a and its transmission
characteristics of the matching network are shown in Figure 3.b.
1N4148
2
0.1u
0.1u
455KHz
455KHz black IF
3
4
S
455KHz
1
3
1
5.1K
BFXO (from modulator)
2
+12V
270
470p
3
47K
2
0.1u
1
S
20
19
18
17
16
15
14
13
12
11
0.1u
6
270
470p
2N2222
1N4148
1K
CW / SSB
5.1K
10n
10p
10n
1
2
3
4
5
6
7
8
9
10
4.7K
1K
10n
40n
AM / FM
FM
AM
+12V
AM
2.3uH
24 turns T-50-6
1n
470pF
1N4148
1N4148
1N4148
1N4148
1n
455KHz black IF
27p
1
50p
0.577uH
12 turns T-50-6
4.7u
BNC
2
RF in (from T/R relay)
680
56pF
0.1u
22K
10n
510
20K
Audio (to audio amplifier)
SA605
50p
100K
BB109
120pF
50pF
100K
100K
RFC
1mH
+
6.2V
1n
15n
150p
2K
0.1u
0.1u
100u
0.1u
100K
10V
0.1u
+
0.47u
0.1u
47u
2N2222A
2
10n
VFO (to mixer and power amplif ier)
2.7K
220K
1K 100K
470
10n
1
10n
BNC
2N918
5.1K
100K
680K
Figure 2: The receiver and product detector.
1
2.3uH
Input
50 Ohm
24 turns T-50-6
56pF
4.5K
470pF
2
(a)
(b)
Figure 3: Front-end impedance matching network: a) scheme, b) power transmission.
The main receiving chain is based on the Philips SA605 mixer and IF system [1].
This integrated circuit (previously manufactured by Signetics with the number
NE605) is a super-heterodyne receiver composing of an internal balanced mixer,
local oscillator and intermediate frequency amplifiers. The 6.2V regulated supply
voltage required at leg 7 is achieved by employing a zener diode.
The local oscillator (L.O.) of the SA605 is based on a varactor-tuned variable
frequency oscillator (VFO). The L.O. frequency should be 455KHz below the
receiving frequency, i.e. 13.545-13.895MHz. In this Colpitts configuration, the
frequency is determined by the coil L3, the varactor CV and the two parallel capacitors
C3a and C3b, as shown in Figure 4.a. The oscillation frequency of the L.O. is
following the expression:
f LO (V ) =
1

C ⋅C 
2π L3 CV (V ) + C3 + 1 2 
C1 + C2 

While using the BB109, I found out that the coil inductance should be L3=0.577µ H
(12 turns on a T-50-6 iron powder toroid yellow core). The capacitor is
approximately C3=150pF made of a C3a=120pF capacitor in parallel with a C3b=50pF
trimmer (see Figure 4.a). In that case, the receiving frequency as a function of the
voltage introduced to the varactor is shown in Figure 4.b. Note that besides the main
14.000-14.350MHz band (455KHz above the L.O. frequency), also image
frequencies of 13.09-13.440MHz (455KHz below the L.O.) are expected to be
detected as a result of a super-heterodyne configuration.
1n
1n
C2=50p
L3=0.577uH
3
12 turns T-50-6
C1=50p
C3a
C3b
Cv
BB109
Vtune
+
4
-
(a)
(b)
`Figure 4: The VFO: a) Resonant circuit, b) Receiving frequency as a function of the
voltage in the 20m bands. The image frequencies are also shown.
The continuous local oscillator sine wave appearing in leg 3 is also transferred to the
up-conversion mixer in the transmitting unit of the transceiver, via a buffering 2N918
transistor in the receiver module followed by 2N2222A transistor (located in the
transmitting module).
Two standard 455KHz ceramic filters are used in the IF stage. The SA605 is
originally designed as an FM receiver; it includes a limiter and an FM quadrature
detector, as well as an RSSI (received signal strength indicator). When an FM signal
is received, the demodulated information is obtained at the unmuted audio output (leg
9). A black 455KHz standard IF transformer is used in the quadrature detection
system. Appropriate design of the RSSI filter, enables also detection of AM signals.
In that case the demodulated audio is obtained at the RSSI output (leg 7).
In order to detect CW or SSB transmissions, an external product detector is added.
Here, a switching two-diode detector was chosen, driven by a 2N2222 bipolar
transistor and an additional 455KHz black IF transformer. The detector, which in this
construction is part of the IF PCB, is connected to leg 14 (the input of the SA505
limiter), where the IF signal is obtained. The 455KHz beat frequency is generated by
a crystal oscillator (BFXO) in the IF board.
The audio amplifier is shown in Figure 5, including the 12V regulated power supply
of the whole receiver unit. The audio amplifier is composed of the LM380 producing
about 2W of RMS power over a 8Ω loudspeaker.
