 |
A
product from the NorCal QRP Club
...
A 30m QRP Transceiver |
About the
AmQRP Club |
The NorCal "38 Special" Transceiver for 30 Meters
| Schematic | Parts List | Manual | Debugging |
| Mods | Lab Tests | 40m Version | Power Mod |
The NorCal QRP Club sponsored the first annual Dayton Building
Contest in 1996. We selected 2 projects, the 49er 40 Meter Transceiver
designed by Wayne Burdick, N6KR, and the Pipsqueak Regenerative Receiver
designed by Paul Harden, NA5N. To add a little spice to the contest and
generate some technical articles, we decided to have a design contest to
go along with the building contest, with the winning designs published in
QQ and QRPp. The winning designers will be awarded a plaque from NorCal
and the winning design kitted. This year, we had two great winners, the
"38 Special", a 30M transceiver designed by Ori Mizrahi-Shalom, AC6AN, and
the "Rainbow Tuner", designed by Joe Everhart, N2CX.
NorCal will kit the "38 Special" and the New Jersey QRP Club will
kit the "Rainbow Tuner". We found that NorCal did not have the time to do both kits, and the New Jersey QRP Club graciously stepped forward to help
us out. QRPers are the winners in the design contest, because there are
now two great $25 kits out there to build.
The point of the design contest was to provide kits for the Dayton
97 Building contest, which is again sponsored by NorCal QRP Club. There
will be 3 prizes in each category, with the judges decision final. The
rules are simple. The project must work, and mods are allowed. You may
build from the kits available, or you may home-brew your own. Chuck Adams,
K5FO is the head judge, and he will select two additional judges the night
of the contest. Judging will be held at the ARCI Hospitality Room at 8:00
on Saturday night of Dayton.
I would like to thank all of the designers for their efforts and
time that they have put in to providing the rest of us with great projects
to build. Good luck builders, and we will all get to see the winners in
Dayton. 72, Doug, KI6DS
GENERAL DESCRIPTION
The "38 Special" is a superhet transceiver for the 30M band. The first thing you'll notice looking at the schematics is the lack of discrete transistors. I have been playing with this concept for some time and the 38 Special was the right vehicle. The transmit section relies on a TTL buffer. I extended this concept to utilize the same TTL chip for other tasks. Although it's an NE602-based superhet transceiver, the "38 Special" incorporates only two NE602s. I reuse the product-detector as the transmit mixer by channeling different signals to that chip on receive and transmit. Also, the traditional LM380/386 is gone in favor of a more versatile dual op-amp circuit for the audio section.The receiver front-end starts with a back-to-back diode switch.
The "38 Special" utilizes 1N4007 diodes for the switch. This diode has a PIN
structure and provides low insertion loss, although it suffers from poor
zero bias isolation and does not offer a strong IMD performance like an RF-rated PIN diode. (1) It is superior to the 1N914 or similar diodes in
this type of design. Next is a toroidal impedance transformer with a 10.1
MHz tuned circuit at its output, providing additional front-end selectivity
to that offered by the transmit output network. Provisions were made to
include a 10 KOhm pot for RF-GAIN control. The 10.1 MHz RF signal is fed
to the input of the NE602 receive mixer, where it is mixed with the 22.1 MHz
VXO to generate the 12.0 MHz IF frequency.
THE VXO
The superhet circuit enabled me to use standard crystals and avoid
the high price of custom ham-band crystals. Many crystal combinations work
for most HF bands. I chose a high frequency first crystal to achieve a high
frequency swing. (2) This required a relatively high IF in the simple
receiver. The NE602 Collpits oscillator required a high DC bias for a large
swing, provided by a 3.9 KOhm resistor at pin 7. Although well below the
value recommended, this resistor provides for stable operation of the NE602.
"Rubbering" the crystal with a varicap allows relocating the tuning pot away
from the oscillator, if desired. A 1N4000-family diode works here nicely as
a varicap. (3)
A little assist from a molded inductor yields a tuning range of 25
KHz. A little hint here for the experimenters, do not replace this inductor
with a toroid. The low Q helps to increase the pulling range. The VXO
signal is mixed inside the NE602 with the received signal to produce an IF
output of 12.0 MHz, which is the difference between the VXO and the RF
frequency. During transmit, a 22.1 MHz signal is taken from the Collpits
oscillator and injected into the input of the transmit mixer.
