24-05-2013, 11:38 AM
(This post was last modified: 24-05-2013, 11:46 AM by Radio Fixer.)
Seems to me you are describing super regeneration invented by Howard Armstrong. http://en.wikipedia.org/wiki/Regenerativ...e_receiver
I restored a US AM / FM set that got the FM with the use of just one extra valve. Not super quality but it worked. It used a circuit called a Fremodyne which was just the super regen principle. Here's an extract from an article I wrote:
"The Fremodyne FM Converter
The single, double triode, valve converts the FM band of 88 to 108 Mc/s to give audio output. When switched to FM this output is amplified by the AM pre-amp and the 35L6 output valve.
Wireless World, of March 1948, which included a short article on the circuit, put the functions like this:
One section of the valve works as the local oscillator necessary for superheterodyne conversion, while the other section functions as:
1) A superhetrodyne convertor to an IF of about 22 Mc/s
2) A super-regenerative I.F. amplifier of high gain.
3) An F.M. to A.M. converter and
4) A detector giving an AF output.
W.W. also said the circuit has been criticised for radiation but the Hazeleltine engineers said that it was 30-40 dB better than an ordinary super-regenerator. This would be one without superhetrodyne conversion and a separate local oscillator stage. Here, this runs at around 22 Mc/s higher than the tuned frequency and is not near the resonant frequency of the input tuned circuit. Another problem was said to be that stations could be tuned in two places, one on each side of the I.F. transformer response with both being correct.
Now from the schematic of Figure 1, how does this clever piece of circuitry actually work?
Below is an explanation ‘worked up’ by me. I don’t know whether it is totally correct?
Fremodyne.pdf (Size: 42.02 KB / Downloads: 91)
From looking at Refs. 3 and 4, I consider the second triode (12AT7 b) to be a VHF Colpitts oscillator. The anode is at AC earth, due to capacitor C27, and so is connected to one end of the oscillator coil, L62, the other end of which is connected to the grid. Now the grid to cathode capacitance and the stray capacity from cathode to earth forms the tapped capacitors, across the coil, associated with this type of circuit. The RF choke, L67, allows DC to flow in the valve but has a high reactance so as not to effect oscillation. Ideally, the resistor R55 should have a value so as not to load unduly the ‘tank’, formed by L62, A8 and the tuning capacitor. No doubt by suitable choice of R55 and the capacitor C29 it is possible to control oscillator amplitude, in ‘grid leak’ fashion.
The first stage (12AT7 a) has to be basically an oscillator that is continually quenched (stopped) at supersonic frequency. Again it looks like a Colpitts with C31 and C32 forming the feedback capacitors.
They are also the IF transformer tuning capacitors, returned via C36. Choke L65b provides a high impeadance DC path.
Mixing takes place, between the input signal and the injected oscillator waveform, at the non-linear cathode to grid of the stage. R54 and L66 provide a high impeadance DC return path for the grid.
Capacitor C33 and resistors R48 and R49 are the quench components. This capacitor, charging via the grid cathode diode, leads to the valve to becoming biased off. If an oscilloscope is connected to C9 negative (or positive), then a sawtooth waveform is seen, of about 25 Kc/s, with audio superimposed upon it. The value of C9 is not critical, it being just an audio coupler with IF shunted away by C34.
As the frequency modulation sweeps across the tuned circuit skirt, the IF current through the valve changes and this is converted to an audio voltage change across R49. R63 and C25 provide de-emphasis.
So what of performance? AM performance is as good as any small radio of the time and is quite respectable. This can’t be said for the FM band. It can only be described as poor and the quality is worse than the cheapest solid state radio. It is possible that I am still not getting the best from the circuit; I was reluctant to start disconnecting and measuring the ceramic capacitors’ birdnested’ around this stage. However, these did look as good as new with solid encapsulation. Also, I have ‘talked’, via the Internet, with US collectors and old radio dealers. They said this performance was about all it was capable of, and some gave a lot of other more disparaging remarks as well!