+12V
7812
1
0.1u
4.7K
MU TE
1
2
3
OUT
IN
3
4.7K
2N3906
Q1
10K
470u
6
LM380
1
2
3
4
5
PH ONES
SPEAKER
+
47u
+
0.1u
+
10u
0.1u
1000u
-
100K
~
-
+
+
4
5
3
~
2
+
1u
+
100K
100K
7
8
Audio (f rom receiver)
1n
Figure 5: Audio amplifier and receiver 12V power supply.
II.
THE IF BALANCED MODULATOR
The modulator section is based on the Motorola MC1496 balanced modulator
integrated circuit [2]. Audio signals from microphone, external auxiliary line or
internal tone generator are combined together by an operational amplifier TL071
(Equivalent to the 741). The microphone signal is fed to a single transistor preamplifier, enabling utilization of a standard 600Ω dynamic microphone or an
electrostatic. The transistor BC149 was chosen due to its low noise performance but
any other small-signal transistor will do well. The level of the microphone can be
adjusted by an external potentiometer, changing the modulation level. A tone of
1,000Hz is generated by a single 2N3904 (or equivalent) transistor phase-shift
oscillator, which can be switched on for transmitter testings. An internal on-board
trimmer is used for setting tone modulation level. The auxiliary input impedance is
47KΩ, similar to that of standard audio inputs. The voltage gain of the TL071 audio
amplifier is 20, producing 300-500mVpeak to the signal input of the balanced
modulator.
The modulator generates a double-side band suppressed-carrier signal centered at an
intermediate, sub-carrier frequency of 455KHz. The sub-carrier is produced by the
beat frequency crystal oscillator (BFXO) based on the JFET 2N4416. The same
signal is used also for the product detection in the receiver. The oscillator, is buffered
by a two stage amplifier, consisting of the common emitter 2N918 followed by a
common collector 2N2222. This is required in order to feed the low impedance
carrier input of the balanced modulator with 60-100mVpeak of 455KHz continuous
wave. A trimmer is adjusted for best carrier null the output. In order to enable CW or
amplitude modulation transmission, violation of the carrier balance is created, by
connecting a 1KΩ resistor from leg 4 to the ground.
The balanced modulator, 455KHz oscillator, audio preamplifier and tone generator
are all constructed on the same PCB including their 12V regulated power supply.
7812
180
1
2N 4416
820
2. 7K
1.2K
2N 2222A
1n
3.6K
OUT
IN
3
+
0. 1u
10u
15n
455KHz
2N 918
0.1u
47
RFC
47K
100K
+
1N4148
6.8V
1mH
100u
12K
240
1n
RCA JACK
0. 1u
2
1
10K
10K
5. 1K
MICR OPH ONE
+
14
13
12
11
10
9
8
1u
MC1496
1
150p
1. 5M
100K
BC149
1
2
3
4
5
6
7
2
3
47K
56n
50n
10K
180
2.7K
7
1
1M
47K
+
2
-
1K
1u
4
5
TL071
6
10K
+
3
1u
1
2
3
+
Tone
10K
10K
50K
100K
10K
100
100
1K
820
1u
+
10n
10n
10n
+
10n
2N 3904
10K
10K
10K
1K
10u +
5. 1K
+
10u
CW / AM
+
10u
1K
~
+
0.1u
10K
~
47K
50K
10m
+
0. 1u
10u
-
AUXILI AR Y
455KHz DSB (t o t rans mit ter)
1n
1n
Figure 6: Intermediate frequency balanced modulator.
III.
THE UP-CONVERSION MIXER AND POWER AMPLIFIER
The unique structure of the transmitter unit shown in Figure 7, is aimed at
simplification of the coupling between the mixer, driver and final amplifier. These
three stages are constructed on a single PCB, installed in a separated enclosure, to
minimize RF interferences to the sensitive receiver.
The up-conversion mixer is the well known SA602 double-balanced mixer and
oscillator [3]. A 6.2V zener diode is employed to regulate the voltage supplied to the
mixer. The double-side band, suppressed-carrier signal from the MC1496 balanced
modulator (in Figure 6), is fed to the oscillator input (leg 6) of the SA602 mixer. A
455KHz ceramic band pass filter is used to reduce inter-modulation products of the
sub-carrier before the up-conversion process. As mentioned before, the internal VFO
of the SA605 receiver (shown in Figure 2) serves also as the local oscillator of the
transmitter. The continuous sinusoidal wave of the VFO, shifted 455KHz from the
transmission carrier frequency is fed to the input of the mixer (leg 1). Two RF
suppressed carrier modulated signals are generated at the output ports of the balanced
mixer, legs 4 and 5, with a 180˚ phase difference between each other. This enables a
direct coupling to the subsequent push-pull driver.