IF FILTER
The IF filter is implemented with a single crystal. It is a few KHz
wide, due to budget constraints. Provisions for a better IF filter are
included in the board layout. The main selectivity is achieved at the
audio stage and the wide IF filter greatly simplifies the alignment of the
receiver. The wide IF filter provides very little "wrong" sideband
attenuation. Due to this, the "38 Special" in the stock form cannot be
classified as a single-signal receiver. It will take a much sharper IF
filter to achieve that. Although you will hear the same signal twice, the
sharp audio filter totally eliminates the "off" signal.
PRODUCT DETECTOR
In the cost cutting tradition I left out the "traditional" third
NE602 for the transmit mixing. Instead, the 38 SPECIAL reuses the
product-detector for the same function. On receive, an oscillator (IF
frequency) is mixed with the IF signal and it results in a low-level audio
signal. On transmit, a signal from the VXO is mixed with the IF frequency
oscillator in the second NE602. The selection of the input signal to the
second NE602 is done by means of a 4066 analog multiplexer. Other than the
switching of signals with the 4066, the receiver is similar in concept to
most NE602-based superhet rigs.
RECEIVE OFFSET
Sharing the product-detector and transmit mixer required a "trick"
to achieve a receive offset. The 38 SPECIAL "pulls" the IF frequency
oscillator about 500 Hz up on receive with a 100 pF capacitor in series with
the 12.0 MHz oscillator crystal. During transmit, this capacitor is shunted
to ground with a parallel forward-conducting diode, so the crystal
oscillates right on its fundamental frequency, resulting in a zero-beat
transmit signal. The down-conversion at the first mixer and this oscillator
pulling up on receive combine to yield the "right" receive sideband at a
lower frequency. So, although the IF filter allows either sideband through,
it is easy to identify the "right" one. This is not an issue when you call
a CQ. The answering station is on the right frequency, if it zero-beats
with your transmit signal.
AUDIO AMP/FILTER
The audio is filtered and amplified by an NE5532A dual op-amp,
instead of the "traditional" LM380 or LM386 chips. (4) The 5532 requires
more external components, but it gives a higher gain, and more important,
the circuit also forms a sharp band-pass filter. From that point of view,
the "38 Special" is superior to most NE602-based radios. This amp delivers
about 60 dB of gain while driving a walkman-style headphones. The filter
offers a 50 Hz -6 dB bandwidth and about 400 Hz at -30 dB. This circuit
uses a dozen more components than an LM386. But they are probably the most
cost-effective components in the whole radio!
TRANSMIT CHAIN
As mentioned before, the product-detector doubles as a transmit
mixer. The signal on the output is filtered by a tuned circuit, and the
10.1 MHz output of 100 mV is used to drive the two-stage transmit amplifier.
A few examples in the literature describe the use of TTL chips (and other
logic devices) for a low-power transmitter. (5)
The most interesting article on the subject was written by Len Smith
and appeared in QST. Len used an octal inverting buffer with eight
individual active devices, but really utilized only five of them. This
circuit gave me the idea of using the leftover devices for sidetone
generation and other tasks.
One of the inverters is biased as a high gain linear amplifier.
The 100 mV at its input comes out as a few volts on swing at the output.
The single inverter is strong enough to drive the final circuit, which is
made of four parallel inverters. Depending on the output matching, this
circuit can deliver well over half a watt of output power. I chose to
leave it at 400 mW for the sake of cool and safe operation of the final.
The board supports additional circuitry (not supplied with the kit) for a
higher output of up to 5W.
THE OUTPUT NETWORK
The use of only 8V supply required very low impedance for the final
to give any appreciable output power. The matching is easily done by an
L-C-L-C type network. (6)
SIDETONE
Two inverters combine in a simple oscillator circuit, as described
in the ARRL handbook. The oscillator is clamped to ground during receive
but is free-running during key-down. The output is attenuated by means of
a large series resistor. There was no need to filter the square waveform,
as the audio bandpass circuit does that anyway.
