10 kHz - 30 MHz


This simple homemade RX gives me much more fun than a much better commercial receiver!

My first (homemade) SSB and CW receiver 10 kHz to 30 MHz
When I made a new start with my old radio hobby, the first item to solve was how to construct something in such a way that the prototype is also useable as the final version. The usual way is to make a prototype, then design the PCB's, make the final version and hopefully a lot of the parts of the prototype can be used again. Finally you will have a nice sample, but without any possibilities for modifications afterwards.
The second item to solve was how to find an easy way to make a nice housing for the equipment with text labels etc.

As I did not have a shortwave receiver anymore, that would be a nice project to try out how it is to work with "dead bug" construction on unetched PCB.
For the housing of the receiver I bought a standard 19 inch cabinet and found a way to make good looking text labels on it with a label printer and the window function.

Did I like the construction?
The "dead bug" construction method on unetched PCB was a very nice method to make a prototype, also useable as the final version. Use of a standard housing with text labels is a very easy and fast method to make a nice enclosure.

And the receiver?
It is a big but not so very complex and stable receiver, used almost daily to receive all kinds of signals such as 28 MHz beacons, amateurs, the clubstation, ships, new 5 MHz amateur bands in the UK, 518 kHz Navtex, DCF77 time signal decoding at 77.5 kHz. It is stable enough to receive all kinds of digital modes.

Recently modified
However, a second shortwave receiver was made, simpler, smaller and with a better VFO stabilizer circuit. Due to the unique construction method it was quite easy to make modifications and improve the VFO and AVC circuit of this receiver as a result of the experiences with the new shortwave receiver.

Block diagram.

Why is it so simple?
There are a few reasons for the simplicity.


The VFO.
big diagram

The same type of VFO is used in the new shortwave receiver, so the story is almost the same.
Many small tuning ranges (37.9 kHz at 10 kHz and 61.7 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 48 to 78 MHz and the VXO frequency is 10 MHz. Frequency variation of the VXO is 7.9 kHz. If you want to calculate the tuning range, the formula is:

(VCO frequency) x (frequency variation of the VXO) / (VXO frequency)

So the actual 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.
The VCO frequency is divided by 16384. The maximum frequency (here 4761 Hz) has to be less than the 7.9 kHz frequency variation of the VXO. The loop filter component values are experimentally determined. Adjustment of the loop potentiometers is done by ear. 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.

The VFO coil
The coil is wound on a piece of a plastic potentiometer axis. It is totally glued in plastic with a glue gun to avoid all kinds of noises when knocking against the enclosure of the receiver. How funny it was to see that the VFO stayed exactly on frequency while the hot plastic of the glue gun was cooling off!!! Normally my VFO's start drifting already at the slightest temperature changes....

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 RF part

The RF part.
big diagram

Attenuator, preselector and HF preamplifier
Instead of broadband input filters, a tuned selective preselector is applied. 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 first transistor of the RF preamplifier is an impedance match, the second one gives some extra gain. The gain can be varied by changing the 560 ohm emitter resistor.

The mixer
The mixer is a Winner! The SL6440 is very good. During tests of the prototype, 5 meters of wire was connected directly to the input of the mixer without any preselection and many 80 meter amateur stations could be received without any overload or intermodulation products!

The IF and BFO
A 5 pole IF ladder filter (3.5 kHz wide but I will retune it to 2.4 kHz soon) is followed by a NE612 BFO mixer. So there is no IF amplifier! I did spent a lot of time to try to make a VXO with an overtone crystal at 48 MHz, but without success.... That is why the BFO works at 16 MHz and is tripled to 48 MHz. The LC circuit in the collector is already tuned to 48 MHz. So the second transistor is not a tripler but an amplifier.
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 LF part

The audio part.
big diagram

The Audio part
At the input is a second SSB audio filter for extra selectivity, it attenuates the high audio frequencies as the crystal filter is quite broad. Tune it narrower will be one of the next modifications.
For CW or digital modes like Hell, there are two filtes, a narrow and a wide one.
The AVC is controlled by a fet in the LF part of the receiver. For muting there are a few 4066 CMOS switches. However, I never used the receiver in combination with a transmitter so I cannot guarantee that it works!
The side tone oscillator was an experiment, it gives an audio tone of exactly 700 Hz, that is also the center frequency of the CW filters.

The Frequency counter

Frequency counter with 8 leds.

The Frequency Counter and mixer VFO-BFO.
big diagram

The Simple Frequency counter
The idea was to have a binary display with 8 leds. The frequency can be found by adding the frequency values of the illuminated leds. With the switch in the kHz position, the frequencies of the leds D7 to D0 are rounded off to:

200 kHz - 100 kHz - 50 kHz - 25 kHz - 13 kHz - 6 kHz - 3 kHz - 1.5 kHz.

Example: Leds D7, D5, D3 and D0 are on: 200 + 50 + 13+1.5 = 264.5 kHz.
However, we want to measure frequencies that are higher than 400 kHz. Therefore, a switch is added so that the led frequencies are higher if the switch is in the MHz position:

12.8 MHz - 6.4 MHz - 3.2 MHz - 1.6 MHz - 0.8 MHz - 0.4 MHz - 0.2 MHz - 0.1 MHz.

The MHz position is not so easy to use, but normally you will use it only to find the desired amateur band. Then you do use only the kHz position, reading the frequency is easy then.
The maximum frequency is 25.5 MHz (all MHz leds on). At 25.6 MHz, all MHz leds are off just as at 0 MHz. 28 MHz is displayed as 2.4 MHz. In practice, that is not a problem. A list of which MHz leds are "on" for a certain amateur band may be useful in the beginning.

The mixer for the Frequency counter
To get a signal equal to the reception frequency for the frequency counter, a mixer is used to mix the VFO and BFO down to the reception frequency. The mixer is followed by a 30 MHz low pass filter. The VFO potentiometer is set to maximum in my version, the potentiometer connected to the 48 MHz is set to a compromise between output signal and spurious. If the spurious is too high, the frequency counter does display random frequencies.


Interior of the receiver.

VFO 48 - 78 MHz with stabilizer.

RF part with 48 MHz crystal ladder filter.

BFO 48 MHz and mixer VFO-BFO for frequency counter.

LF part with CW filters and side tone oscillator 700 Hz.

The simple frequency counter with 8 leds.


It is a nice receiver with a pleasant sound. Due to the very good SL6440 mixer, the 3rd IP is approximately +8 dBm, strong signals are not really a problem! The narrow CW filter is very good for the suppression of broadband and narrow band interference. Of course it does not have all the features of a good commercial receiver, there is for example no Noise Blanker or Notch filter. But I had a lot of fun with this receiver, know how it works, can repair everything, it is easy to make modifications and finally, the second output of the mixer for the counter can be used as a signal generator from 0 to 30 MHz (but it is not spurious free enough for a transmitter as it was intended for!).