is minimal. By keeping the number of active devices low (3 devices
between the antenna and the audio amplifier), very good fidelity
is achieved. The circuit is kept at a low impedance and broadband
everywhere except the front-end. This helps in stability.
The front-end uses a low-noise
FET. We have used a BFW11 (because the local component shop ran
out of BFW10). They have slightly different characteristics. Almost
any FET can be used if it is biased properly. The FET should be
biased for exactly half the pinch-off voltage. Wes Hayward (W7ZOI)
has described the proper way to bias a FET Mixer for proper operation:
► Short gate and source
and measure the current that flows through a 560 ohms resister
connected to +12V through the drain. This gives the exact Idss.
► Place a 10K resistor
between the source and the ground. Keeping the gate grounded and
the drain still connected through the 560 ohms resister, measure
the voltage between the source and the ground. This gives you
the pinch-off voltage.
► The FET has to
be biased such that the voltage on the source is exactly half
the pinch-off voltage and there is half of Idss current flowing
through the FET. Such a scheme assures you that the FET is driven
between pinch-off and maximum drain by the VFO injected at the
source. This gives the proper switching action for the mixer to
operate as well as maximum gain. We measured the pinch-off voltage
for BFW11s as 2.1 volts and Idss as 5mA. A standard 1K resistor
at the source gives the proper bias.
It should be noted here that we
first tried a double balanced diode ring mixer at the front-end.
It has a number of spurious responses that literally made it impossible
to use the receiver. We tried to properly terminate the diode
ring mixer by inserting attenuators between the mixer and the
Ladder filter’s pre-amp. It didn’t cure the problem. When we changed
to the FET mixer, the noise figure improved, the receiver’s dynamic
range, while unmeasured, was never found lacking in the last one
month of extensive usage at VU2PEP.
The output of the IF amplifier is detected in a balanced
detector using just two diodes. Here gain, we break a common myth.
You will see most of the HF receivers employing a two diode balanced
detector with the BFO fed to the center tap and the incoming signal
applied through the primary winding of the detector transformer.
This is wrong. The signal applied through the primary winding
should strong enough to switch the diodes on and off
about 0.6 across each diode, that is, 1.2 volts across the winding).
This roughly translates to about 5 mW power. The diodes switch
the low level signal coupled at the center-tap of the coil to
the detector output. Therefore, in our design we have applied
the local oscillator through the primary of the transformer and
the incoming signal from the IF stage to the center tap.
There is a 100
ohms preset used to null the local oscillator from appearing at
the output. This is of importance during transmit
where the balanced detector also doubles up as the transmitting
An audio pre-amplifier follows
the detector. The capacitor of 220 pf between the base and the
collector ensures that the ‘hiss’ is kept down. The audio amplifier
used is an LM380. Almost any audio amplifier can be used. We have
tried everything from the PC’s ampli-speakers to a Sony amplifier
to a TBA810 amplifier. We would recommend using a high fidelity,
low cost amplifier like the TBA810 if you plan using a speaker.
If most of your work is with headphones (to save your companion
from the late night QRM), we recommend the LM386.
starts with the modulator using a 741. There is a three resistor
network that biases the electret microphones. We use a Phillips
‘walkman’ style headphone with built-in microphone for our work.
The electret microphone requires a bias that provides 5V as given
by the circuit.
The balanced modulator also had
two 22pf trimming capacitors for nulling the carrier. They were
later found unnecessary (as long as both the diodes are purchased
from the same roll) and removed. If you do find balance a bother,
feel free to add a 22 pf trimmer to one side and a 10 pf fixed
to the other side as indicated in the schematic.
The output of the balanced modulator
is routed to the common IF amplifier through a buffer amplifier
using a BF195. This serves to keep the carrier leak from the modulator
out of the IF string during the reception mode.
The balanced detector of the receiver
also doubles up as a mixer during transmit. It is important to
balance out the VFO energy at the output by setting the 100 ohms
trimmer properly. We noticed a 50mW residual out-of-band output
from the transmitter when the VFO is unbalanced. The power chain
is an interesting broad-band amplifier. You can use this in virtually
any transmitter of up to 7 watts (and higher with more than 12
volts supply to the final stage). Three stages of broadband amplifiers
feed an IRF510 PA. It is an interesting twist that the driver
2N3866 transistors cost more than the IRF510! The IRF510 should
be biased for 80mA of standing current during transmit with the
microphone disconnected (no modulation) and carrier nulled by
the trimpot of the balanced modulator.