ADDITIONAL LOGIC
One inverter of the octal buffer chip forms the receive/transmit
logic. It inverts the logic state of the "key" line, so when key is down,
this signal is at full Vcc and when key is open, this signal is at 0V.
The availability of the T and R signals simplifies the implementation of
the T/R circuitry.
ALIGNMENT
The "38 Special" was designed with a novice builder in mind. There
are only two alignment steps, although more parameters could be tweaked by
the experienced builder. The receiver alignment consists of peaking the
front-end trimcap for the highest receive audio.
The transmitter alignment is a bit more tricky. The radio
transmits 10.1 MHz signal, when it has a 12.0 MHz IF signal in the output
of the transmit mixer. This requires care when tuning the transmit filter
trimcap. One way of doing that is by listening to your signal on another
receiver. Tune close to your center frequency and look for the adjustment
that results in an output with the least close-in spurs. The two alignment
steps do not require any test equipment, although having such equipment can
improve the alignment.
THE NEXT STEP
The final design reflects cost-cutting and other changes to
simplify the alignment and kitting of the design. It is a superhet with
offset and sidetone at a 40-9er price! As such, it has limitations, of
course. I look at it as a product and a development platform. There will
be many that will assemble the basic unit and have lots of fun with it in
the stock form. But there will be those who want to do things their way.
This radio was designed for both.
I will not continue its development. I leave that to the hackers
and tinkerers out there. The "38 Special" was designed to continue the
tradition that started with the 40-9er - mods by the dozen. There are many
possibilities. In fact, some mods are being developed as we speak. I
certainly encourage people to do just that. All I ask is that you share
with the QRP community and let us all know of your adventures...
For those that build it stock or custom, plain or modified, hot or
low key - have fun. I can't wait to see the entries for Dayton and
Pacificon next year. I also hope to get more activity on the 30M band.
Maybe we will finally know what the propagation properties really are on 30...
ACKNOWLEDGEMENTS
Again, many thanks to Doug Hendricks for the encouragement. Special
thanks to Dave Fifield, KQ6FR, who built the second prototype and beat me to
the first QSO - 549 on 200 mW from San Jose to Spokane! Dave is a master
builder, both of circuits and enclosures. He came up with many additions
and suggestions that made this radio nicer and more robust. Special thanks
to the XYL, who became a "radio-widow" during the last month...
NOTES:
(1) QST, Dec 1994 pp. 25-27.MANUAL UPDATE
I made a mistake in the 38S Manual that I did not
catch before I had 1000 of them printed. Ok guys, on page 10 on the 38 Special
Parts list, L3 says 12 Turns, and L4 says 15 Turns. It should say L3 is 8 turns,
and L4 is 12 turns. The schematic is right, the instructions are right, but, the
@#$%#%$^ parts list is wrong!! Please correct in your manual when you get it.
Also, when you tune up your transmitter. It is much easier to use a scope than
any other method. I thought of two ways those of you who don't have a scope can
do this. One, find a friend, acquaintance, enemy, old girl friend, new girl
friend, mistress, neighbor whatever the case may be, and ask them to let you put
your rig on their scope. Two, complete the rig. Put it in the case. Take it to
your nearest store that sells scopes. Say, something like this:
"Hi, do you mind if I try your scope out?" Say that you have just
built this QRP radio, and you want to try out the scope that you are thinking of
buying (not a lie as every ham who has ever built a rig has thought about buying
a scope). Then hook the probe to the center connector of the antenna, the ground
to the case, attach a dummy load, plug in your battery and key, and then key the
rig. Adjust TC-2 for the cleanest waveform. It may or may not be the point of
highest power out. It should take about 10 seconds. The guy will be impressed
that he can demo the scope, you will have your radio tweaked, and all will live
happily ever after.
If anyone actually does option number 2, please post an account to the net. We
would love to hear it. You can adjust the rig without a scope, but it is much
easier with it. Don't despair if you don't have a scope, you will have good
directions on how to do it in the manual. But, please check your alignment with
a scope the first chance you get. Almost any scope will do, and you will learn
something too!
AVAILABILITY
Sorry, the kit is sold out and no
longer available.
Material
and concepts presented on
These pages are designed and maintained by George Heron, N2APB
Page last updated: April
15, 2004