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The miniPIG the multiPIG and the "UGLY"
PART 1 - Preface:
This project came about when Flying Pig Club member Rick, WB6JBM made the comment
"we should have a club project to build". We talked about building a qrp radio
that was a bit different from the normal small rig projects; something that could
be used for competition in the field and at home. A ten meter rig seemed to fit
the bill as Rick had no 10 meter capabilities :-).
I gave the idea a little thought. Doing a vfo rig on 10 is a bit ugly. So, I
thought how about VXO. I looked around and found this odd crystal 8,064 KHz.
I'm not sure where it is used, but it would mix very well with a 20 MHz crystal
to cover the 10 meter QRP frequency. So I ordered ten 8.064 crystals from
radioshack.com (actually manufactured by International Crystal). I built up a
little mixer using a NE602 and bingo, we had coverage from 28,057 to 28,067 and
the miniPIG-10 was born.
After building the prototype, miniPIG#2 was built into an aluminum enclosure.
Included are a K1EL keyer, capacitive touch paddles and a balanced antenna tuner.
This rig has been to Key West and Tampa Florida, the Bahamas and to many field
contests, logging mostly European DX with it's one watt power output. The first
3 qso's were with as follows on December 5th, 1999 with a measured output of
900 milliwatts.
1. Dave, FY/DJ0PJ rcvd-569 Kourou, French Guiana - dipole at 30 ft
2. Mike, WB4HUC rcvd-579 near Austin, TX - nice long qso
3. Art, GD3FXN rcvd-539 Douglas, England - went qsb
Needless to say, I was very happy with the operational results of the miniPIG-10.
The multiPIG uses the miniPIG-10 tranceiver design, but incorporates a PLL to
replace the miniPIG VXO. This would allow multiband operations. Also, all the
band specific components are built on seperate modules. Also included is a
frequency counter with an IF offset to display the operating frequency.
Credit for the design of the miniPIG goes to all the great QRP and Ham tech
manuals I've read over the years, mostly to W7ZOI Wes Hayward and W1FB Doug
DeMaw for "Solid State Design for the Radio Amateur".
Wes is a member of the Flying Pigs, and if Doug were with us today, I'm sure he
would be an FP too. There were also circuits used from "The Radio Amateur
Handbook" published by the ARRL. My only contribution to the design was to glue
together all the various circuits with some slight tweaking and being lucky to
find the 8.064 crystals.
PART 2 - multiPIG Design Criteria:
Receive from 0 KHz through 30 MHz.
Transmit on ALL HF bands 160 - 10 meters, CW only
Variable 4.9152 MHz band pass filter
Click-free audio derived AGC with S-meter
Radio "system" will utilize band modules like the TenTec Scout.
PLL MAIN TUNE will double as a Signal Generator
FREQ DISPLAY will double as a Frequency Counter
Main tuning via a VXO Reference Oscillator
Built-in K1EL memory keyer (K10 chip)
Built-in Capacitive Touch Paddles
5 watts out, fully adjustable
SWR indicator and BALANCED/UNBALANCED ANTENNA Tuner
Completely modular, designed with the experimenter in mind
The main radio design is taken from the miniPIG-10
PART 3 - multiPIG Tranceiver:
This is the heart of the miniPIG tranceiver. The circuit discussed here is a
general purpose radio component that can be used for most any HF radio project
requiring CW and/or SSB modes of operation. The chosen IF is 4.9152 MHz.
The frequency was chosen because of the redily available inexpensive crystals
and a compromise frequency to eliminate out-of-band image signals and those
unwanted birdies.
BFO - The BFO uses one 4.9152 MHz crystal to produce a 4.9140 MHz when in
receive mode and 4.9135 MHz when in transmit mode. In the RCVE mode, C3 is used
to adjust the BFO to 4.914 MHz. When in XMIT mode, C5's capacitance is added to
the BFO oscillator to lower the frequency by 500 Hz for the XMIT frequency by
turning on Q1. There is a small interaction between C3 and C5, but you should be
able to set the capacitor within 3 tries. The actual TRUE frequencies mentioned
above are determined by the bandpass of the crystal filter, X2-X6.
I'll get back to that later.
The BFO oscillator is Q2 followed by a tuned output amp Q3. The output of L2
transformer is about 10 milliwatts. This signal is switched to either the
transmit mixer U3 or the receive audio detector U1 via 1N5767 PIN diodes. When
these diodes are turned on with a 7 milliamp current, the exhibit an RF resistance
of about 5 ohms. Both the XMIT and RCVE control lines are at 8 VDC when active.
MIXERS - The receiver front end mixer U6 is an old standard SBL-1 from
mini-CIRCUITS. These devices are proven to be the best bet for the money when it
comes to reliability and being able to handle a large dynamic range of signal
inputs without excessive distortion products. This is critical when working
along side other rigs such as Field Day ops or even another ham running a KW down
the street. The transmit mixer U3 is also an SBL-1. It was chosen because the
low impedance characteristics. There is less chance of stray signals getting into
the front end of the transmitter signal chain with a low impedance mixer. The
output of U3, the transmit mixer goes to the BAND MODULES. The input of U6, the
receiver mixer also comes from the BAND MODULES. Both mixers are also fed signals
from an HF PLL oscillator.
POST MIX AMP - The front end receiver mixer signal is passed through a 4 dB
attenuator pad, R36-38 to help terminate the front end mixer U6 to an aproximate
50 ohm impedance. Q7 is a high power class A amp running at approximately 25
milliamps. It will get warm to the touch.
NOISE BLANKER - This circuit is still in the design mode. If you do not plan on
using a noise blanker, then you can remove transformers L6 and L8 and add a
connection from R46 to D12.
