10 kHz - 30 MHz
KLIK HIER VOOR DE NEDERLANDSE VERSIE
The simple shortwave receiver for CW and SSB.
No AM reception
The receiver does not have AM reception. But if you want, you can receive AM in SSB mode. The advantage is that there is no distortion due to selective fading.
Speech sounds acceptable, just like SSB, music sounds like an old 78 rpm gramophone...
The RF part
The RF part.
The mixer is a balanced transistor mixer, not because that it is so good, but because I was curious to see if it is really as bad as they say. Well, it's performance is acceptable together with the preselector and input attenuator.
It is single balanced. A double balanced mixer will also attenuate 36 MHz signals from the antenna and noise on that frequency generated by the RF preamplifier. That will improve the sensitivity at 28 MHz. Another advantage of such mixers are a better buffering of the VFO signal. I had to add a resistor of 820 ohm between the RF preamplifier and the mixer to avoid that the VFO frequency changes when extremely strong signals (more than 0 dBm) were fed to the antenna input. My advice: Take a NE612 or even a SL6440 if you want to have a real good mixer, do not copy this one.
The 36 MHz ladder filter adjusted for maximum sideband suppression of 40 dB at 600 Hz. The upper trace is the filter curve, the lower trace the suppressed sideband.
The 36 MHz ladder filter adjusted for maximum flatness. Sideband suppression is less but still 25 dB at 600 Hz.
The IF and BFO
A 5 pole IF ladder filter (2 kHz wide) is followed by a Poljakov (or RA3AAE) mixer with two diodes. The BFO frequency for such a mixer is half it's working frequency (half the IF frequency: 18 MHz). The advantage is that it is easy to make a VXO for 18 MHz but not for 36 MHz as you need an overtone crystal for that frequency. It is not possible to make a good VXO with an overtone oscillator.
The IF gain control is not really an IF control as it is in the LF part of the receiver. But it has the same effect: Control the AVC voltage.
The AF part
The audio part.
(VCO frequency) x (frequency variation of the VXO) / (VXO frequency)So the range of the actual small frequency band that can be tuned by the 10 turn potentiometer is depending on the VCO frequency and the frequency variation of the VXO.
One important remark about that: The Up switch does only work properly if the VXO is tuned to a high frequency. For the lower VCO frequencies it works already when the VXO is tuned to its center frequency. And for the Down switch it is just the opposite.
Sometimes a (not very stable) locking occurs while there is an AC ripple on the loop (check with an oscilloscope on TP2). But you can solve this problem with some DC offset, adjust the 10k potentiometer a little to get this DC offset.
The Frequency counter
The Frequency Counter.
The start of the Receiver project.
Only some drawings and components. Will it ever work?
Notes for alignment
Adjust the trimmer of the 36 MHz reject filter for maximum attenuation of a 36 MHz signal when the receiver is tuned to the 10 meter band.
Adjust the 100 ohm potentiometer of the mixer for minimum noise for reception below 100 kHz.
Adjustment of the ladder filter is more complex. Input and output trimmers are tuned to maximum signal.
The four other trimmers (30-40 pF? I do not know) are set to 50 percent of their value. The AVC is switched off, the audio output from the loudspeaker is connected to the audio line input of the PC soundcard VIA A RESISTOR of 10k ohm.
An audio spectrum analyzer program is running on the PC.
While tuning around the 10 MHz clock signal of the frequency counter (or another carrier), see how the level varies when the tone height changes. Adjust the trimmers for best filter shape. At each audio frequency that has to be adjusted, find out which trimmer has most influence.
Well after two hours I had a shape that was a little optimized for CW and that had a quite good sideband suppression. It's performance is very acceptable!
Do not forget to adjust the BFO frequency of 18 MHz to the filter edge!
The SSB filter (green) and
the suppressed sideband (orange).
The CW filter (green) and
the suppressed sideband (orange).
It works! Although not as good as an expensive commercial receiver, a lot of DX is heard on all amateur bands. Selectivity and sensitivity are good. Tuning is very comfortable. CW and SSB filter are okay. AVC works good.
As the mixer is not a very good one, I have to use the input attenuator in the evening at 40 and 30 meter. But also with the extra input attenuator, sensitivity is good enough to hear the atmospheric noise.
Even with 20 dB attenuation, the sensitivity on 30 and 40 meter is sufficient. The advantage is that the 3rd IP also increases with the attenuator value.
A lot of hours are already spend to listen to this receiver with pleasure. Even to AM transmissions! The receiver is stable and the CW filter sounds good, not too narrow. Operation is simple, no manual needed for all kinds of "hidden" controls.
Of course there was an error: the 56k and 10k resistors of the under-voltage circuit of the frequency counter were exchanged (diagram is correct, not the photo of the frequency counter). The result was that the EEprom values changed sometimes during switching off the receiver.
SOFTWARE FOR THE FREQUENCY COUNTER
"FREQRX02.ZIP" WITH "FREQRX02.ASM" TO PROGRAM THE FREQUENCY COUNTER
PHOTOGRAPHS OF THE RECEIVER CONSTRUCTION
Top view of the interior of the receiver.
Bottom view of the interior of the receiver.
RF preamplifier, mixer with trifilair coil on plastic rod (no ferrite!)
ladder filter, BFO with Poljakov (RA3AAE) mixer, LF preamplifier
The frequency counter with SMD chips.
Thin wires are soldered to the pins of the chips before mounting them.
During soldering such a thin wire, the pin is isolated from the others by aluminium foil.
The SMD chips are glued on a piece of wood (from a match) on the PCB.
Pieces of a glue stick are used to fix the resistors etc. (melting them with the soldering iron).
Back side of the receiver
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