Help understanding the regenerator circuit in a crystal radio

Discussion in 'The Projects Forum' started by epsilonjon, Jul 21, 2012.

Feb 15, 2011
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Hello. I am planning to build a crystal radio for the first time and i'm following a little book I found on the topic. The design comes in stages, and I think I understand everything until the "regenerator" circuit is added instead of the simple crystal set stage. so he is going from the top circuit to the bottom one (ignoring the amplifier stages that follow):

1) In the top circuit i'm thinking of the antenna as a voltage source connected across the tuning circuit, so the resonant frequency is f = 1/(2pi√(LC)). In the bottom circuit you have the two extra capacitors coming off the top and bottom wires. I don't think these change the frequency equation much, but am I wrong?

2) It's difficult to see how the feedback is working. We want positive feedback right? So we need a 180° phase shift from collector to base (since the amplifier is inverting)? Just looking at the feedback through the 500pF capacitor I don't see how you can get this

3) Is the purpose of the RFC to allow the lower frequency modulating signal (the audio) and the bias voltages, whilst blocking the high frequency carrier?

I have a few more questions but maybe once I understand those three I'll be able to answer the rest myself.

Thanks for any help

2. KL7AJ AAC Fanatic!

Nov 4, 2008
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Hi Jon the Epsilon:

The regenerative receiver is actually an oscillator that operates JUST on the verge of self oscillation. At this point, nearly infinite gain and infinite Q (selectivity) is possible. The 10pf variable cap adjusts the amount of feedback. You need a safecracker's touch to do this right, but if you've never experienced a "genny' receiver, you'll be astonished at the sensitivity!

In many cases, the regeneration and detection are combined in the same stage.

Hope this helps!

Eric

3. sparky12x New Member

Jul 21, 2012
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When you add a regenerative stage it is no longer a crystal radio but is now regenerative receiver. The top schematic would be called a transistor radio

4. t_n_k AAC Fanatic!

Mar 6, 2009
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Q1. You are correct

Q2. The feedback is tricky - I think the small tap at the bottom of the tuning inductor actually provides the 180° phase shift to form the base drive signal. I think the 500pF cap is providing feedback for the demodulation/mixing function. The 10pF tickler also allows one to bring the feedback gain close to but not into the unbounded oscillation condition - as KL7AJ indicated.

Q3. Correct

Last edited: Jul 21, 2012
5. cork_ie Member

Oct 8, 2011
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Oh my God, this brings back some memories. My first radio set built on a wooden board with cup screws. I recognise the pics from 45 years ago straight out of the Ladybird book "Making a Transistor Radio"

OC71 and OC45 were germanium transistors and you are unlikely to find them nowdays.
A regenerative receiver was a popular method of increasing sensitivity & selectivity in radio sets in the interwar period i.e. 1920's & 1930's before the invention of the Tetrode facilitated the economical use super-heterodyne mixer sets.
A regenerative receiver amplifies the incoming signal over and over by introducing a feedback loop, just to the point of oscillation.This dramatically increases the gain of the amplifier at the tuned frequency , with little or no amplification at other frequencies. It increases the radio's sensitivity to weak signals and hence improves it's selectivity by swamping out other signals at close-by frequencies which caused interference and fading.

To prevent oscillation and it's characteristic howl (like a microphone held too close to a speaker on the same audio circuit) an additional knob was fitted to regenerative radio sets to control the feedback loop gain. An oscillating set would also tend to retransmit the selected frequency via the receiving antenna and cause interference to other users.

6. KL7AJ AAC Fanatic!

Nov 4, 2008
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Hi Corkie:

There is still a vast supply of NOS 1n34 germanium diodes. I think they originally made a few trillion of them! However, even better than germanium diodes are Schottky (Hot harrier) diodes, readily available for microwave engineering tasks. The reason the ancient cat's whisker was so sensitive was that it was actually a hot carrier diode.....a metal wire against a semiconductor.

Eric

7. MrChips Moderator

Oct 2, 2009
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Yeah, I grew up on OC44, OC45, OC71 and OC72 for breakfast!
I still have a number of Clive Sinclair's Transistor Circuit Manuals.

Feb 15, 2011
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I have read a little on self oscillators - the top feedback loop reminded me of the Hartley oscillator a bit, but it's more difficult to see how this one works!

You're spot on with the book I found it at a car boot for 50p!! There are a few of the germanium transistors on ebay - a bit expensive though. Hopefully they still work after all this time! Btw i'm not too experienced with picking specific transistors/diodes yet as I haven't gone much beyond general characteristics of fets/bjts, and am only just starting to read about the physics of the devices. Could you explain what is special about germanium ones? Could I replace with silicon pnps or would it be too tricky to find ones with similar values?

I was thinking that the diode in the bottom feedback circuit gives you half-wave rectification and then the RFC filters out the carrier wave. Is that along the right lines? I don't see how it ends up back in phase at the base though :s

9. t_n_k AAC Fanatic!

Mar 6, 2009
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The tapped tuning inductor has mutually induced voltages as per the polarities shown [at a particular instant] in my edit of your original image.

The inductor tap point being tied to common [ground] potential leads to the phase inversion [180°] at the base feed point.

I suspect there is a form of 'rectification' in the diode string pair - the diodes probably introduce non-linear mixing of the incoming AM signal and the amplified carrier frequency to produce a complex base input current having [among other things] the modulation signal itself.

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Last edited: Jul 22, 2012
10. KL7AJ AAC Fanatic!

Nov 4, 2008
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Actually, most regen receivers are based on the ARMSTRONG oscillator. This uses a "tickler coil" with no direct connection to the tank circuit except by magnetic induction. It's actually a bit easier to analyze than the Hartley oscillator....and actually easier to make OSCILLATE if you are using less than optimal components!

In a Hartley oscillator, the actual tap point can be critical, as it has a profound effect on tank coil Q....a little too "high" a tap point, and the Q goes right down the drain...and won't oscillate! In addition, the Q is also dependent on frequency...ideally, if you want to make a tunable oscillator, you want to tune the inductance and capacitance simultaneously...but nobody actually does that!

Fun stuff!

Eric