It needs strong signals and careful tuning to minimise distortion and low level background whistles, to which it is prone. It does have reasonable rejection of AM noise, presumably the high gain of the super-regenerator, means it acts as a limiter. I have a fluorescent bench light that generates stacks of interference (I must get around to adding some line filters); it is so bad that I cannot have an AM radio in use in the same room. On the FM band of the Gilfillan, although it can be heard when switched on, it is at a very low level and not objectionable. So in 1948, buyers would have had a radio with which they could listen to the new FM stations, with a fair degree of AM noise rejection, for just $34. It was one of the lowest priced FM receivers. "
Gary
I restored a US AM / FM set that got the FM with the use of just one extra valve. Not super quality but it worked. It used a circuit called a Fremodyne which was just the super regen principle. Here's an extract from an article I wrote:
"The Fremodyne FM Converter
The single, double triode, valve converts the FM band of 88 to 108 Mc/s to give audio output. When switched to FM this output is amplified by the AM pre-amp and the 35L6 output valve.
Wireless World, of March 1948, which included a short article on the circuit, put the functions like this:
One section of the valve works as the local oscillator necessary for superheterodyne conversion, while the other section functions as:
1) A superhetrodyne convertor to an IF of about 22 Mc/s
2) A super-regenerative I.F. amplifier of high gain.
3) An F.M. to A.M. converter and
4) A detector giving an AF output.
W.W. also said the circuit has been criticised for radiation but the Hazeleltine engineers said that it was 30-40 dB better than an ordinary super-regenerator. This would be one without superhetrodyne conversion and a separate local oscillator stage. Here, this runs at around 22 Mc/s higher than the tuned frequency and is not near the resonant frequency of the input tuned circuit. Another problem was said to be that stations could be tuned in two places, one on each side of the I.F. transformer response with both being correct.
Now from the schematic of Figure 1, how does this clever piece of circuitry actually work?
Below is an explanation ‘worked up’ by me. I don’t know whether it is totally correct?
Fremodyne.pdf (Size: 42.02 KB / Downloads: 91)
From looking at Refs. 3 and 4, I consider the second triode (12AT7 b) to be a VHF Colpitts oscillator. The anode is at AC earth, due to capacitor C27, and so is connected to one end of the oscillator coil, L62, the other end of which is connected to the grid. Now the grid to cathode capacitance and the stray capacity from cathode to earth forms the tapped capacitors, across the coil, associated with this type of circuit. The RF choke, L67, allows DC to flow in the valve but has a high reactance so as not to effect oscillation. Ideally, the resistor R55 should have a value so as not to load unduly the ‘tank’, formed by L62, A8 and the tuning capacitor. No doubt by suitable choice of R55 and the capacitor C29 it is possible to control oscillator amplitude, in ‘grid leak’ fashion.
The first stage (12AT7 a) has to be basically an oscillator that is continually quenched (stopped) at supersonic frequency. Again it looks like a Colpitts with C31 and C32 forming the feedback capacitors.
They are also the IF transformer tuning capacitors, returned via C36. Choke L65b provides a high impeadance DC path.
Mixing takes place, between the input signal and the injected oscillator waveform, at the non-linear cathode to grid of the stage. R54 and L66 provide a high impeadance DC return path for the grid.
Capacitor C33 and resistors R48 and R49 are the quench components. This capacitor, charging via the grid cathode diode, leads to the valve to becoming biased off. If an oscilloscope is connected to C9 negative (or positive), then a sawtooth waveform is seen, of about 25 Kc/s, with audio superimposed upon it. The value of C9 is not critical, it being just an audio coupler with IF shunted away by C34.
As the frequency modulation sweeps across the tuned circuit skirt, the IF current through the valve changes and this is converted to an audio voltage change across R49. R63 and C25 provide de-emphasis.
So what of performance? AM performance is as good as any small radio of the time and is quite respectable. This can’t be said for the FM band. It can only be described as poor and the quality is worse than the cheapest solid state radio. It is possible that I am still not getting the best from the circuit; I was reluctant to start disconnecting and measuring the ceramic capacitors’ birdnested’ around this stage. However, these did look as good as new with solid encapsulation. Also, I have ‘talked’, via the Internet, with US collectors and old radio dealers. They said this performance was about all it was capable of, and some gave a lot of other more disparaging remarks as well!
It needs strong signals and careful tuning to minimise distortion and low level background whistles, to which it is prone. It does have reasonable rejection of AM noise, presumably the high gain of the super-regenerator, means it acts as a limiter. I have a fluorescent bench light that generates stacks of interference (I must get around to adding some line filters); it is so bad that I cannot have an AM radio in use in the same room. On the FM band of the Gilfillan, although it can be heard when switched on, it is at a very low level and not objectionable. So in 1948, buyers would have had a radio with which they could listen to the new FM stations, with a fair degree of AM noise rejection, for just $34. It was one of the lowest priced FM receivers. "
Gary
