To create is to live

Can also be used as RF-millivoltmeter

Published in SPRAT#200

In many of our short wave radios the simple diode envelope detector is used for amplitude demodulation. It is build from a single diode and some passive parts. It is a very low cost and simple circuit block. Because of this favourable characteristics it can be found in many radios. However, it also comes with drawbacks. Detection of low signals are not handled very well. A Schottky diode has a turn on voltage of circa 0.2V – 0.3V and a standard silicon diode has a knee voltage of around 0.6V. Although it is true that the diode will start to conduct before this, the transfer characteristic is highly non-linear (the so-called quadratic region). It will enable detection of lower level AM signals but at the expense of increased distortion at these levels. The proposed circuit seeks to remedy this.

Concept
The core of the problem is that the voltage to be rectified must be above the threshold value. However, when a current instead of a voltage needs to be rectified, the threshold voltage is no longer important.
In a common emitter amplifier stage, changes in input voltage are translated to changes in the collector current. These current changes are then translated by the collector resistor to changes in the collector voltage. If the collector resistor is swapped with a DC current source as load (figure 1), changes in the collector current (the AC current) will be forced to the load. In this circuit the load of Q1 is formed by a diode rectifier circuit.
Because this stage is current driven, the threshold voltages of D1 and D2 have become largely irrelevant. The resistors R2 converts the rectified current back to a voltage. Another way of looking at this stage is to say that due to the high dynamic resistance of the current load, the open loop gain of Q1 is extremely high. The closed loop gain is determined by R2 and R1 such that Av = R2/(r_be + R1). The values for R1 and R2 are chosen quit low to help in achieving a high bandwidth. With the current values, this translates to a gain of circa 2x.

Detailed Circuit Description

Please consult figure 2 for the implementation details. The circuit consists of a number of blocks: Q1 is a voltage to a current converter, Q2 and Q3 form a DC current load for Q1, D1 and D2 are the current driven balanced rectifier and A1 is the final amplifier stage.

The collector load of Q1 is formed by a DC current source build with transistors Q2 and Q3. This configuration functions as a static dynamic current source. Capacitor C5 keeps the voltage across, and thus the current through, R4 constant. Q2 and Q3 form effectively thus a current source. It is ‘dynamic’ because it automatically adjusts to slow changes in the standing current of Q1. It is ‘static’ because it does keep constant for rapid fluctuations.

For RF frequencies, the collector-base capacitance of Q2 might limit the impedance seen by transistor Q1. Inductor L1 helps to increase the collector impedance. A FT37-43 toroid based inductor was found to work quit fine in this application. A 220uH RFC choke was used but with less satisfactory results (see measurements below). Alternatively, L1 good be left out and Q2 and Q3 could be replaced with good (low capacitance) RF transistors. This ideas was not further investigated, however.

The maximum input voltage is determined by the DC emitter current and is Vi,max< Ie x R5 or 237 mVp with the current values

Capacitor C6 and C7 are the smoothing capacitors. With the values given, the corner frequency is circa 15 kHz. The average voltage after smoothing for a half wave rectified sine wave is given by the formula Vavg= Vp/ π = 0.318 xVp.

Operational amplifier A1 is setup as a difference amplifier with a gain of 10x. A1 has two outputs: the ‘RF level’ output gives the average output of the RF carrier minus the AM demodulation. This can be used to drive the AGC in an amplifier or to drive the meter circuit in RF-millivoltmeter applications. Keep in mind there is a 5V offset. The ‘AF’ output gives the demodulated output signal.

Even though it is a RF circuit, the properties of most parts are not very critical. However for Q1, D1 and D2 components with good HF properties must be used! Luckily enough the MPSH10 and the 1N5711 can be had from the GQRP club sales.

Measurements

Some measurements were done with regards to the AM detector sensitivity. The results are summarised in the table hereafter. Below 10MHz the minimum signal that can be demodulated without distortion is < 20mVpp (!). Slowly rising with the frequency. As said before, the upper limit is set by the DC emitter current of Q1 and is conservatively set a ~5mA. This can easily increased by lowering resistor R6.

Also some measurement were done to test the circuit as a plain RF level detector. The results are shown in the graph below. It shows excellent linearity from very low voltages all the way to the maxim input voltage. There is some frequency dependence as might be expected. If the circuit would be calibrated at 15 MHz, the rectified output deviation between the lower and upper extremes of the frequency range would be circa 4%. For an AM detector, the small change in sensitivity would be irrelevant. To combat this affect a small capacitance might be added over resistor R5.

Preamplifier
If you want to use the circuit as a sensitive RF-millivoltmeter, the circuit of figure 3 can be used as wideband unity gain preamplifier. The input resistance is given by R1. If a high impedance is required, it is important to use proper HF construction to minimize stray capacitance. Of course you can also use the preamplifier to minimise loading of the AM detector on the previous stages.
The maximum input voltage is > 600 mVpp. The measured -3dB bandwidth is > 60MHz.

Meter circuit
A possible setup for the meter circuit is given in figure 4. The actual resistor values might need to be changed to accommodate for different meter properties.

References

• RF Diode Detector / AM demodulator; https://crystal-radio.eu/diodedetector/endiodedetector.htm
• AM synchronous demodulator; Electronics & Wireless World, September 1989, page 858ff.
• Sensitive wideband linear a.c.-d.c. convertor; Volume 122, Issue 3, March 1975. K.F. Knott.

The G2NJ Trophy is sometimes awarded for a really good technical article, and sometimes for an outstanding contribution to international QRP. This year’s winner described a Cross-coupled Double Balanced Product Detector in SPRAT 191. The winner is Cor, PA3COR”
Steve G0FUW, Sprat 194

Needless to say it was a complete surprise! I feel very honored with this!
I looked up who has been awarded this trophy in recent years and then I felt completely hunbled! I can hardly believe it…

You can find the original article HERE.