CRYSTAL FILTER - The filter is comprised of crystals X2-X6 and D7 thru D12. The
varactor diodes used, MVAM108 exhibit a capacitance of about 100-500 pF when
reversed biased from 8 to 1 volts. This will give you a bandpass of about 3 KHz
to 300 Hz. The lower the voltage, the lower the bandpass. One characteristic of
this circuit is that as the bandpass gets tighter, so does the center frequency
go lower. If we decide that a 500 Hz bandpass is the average listening bandpass,
then we would measure that center frequency and use the same frequency for the
BFO when in the transmit mode. Example... 500 Hz bandpass freq = 4.9135 MHz then
the XMIT BOO freq is also 4.9135 MHz and the RCVE freq is the XMIT freq + 500 Hz
or 4.9140 MHz. To find the center frequency of the crystal filter, set the
bandwith pot R32 to about 2 volts. Connect a wire from the cathode of D2 to R45.
Connect an oscilloscope to the anode side of D7. Then adjust C3 for a maximum
signal on the oscilloscope, and then measure the frequency at R45 with a frequency
counter. Record the frequency. Remove the oscilloscope and freq counter and the
jumper from D2 to R45 and then connect the freq counter to the cathode side of D2.
Place the module into receive mode and adjust C3 for the recorded frequency + 500
Hz. Then place the module into transmit mode and adjust C5 to the recorded
frequency. Repeat the adjustments until both C3 and C5 are adjusted to give the
correct transmit and receive BFO frequencies.
IF AMP (MC1350) - This IOF amp IC will provide about 60 dB of receiver gain. L5
is a matching transformer to step up the impedance from the crystal filter from
about 150 ohms to 1350 ohms for the IF amp input. The IF amp has an AGC input of
5-9 volts to control the gain. Maximum gain is a 5 volts or less.
AUDIO DETECTOR (NE602) - The audio detector is an NE602 mixer, providing about 14
dB of gain. The BFO injection is via C12. The suggested BFO injection is about
250 millivolts BFO signal. Currently, I have not attenuated the BFO signal to
meet the 250 millivolt suggestion. This could be accomplished by reducing the
value of C12 until a measured 250 millivolts is at PIN #6 of the NE602 IC.
The audiuo output of U1 via PIN #4 goes to a MUTE switch Q5, which is controlled
by Q4 and XMIT signal.
AGC AMP (LM358) - A portion of the audio signal is passed to U4, the AGC amp.
The gain of the AGC amp is determined by C16 and R16. The output of the 1st amp
is detected by D4 and buffered by the 2nd amp. The RC time constant R20 and C25
determines the AGC delay time. With no audio signal present, R19 is adjusted for
5 VDC at R25. When an audio signal is present, it is detected by D4 and the
voltage at R25 increases to reduce the gain on PIN#5 of U5.
AUDIO AMP (LM380) - The audio amp is a common LM380 IC. It provides more than
enough audio for a speaker.
PART 4 - multiPIG Band Modules:
What makes the multiPIG versatile are the band modules. I have designed 2 versions;
the first used a terminal block to connect signal and power wires between the modules
and the main tranceiver; the second uses surplus PC 16 bit ISA sockets and connectors.
Obviously, the second method is prefered whan changing bands.
I looked around the electronics catalogs for PC ISA sockets, but found their prices
to be outrageous. One of the Flying Pigs (can't remember who) suggested using a
propane torch to de-solder ISA sockets from an old PC motherboard. This worked
very well. Took about 1 minute to remove each socket from the motherboard.
The male connectors are constructerd for the plug-in modules are constructed from
old 16 bit cards, which are cut using a hack-saw. First I removed all the components
near the edge connector and then cut the board all the way across about 1 inch up from
the card edge connecting fingers. I then mounted a copper clad board 6" x 3" to the
edge connecter with two small bolts after drilling through both boards.
[ show pictures here; plug-1.jpg, plug-2.jpg, plug-3.jpg ]
The transmit amp chain is relatively simple. The output from the SBL-1 transmit mixer
is fed to a 4 dB attenuator pad and then to a 2-pole sharp bandpass filter, L1 & L2.
The filtered signal is amplified by a buffer circuit using 2N3904 NPN's, Q1 & Q2.
This amp employs negative feedback via R8 to give very stable performance. The output
is controlled via the 500 ohm pot R11, to set the desired final output power.
The driver, Q4 is a 2N3866 VHF transistor using a tuned tank circuit with a low-Z
output link to the final, a 2SC1969 transistor that can handle 10 watts output.
The max I can get out of this circuit is about 8 watts on 160 meters, but I trim the
output back to 5 watts with pot R11. The heat sink, from a 486 uP cooler, is mounted
on the back of the copper board, held together with a 6-32 nut and screw. The collector
from the final is center-tapped/soldered to L10, a 10 turn FT37-43 ferite toroid, and the
output of the final is filterd by 2 double filter L11 and L12. L13 is used to set a
ground level for PIN diode D3.
The pin diodes D1,D2 and D3 make up the tranceive antenna switch. Their resistance is
about 5-10 ohms when forward biased with 5-10 milliamps and a very hi resistance without
forward bias.
The RF amp uses a MRF911 VHF very low noise transistor. You could also sub a MRF901
which is a bit more common. Bothe the input and output are double tuned to reject any
image frequencies.
This RF module design was used on 160, 40 and 20 meters. On 20, I only get about 4
watts output, but so what - good enough for QRP :-) I'll post the band critical parts
with winding info and cap values later at http://home.cinci.rr.com/w8diz/multi/
PART 5 - multiPIG QSK and Keyer:
PART 6 - multiPIG Freq Counter:
PART 7 - multiPIG PLL:
PART 8 - SWR - Antenna Tuner:
PART 9 - Conclusion:
