Set 6
FM crystal receiver.

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This receiver can receive stations in the FM band (87-108 MHz).

Schematic description:

The receiver has a tuned circuit made with variable capacitor C1 and coil L1.
Coil L1 is a 285 mm long copper tube, the outside diameter is 12 mm.
Via coil L2 the input signal is coupled to L1.
L2 and L1 are placed parallel over a distance of 25 mm.
The wire diameter of L2 is 0.5 mm, and the spacing with L1 is about 1 mm.

On both sides of L1 are placed copper plated PCB boards.
The spacing between the boards and L1 is 10mm
The spacing between L1 and the bottom plate is also 10mm.

The bottom plate is also made of copper plated PCB board.

The "earth" side of L1 is via a small piece of PCB connected with the bottom plate. There must be very good contact between bottom plate and L1, so solder it very carefully.

The copper tube (L1) must be clean and free of oxide, otherwise selectivity en sensitivity will decrease.

The transformer is model TR10.16 made by Visaton.

This receiver is a slope detector. The receiver must be tuned a little bit beside the station’s frequency. The input signal is placed on the slope of the filtercurve, a FM modulated signal will now also obtain some AM modulation. This AM modulation can be detected by a diode.

For reception I use a 3 elements directional antenna. It has a gain of about 5 dB.

With the antenna placed only 1.5 meter high, I could receive the following stations:

The output signal is fairly weak, so use a sensitive speaker, for instance a driver unit.

Sensitivity test

On the picture below you see the test setup for measuring the sensitivity of the receiver, on the right you see a VHF signal generator.

At an input power of -40 dBm and a frequency of 100 MHz the DC voltage over R1 is 1.6 mV.

Diode test

In this receiver I tested the following diodes:
- One OA95 germanium diode (Rd = ± 45 kΩ)
- One HSMS2850 schottky diode (Rd = ± 9 kΩ)
- One BAT62-03W schottky diode (Rd = ± 225 kΩ)
- Two BAT62-03W diodes parallel (Rd = ± 112 kΩ)
- Four BAT62-03W diodes parallel (Rd = ± 56 kΩ)

At several load resistor values between 10 and 100 kΩ , the rectified output voltage is measured  (in milliVolt).
And from this the output power in the load resistor is calculated (in pico Watt).
The measurements are done at 100 MHZ and -35 dBm input power in the receiver.

The following table gives the results:

This graph shows the power in the load resistor as function of the load resistor value.

We see, the HSMS2850 gives the most output power.
The OA95 has the worst performance, probably this germanium diode is not fast enough for 100 MHz signals.
 

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