OH1TV OH1O OH1WX

Direct conversion receiver for 20m CW

OH1WX QRPp station for 20m CW as of 30.1.2012

From left: SWR detector, SWR indicator, Transmitter Minimal 1, Receiver, Keyer with capacitive touch paddle,

keyer speaker, RX speaker. Back from left: UTC time, transmitter current meter.

I had earlier designed QRPp transmitter "Minimal 1", which delivers 1W output on 14.060MHz and works fine.

I wanted to make a pair for it, a receiver in the same spirit. Soon it was clear that it is impossible to

make a receiver which is as simple as the transmitter is if you want some quality and working convenience.

I wanted the receiver be so good that it will used also after the first experiments.

To keep it simple, direct conversion concept was chosen. The drawback is that is will receive both sidebands.

The advantages are that neither RF-preamplifier nor IF-amplifier are needed and a good CW filter can be done on audio

frequencies. Because almost all active stages are on audio, construction is easy.

The overall picture with gain distribution is given in the Block Diagram.

In DC-receivers the mixer is critical, because it may act as an AM-detector when very strong signal is present.

Just below 20m band there is a broadcasting band providing multiple strong signals. After testing some

alternatives I ended up to double balanced passive mixer with Schottky diodes. It is 99% free from

AM-interference in my location when using 2-element yagi. CMOS mux might be better but adds compexity.

Note that often used 74HC4052 work only up to about 10MHz. The better one's are available only in SMD versions.

Mixer output impedance in less than 100ohms. The amplifier following the mixer shall have very low noise

with such a low generator impedance. The best op amp in that respect I found is LT1028. Its own noise

is at the same level as in 50 ohm resistor. I have a switch in the LNA's feedback loop which allows 12dB

additional gain in quiet band conditions.

The CW filter is a 7th order Cauer low pass filter with coils and capacitors. I used Vogt common mode 39mH

filter toroids and connected the windings is series to get 156mH coils.

The filter is very steep and compares well with xtal filters.

After the CW filter there is a post amplifier to provide correct termination for the filter and give additional gain.

Broad band noise shall be kept down after the CW filter. Therefore the post amplifier is also of low noise type.

Audio power amp is with standard LM386. Simple AGC is combined with it. Mosfet 2N7000 serves as variable resistor.

AGC control voltage is generated with full wave rectifier. Audio volume control is with 50ohm potentiometer in the output.

Because AGC eliminates the amplifier overdrive this arrangement can be done. The advantage is that broadband noise

is attenuated when reducing audio level. Also adjusting speaker or headphone levels is easy.

The VFO was taken from an earlier project years back. BC547 transistor could be replaced with better BF199's.

In construction TEKO RF- shielded boxes were used for the RF parts and LNA. The rest were built on

pieces of PCB-material ugly style.

Sensitivity with 500Hz bandwidth is -132dBm, which is below S0 level. Connecting antenna clearly adds noise.

RF preamplifier would not improve the signal.

30.1.2012

Pekka, OH1TV, OH1WX on QRP