The driver is based on high frequency bipolar junction transistors, the 2N3866
followed by the 2N3553. This stage is biased to operate in a class A mode, drawing a
quiescent current of 80-100mA from a 12V regulated power supply. The total power
dissipation at this stage is approximately 1W, requiring installation of heat sinks on
the 2N3553 transistors. The switch, operating the mixer and driver, also serves as the
PTT of the transmitter. The driver is coupled to the power amplifier section via a 3:1
transformer, reducing the impedance by a factor of 9 as explain in [4]. The primary
supplies the voltage to the collectors of the 2N3553, while the secondary applies the
bias to the gates of the MRF150 field effects transistors of the final amplifier. The
transformer is made of a 22AWG enameled wires wound through a BN-43-202
ferrite core, 3 bifilar windings for the primary and a single winding for the secondary.
Two MRF150 field effects transistors (FETs) serve in the final push-pull power stage.
The MRF150 transistor is an N-channel, enhancement-mode FET, designed primarily
for delivering 150W, with 45% efficiency at 50V. It is often used in broad-band
linear amplifiers operating in the HF band [5-8]. In the present design both gates are
biased from the same regulated 5V source. The positive bias is adjusted by a 10KΩ
trimmer for a quiescent current draw of 200mA from the 50V power supply. In order
to avoid oscillations, two 1Ω are used, isolating the paralleling inductance, from the
gates as noted in [6-7]. The power transistors should be mounted on a proper heat
sink for efficient removal of the dissipated heat.
T / R Control (to T/R relay)
470
7812
1
0.1u
100u +
6.2V
RFC
390u
6.8K
220, 10W
OUT
IN
+
470
0.1u
10u
500mA
Driv er
5A
Power Amplif ier
3
0.1u
22V , 5W
1.2K
R ed
RCA J ACK
1
455KHz
1
1.8K
10n
3
2N 3866
2
8
7
6
5
2N 3553
1
2
3
4
220
10
1
4
2
5
3
6
0.1u
1.2K
220
15n
100p
100
15n
10
BN-43-202
100
1
10K
1: 3
1
2
3
4
4
10K
10 bifilar turns FT-50-43A
2
1
100
3:1
1.2K
3
MRF150
SA602
RF out (to T/R relay)
1
2
455KH z D SB (from modulator
Yellow
BN-43-7051
MRF150
BN C
10n
1.8K
1
2N 3866
10n
10K
A
1M
7805
IN
OUT
82
1
Bias
+
~
3
5A
+
1N4005
6.8K
~
2
0.1u
Green
10m , 63V
+
10n
0.1u
0.1u
10u
0.1u
0.1u
-
VFO (from receiver)
2N 3553
1n
(a)
(b)
Figure 7: The up-converter and power amplifier stage a) schematics, b) photo.
IV.
T/R relay and Filter
RF out (f rom transmitter)
BNC
1
7812
1
PHONEJ ACK
1
2
3
320n
ANTENNA
1
220p
2
5
4
4N26
4N26
2N3019
+
~
~
-
0.1u
10u
12turns T-68-6
1N914
1K
10
+
0.1u
320n
SO239
2N3019
+
668n
1
8t urns T-50-2
470
5
T / R indicator
5. 1K
1
OUT
4
IN
T/ R control (from transmitter)
3
2
22V , 5W
0.1u
1000u, 63V
1n
Filter calculation from
http://www.wa4dsy.net/filter/hp_lp_filter.html
220p
8t urns T-50-2
2
T/ R RELAY DPDT
4
3
5
8
6
7
1
2
2
RF in (to receiv er)
V.
References
1. A. K. Wong: "Reviewing key areas when designing with the SA605", Philips
Semiconductors application note AN1994 (November 2007)
2. R. Hejhall: "MC1496 Balanced Modulator", Motorola Semiconductors
application note AN531/D (January 2002)
3. "High sensitivity applications of low-power RF/IF integrated circuits", Philips
Semiconductors application note AN1993 (August 2007)
4. H. Granberg: "Broadband transformers and power combining techniques for
RF", Motorola Semiconductors application note AN749/D (January 2002)
5. H. Granberg: "MOSFET RF power in the kW level: An update", QST Part 1
(December 1982) and Part 2 (January 1983)
6. H. Granberg: "Get 600 Watts RF from four power FETs", Motorola
Semiconductors application note EB104 (1983)
7. H. Granberg: "Get 600 Watts RF from four power FETs", Motorola
Semiconductors application note EB104/D (1993)
8. T. Sowden: "Hombrew solid-state 600WHF amplifier", QST (June 2006)
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