LR VLF Q-MULTIPLIER
(Also known as a Regenerative Receiver)
A Lightning Radar Project
| This is a page
describing a Q-multiplier / Regenerative Receiver built to pull weak
signals up out of the mud, and shut out adjacent signals and
noise. This circuit is based on the well known Wien Bridge
oscillator circuit but modified into a Q-multiplier / Receiver for
Combining two Circuits
This circuit was designed to be a stable receiver with a narrow band pass at 10 KHz. After researching receivers for 10 KHz, none could be found that met those needs. Inductor size and stability are real killers at this frequency for a non-heterodyne type receiver. The Gyrator circuit is used by some designs to over come the inductor size but by their nature are tricky to implement and stability is terrible.
Since we are only talking about an oscillator really, research will show that the most stable of these seemed to be the Wien Bridge oscillator that in effect created the HP company of today. We will not be using it as an oscillator the way it was intended however.
This oscillator has what we need in stability and sine wave ability. You also don't need large coils or tricky Gyrators in this circuit. But its not a receiver! So now we need to turn it into a receiver. If you go back to the 1920-1930 era you will find that the main receiver in use was a very simple but highly effective device for its day. This is the Regenerative receiver. As more stations appeared on the air , it fell out of favor for several reasons and the superhet methods replaced it after WW2.
This type receiver is really a starved oscillator. An amazing thing happens if you bring an oscillator right up to the point of oscillation. It develops a lot of gain, and its centered around a small band pass. In fact it can go beyond what we can use because of band pass ringing that always occurs in extremely small band passes. So what if we take the very stable, Wien Bridge circuit and limit its gain. Then inject our signal , the same as in a Regenerative receiver. Well some neat things happen.. We have a stable receiver parked on 10kHz with a narrow controllable band pass, that is quite Hi Fi actually. It also has no filter delays that can mess up system timing on LR sync, like band pass filters have been found to do.
Although its not shown on the picture on the right, it was necessary to add a buffer on the front end of the receiver. This keeps the amplified signals from making their way back to the antenna and crossing over to the other channel. This is just a single chip wired as a pass through buffer. The amplified signals cant make it backwards through this circuit.
Making it actually work
There are a few critical things to note, when using this otherwise simple circuit. The first thing is temperature stability. The first prototype was built with common ceramic capacitors and carbon resistors and it was unstable.. After adjustment, the settings would only last a few hours. By replacing those components with Polystyrene capacitors and metal film 1% resistors , it can now go weeks and stay on frequency.. Its so stable I'm really not sure how long it would go if not forever.. Now this is in a temperature stable home environment, not in an uncontrolled garage or something. This is meant to be used next to your keyboard or such.
The second thing is to adjust the gain / bandwidth. The way the receiver works puts you in harms way for going into oscillation. This was a walking on egg shells thing until some things were figured out. First, after a while it became apparent that we cant actually use that narrow of a band pass near where it goes into oscillation.. By watching the LR scope for ringing sine waves, you get those , then back off until they are gone.. That gives you the most narrow band pass useable.. At that point and wider it will stay stable. That level can be set by using film 1% resistors of the size needed for the chosen setting. Reading the adjustment pot will give you that value. It's best to stay back a little from the cliff edge of oscillation as it is not necessary.
One last thing then needs to be addressed some how.. We need to make sure both channels are adjusted so they have equal gain. Other wise your antenna beam will be turned in some direction..
Fortunately that is easy to accomplish. By switching both channels to one loop, they then receive identical signals.. When they are adjusted to over lay each other on Spectrogram 16, then we have a perfect match..
Read about the Wien Bridge circuit. and Regenerative receivers.
Schematic Diagram / Circuit Description