455 kHz Oscillator / BFO
using a ceramic filter
by IK1ZYW 2001

Note 1: you need a bit of knowledge on how a supereterodyna receiver works to understand the idea underneath this project. You also need a 455kHz ceramic filter to build it
Note 2: depending on ceramic filter's bandwidth, this project should work with all receivers with an IF between 450 and 460 kHz, just substitute your IF value to all 455s below.

Concept

What I needed was a way to demodulate SSB and CW with those AM only HF commercial receivers. The solution is to insert a BFO before the last IF stage of the receiver (that is usually working at 455kHz). You can find several good projects on Internet. But I did not want to touch the receiver circuits, since it was not mine... Then I read an article about inserting the 455kHz carrier in a less polite way [1] :-) : just put a 455kHz oscillator close to the last IF stage of the receiver, and for simple RF coupling you achieve the same result. That's the way I followed.

Problems

The solution proposed in [1] needs a 455kHz IF coil (color red) to work. I wasn't able to find any in my neighborhoud or scrap-box.

My solution

The idea underneath is the following. Through the filter I can get a 455kHz sine wave in output, whatever I throw into the filter. Therefore I can generate a square wave at a lower frequency and take only one of its harmonics. I chose to have the oscillator work at one-fifth of 455kHz, so around 91kHz. In order to have more flexibility I made the oscillator work from some 70 to some 130 kHz using a 1kohm variable resistor in series with a 2.2kohm on the feedback line of the inverter. Oscillator's base frequency can be obtained with:

So it is a simple, mono-frequency, frequency multiplier.
Some facts:

• the base-frequency oscillator excursion should be within 1/4 and 1/6 of 455kHz, otherwise you get 2 or more points that produce a 455kHz harmonic
• substituting the 455kHz filter with a 10.7MHz one you can obviously get that frequency in output, although I suggest to recalculate R and C for an easier tuning (remember, 1kHz change in the base oscillator frequency corresponds to n kHz at n^th harmonic!)

Tuning

Unless you can use a frequency counter, connect a wire (at least 10cm) to the filter output and put it close to a HAM receiver tuned on 455kHz, USB or LSB. It is also wise to use an indoor antenna or whatever piece of wire that can be put close to our BFO. Start turning slowly the 1kohm variable resistor until you hear the typical carrier whistle on the receiver. There you are close to 455kHz. Tune the oscillator until the beat disappears as it is a too low-frequency note to be heard by a human hear (so you have to go from a high-pitched tone to a lower one, until nothing is heard). Switch to the other SSB mode and make sure nothing can be heard. You are on the right frequency when:

• neither on USB nor on LSB a beat can be heard
• on both USB and LSB, moving receiver's frequency up or down (depending on U/LSB) brings back the beat

Without a HAM receiver tuning is more difficult. As soon as I experience it, I'll write something here.

Usage

Some notes on the receiver. I could get hold of two of them with analog tuning (no PLL). The first, more recent from Sonoko, doesn't use an IF at 455kHz but at 465kHz, therefore this circuit is unuseful. The second RX, older with sensitivity problems, allowed me to decode SSB transmissions. But, althought the BFO frequency was VERY stable (monitoring with the HAM HF receiver), RX's VFO turned out to be rather unstable, thus forcing to continuous tuning corrections or, once stabilized, to listen to a chirped voice. This might be due to a non-stabilized power supply to the receiver (I was using a cheap multi-connector multi-voltage transformer), since I ran out of batteries (PS: this proved to be wrong since the problem persists even with battery powering the receiver, so the receiver is instable).

See my operating notes for some help.

Construction hints

• Don't forget to put two bypass capacitors around the 5 volt voltage stabilizer (I used 10uF on 12V line and 47uF on 5V side).
  
• Don't forget to force at +5V or 0V unused inputs of the 74HC14 chip!
  
• Please do some Internet searches if you don't know the pin-out of your ceramic filter or of 74HC14: I do NOT consider myself responsible of any damage from burned components to house-on-fire if you decide to reproduce my project.
  
• With a 78L05 as stabilizer my prototype sinks 8mA at 12V supply. I assume you know how to setup the 12-to-5 volt circuit.
• Inserting the BFO into a metallic box is a nice idea :)
• I succesfully powered the BFO with a cheap 9V/500mA power supply (I think it was the one from my 14k4 modem). Given the very low current absorbed, no 50/60Hz can be heard.
• If you need to find the (last) IF frequency of your receiver... tune a station with a known frequency, then with another receiver go to look for the local oscillator weak signal 455kHz higher (or whatever you suspect the last IF is). Example: tuning on a 1MHz MW station will generate a carrier at 1.455 MHz, if the last IF is 455 kHz.
• You can use a 1k multi-turns trimmer instead of the variable resistor: much easier tuning and better stability are guaranteed!

Component list