History of Development

Ever since I became familiar with broadband amplifiers, I had been trying to design a superhet with one amplifier that did all the work from RF amplification to the IF and even audio.

One day I was studying the MOSFET single balanced mixer in DeMaw's Design Notebook and discovered that just one change on the gate 2 circuit would change it from a mixer to an amplifier, or to an AGC controlled amplifier.

I immediately built four of these mixers and made the changes to make my single circuit superhet, and it worked great!

Using a balanced amplifier at all the stages seemed excessive, so a single MOSFET amplifier was designed that would work in all places. Two single amplifiers could be put together to form the balanced mixer. This design was equally successful.

The First Universal MOSFET Receiver

Then I discovered through my research an article in the January 1975 issue of Ham Radio titled "Low Cost Printed Circuit Boards, A Simple Technique for Building Low-cost Printed-circuit Boards for Your Homebrew Amateur Equipment".

The article also described the building of a MOSFET receiver that used one circuit board layout for the RF, IF, Mixer, and Product Detector. The author stated "I have never been able to obtain such high performance as easily in homebrew gear." (Page 25)

Circuit details for the VFO, BFO, AGC and Audio were not included in the article. The article concentrated on building the boards and didn't give complete details on building the receiver. In my opinion, this was a real shame because the receiver was a real performer.

As usual, when you think you have discovered something, you find it has already been done by someone else.

Circuit details for his receiver follows:

Universal Printed-circuit Board
MOSFET Mixer Stage
RF or IF Amplifier
High-gain Product Detector
Grounded-gate RF Stage

DX Receiver for the Ham Bands

"DX Receiver for the Ham Bands", by Ovi Florea, Ham Radio, December, l976, Page 10, was the second most important article involved in the development of the LED MOSFET Receiver. The "DX Receiver" is another all MOSFET design, using a single balanced RF amplifier, a single balanced mixer, and a MOSFET 455kHz IF strip.

The LED MOSFET Receiver is considered a 'buildable' version of the "DX Receiver". If a .01 capacitor is put on Gate 2 to ground on the LED MOSFET Receiver single balanced mixer, you have a single balanced RF amplifier very similar to the RF amplifier in the "DX Receiver". Using this single balanced RF amplifier in the input circuit, the LED MOSFET Receiver and the DX Receiver would be almost identical.

The DX Receiver uses tuned circuits on all of its amplifiers whereas the LED MOSFET Receiver uses broadband circuits. A broadband receiver is very easy to build and can be used with any crystal filter and VFO frequency with very few changes.

The DX receiver uses a tunable 4-pole RF filter which is very selective (50 khz wide at 20-db down) versus the bandpass RF filter that the LED MOSFET Receiver uses. The DX Receiver needs a six-gang split-stator variable capacitor to build the rf filter, which is almost impossible to find and then very hard to properly track through all the stages. When I have worked contests, all the signals seemed to be stacked in a 20 to 30 khz range, so the benefit of a tunable RF filter is questionable over a much "easier to build" bandpass filter. A bandpass filter has less loss than a tunable RF filter, making the bandpass RF filter look better overall than a super selective tunable RF filter.

The DX receiver used a variable injection control at the product detector. I quote from the article, Page 16, "Theory says that, for minimum distortion a product detector requires carrier injection at least 10 times the level of the incoming signal at the input port. What would happen upon lowering the injection level? Decreasing the level caused strong perturbing signals to become unreadable, while weak DX signals were still crystal clear. One helpful finding was that the audio output versus bfo level dropped about 10 db faster for the stronger signal compared to the weak one."

The variable injection technique at the BFO is the same for both the DX Receiver and the LED MOSFET Receiver. This feature was also incorporated in the LED MOSFET VFO Variable Gain Amplifier, which helped fine tune the receiver for weak signal reception. Another weak signal feature of the LED MOSFET Receiver is changing the input impedance of the RF Amplifier following the first mixer.

Details of the front end of the DX Receiver follows:

Block Diagram of Front End

Front End Circuit Diagram

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Last Update: 01/20/2000
Web Author: David White, WN5Y