SHORTWAVE RECEIVER
10 kHz - 30 MHz


The simple shortwave receiver for CW and SSB.

SSB and CW receiver 10 kHz to 30 MHz
This receiver was made with use of the experiences of my first receiver. This new one has some improvements and modifications: Enough ideas to justify the construction of a new receiver!

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...


Block diagram.
big diagram

Description
The receiver has a 36 MHz IF with a 5 pole ladder filter and an almost drift-free VFO, it has Xtal stability, tuning is very comfortable.
The BFO frequency for a ladder filter should always be higher than the IF frequency, due to the asymmetric shape of that filter. To be able to listen to LSB signals below 10 MHz, the VFO runs from 36 - 10 MHz to 36 + 30 MHz. For LSB, the VFO frequency is below the IF frequency. So there is no USB LSB switch!
The AVC is derived from the audio.
At the input of the receiver is a RF attenuator and preselector.
For the frequency display, one 7 segment display is used. (see the page about a Simple frequency counter with one 7 segments led display).

The VFO


The VFO.
big diagram

The VFO
The same type of VFO is used in the "MyTRX" transceiver.
Many small tuning ranges (23.6 kHz at 10 kHz and 52 kHz at 30 MHz) can be tuned by the 10 turn potmeter. Press the up/down switches to go to the nearest next small tuning range. For large frequency changes, set S1 to coarse tuning. Otherwise you have to press the up switch 1000 times to go from 10 kHz to 30 MHz!!
The system is based on a frequency locking system with a sampler with a VCO (Voltage Controlled Oscillator) and a VXO (Variable Xtal Oscillator). Harmonics of the (VCO/16384) are locked to the VXO. The VCO runs from 26 to 66 MHz and the VXO frequency is 8867 kHz. Frequency variation of the VXO is 7 kHz. If you want to calculate the tuning range, the formula is:

(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.
No ceramic resonator is used in the VXO as is in the "MyTRX" transceiver but a Xtal for better stability. Stability with a ceramic resonator is not sufficient for this design. In the MyTRX the VCO frequency is divided, giving better stability of the final frequency.
As the tuning range of the VXO is less than that with a ceramic resonator, the VCO frequency is divided by 16384 instead of 4096. The maximum frequency (here 4028 Hz) has to be less than the 7 kHz frequency variation of the VXO. Due to another division ratio, the loop filter component values are adjusted experimental (higher R and C). Adjustment of the loop potentiometers is possible with an oscilloscope at TP2, but you can do that even better with your ears. Tune to a strong carrier at a low VCO frequency and adjust the 100 ohm potentiometer by ear for the best distortion free audio tone. Even very small instabilities in the loop that are not noticed with the oscilloscope are heard by ear when tuning to a strong signal.

Up-down switches.
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 HF part


The RF part.
big diagram

Attenuator, preselector and HF preamplifier
The attenuator improves the reception considerably in the evening on the 40 and 30 meter band while the sensitivity is still good enough to receive weak stations. The low-pass filter suppresses the mirror frequencies and the 36 MHz IF frequency. A trimmer is added for extra suppression of 36 MHz signals received via the antenna.
Instead of broadband input filters, a tuned selective preselector is applied to improve the performance of the receiver considerably. The inductances are readily available parts and mounted directly on the bandswitch. For the band below 150 kHz there is a simple RC low pass filter.
The HF amplifier is certainly not a top performance transistor amplifier, but good enough due to the selective preselector. The first transistor is an impedance match, the second one gives some extra gain. The gain can be varied by changing the 220 ohm emitter resistor.

The mixer
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 IF and BFO
A 5 pole IF ladder filter (2 kHz wide) is followed by a Poljakov 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.
big diagram

The Audio part
At the input is a second SSB audio filter for extra selectivity and high frequency audio noise suppression. It is followed by a CW filter (600 Hz high pass and 800 Hz low pass).
The AVC is controlled by a fet in the LF part of the receiver. It is also used for mute during TX. Even a side tone oscillator is included, so that the receiver can be used with a transmitter.

The Frequency counter


The Frequency Counter.
big diagram

The Frequency counter
Similar to that used in the "MyTRX" transceiver (see also the page about a Simple frequency counter with one 7 segments led display if you want to know how you can read the frequency while using only one display). There is an extra input to measure the frequency of the external transmitter. If a DC voltage is applied together with the RF signal, the counter measures the frequency of the transmitter. This DC signal is switched by the tune switch of the external transmitter.
The exact IF frequency is programmed by zero beating the VFO at 36 MHz (0 Hz reception frequency). This value is stored in the internal eeprom.

Results


The start of the Barefoot Technology receiver project.
Only some ideas, drawings and components. Should it ever work?

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 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

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!

MEASUREMENT DATA

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