The use of the MOSFET QSK switching greatly reduces or eliminates IMD problems which can be caused by a
passive series L/C network with back to back limiting diodes at the L/C junction. This scheme is used in many QRP rigs,
including the previous versions of this rig. It was found that even when using two diodes in series, enough voltage could be
produced at the high impedance L/C junction by strong SWBC stations to cause the diodes to conduct. This would then
cause IMD. To further improve performance in the presence of strong SWBC signals and to improve image rejection, double
tuned input circuits are used on all bands. The first coil in the filter needs to be link coupled, so a toroid is used. The second
coil in the circuit is directly coupled to the input of the first mixer, so a RFC is used here to eliminate the winding of six toroids.
The first mixer input is connected to the top of the tuned circuit, so it provides us with some passive gain.
The input signal is then mixed with the LO frequency in U1 to provide an IF of 4.9152 MHz. The IF is filtered by four crystals.
The filter is terminated by a 470 ohm resistor, which helps flatten out the filter response. Without the terminating resistor, the
filter has noticeable peaks in the response.
The use of four crystals provides significantly better performance than the three crystals many other QRP rigs use.
Four crystals provide somewhat more selectivity, but more importantly, they provide much better rejection of the opposite side
band. You will be much less likely to be tricked into trying to contact a strong station on the wrong side band with the four
crystal filter than with a three crystal filter.
The IF is then mixed in U2 with the BFO oscillator to produce the audio base band signal. 1000 pF caps from the
output pins to ground helps eliminate RF mixer products and reduce high frequency hiss.
The output of the BFO mixer drives a differential input op amp. Using differential instead of single ended input
effectively doubles the output of the mixer for a 6 dB voltage gain. It also eliminates any common mode signals which might
be on the output of the mixer. This first stage provide a voltage gain of 33. A dual diode is connected across the amplifier
feedback resistor to clamp strong signals. These diodes keep the following AGC amp from being overdriven, which causes
distortion and charges the AGC hang cap to the point it takes a few seconds to recover.
The output of the first audio stage then goes into a Panasonic audio AGC amplifier, which provides an additional 26
dB of audio gain, before AGC action starts. This part was meant to be used as a microphone preamp and AGC in cell
phones, but it works well here to eliminate the need for a volume control, which we have no room for.
The output of the AGC chip then goes into a SPDT analog switch. This switch is used to mute the receiver by
disconnecting the preceding audio stages and connecting the side tone to the audio output stage.
Finally, the audio goes into the head phone driver stage, with up to 105 mw of output drive, which is also configured
as audio band pass filter, with a gain of 1 and 600 Hz center frequency. The Q is a modest 5 and this helps to peak the CW
beat note. A 10 uH choke and .01 uf bypass cap keeps RF out of the audio amplifier. Without this RF filtering on the audio
output, RF pickup on the headphone leads could cause raspy side tone when using end feed or open wire feed antennas.
Transmitter:
The transmitter is about as simple as you can get. The square wave output of the DDS is buffered by a pair of
74AC02 NOR gates, connected in parallel. Singe the output from the DDS is high impedance when turned off, a pull up
resistor is added to keep the output of the NOR gates low when not transmitting. Two gates are used in parallel as that
lowers the output impedance and is better capable of driving the gate capacitance of the power output FETs.
The RF power amplifier is comprised of three BS170 MOSFETs in parallel. The BS170's have a relatively high “on”
resistance, so using three of them in parallel reduces their effective resistance and boosts efficiency. They also share the
load, enabling higher power output. PA efficiency is about 70 to 75%
The drain to source break down voltage of the BS170's is a relatively low 60 volts. This can easily be exceeded
when operating at 5 watts with a moderate SWR. Therefore, a 46 volt zener diode is added across the drain to clip the
voltage. Even though a 1 watt diode is used, under conditions of very high SWR, the current can get high enough in the
diode to cause it to short out. Therefore, very high SWR conditions should be avoided!
.A low pass filter connects the output of the PA to the antenna. This filter provides both impedance matching and
harmonic suppression. The second coil in the filter is tuned to the second harmonic with a parallel cap. This greatly reduces
the second harmonic and helps increase efficiency of the PA. An additional small inductor, 0.15 uHy is placed in series with
the output of the LPF and the antenna terminal. This inductance, in combination with the connecting coax capacitance
eliminates VHF spurs which can leak through the HF filter.
The PA is keyed by supplying power to the PA though a P Channel power MOSFET. A 0.01 ufd capacitor connected
between the gate and drain output, in conjunction with the gate resistors develops a fairly linear 2 ms turn on and turn off
ramp time so reduce keyclicks. A 5.1 volt zener connected between the supply voltage and the junction of two resistors in the
gate leg resistor divider chain ensures a known voltage across the gate to keep the turn on and turn off ramp times
consistent regardless of supply voltage.
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