FM Converter
This page describes the FM converter part. A converter consists of two things: an oscillator and a mixer. In my design, both are contained in the same tube: an ECC85. One triode acts as a Hartley oscillator, and the other triode acts as a mixer.
Oscillator: Colpitts
Before the current design, I had tried a Colpitts oscillator. The (partial) schematic is shown below. This oscillator did not work. Thanks to Peter Dolron and his expensive equipment, we found out that the RF choke (L1 in the schematic) was resonating at 6MHz. This means that its impedance at any signal with a frequency higher than 6MHz is very low. This is bad. The choke was supposed to have a HIGH impedance for frequencies up to 150MHz!
Colpitts. L1 resonated at 6MHz.
For an impedance of 1kOhm at 100MHz, the choke needs to be 1uH. This turned out not to be practical, because the windings had to be spaced far apart in order to keep the resonance upwards of 150MHz. A 1uH inductor would have been quite large. So, I switched to a Hartley-type oscillator.
Oscillator: Hartley
The new oscillator is in a Hartley-configuration. This means that no big choke is required.
Winding the oscillator inductor is fairly simple. I wound it on the rear part of a 4mm drill. The tap was simply soldered on. I used wire from an old 50Hz transformer. To get the insulating material off (for the connections), just scratch the wire with a sharp knife, then put a blob of solder on your soldering iron and heat the scratched area. The insulating material should burn right off. The tap is at N = 2.
Here are some more pictures from Max at http://hem.passagen.se/communication/:
Digital tuning
In order to be able to digitally tune to a station, we need to tune two things: the oscillator frequency, and the input filter frequency. Currently I'm focusing on the oscillator.
Both LC circuits are tuned using a varicap. This is a diode, which has an internal capacitance that varies with the reverse voltage applied. This voltage is controlled by a PLL (phase locked loop) IC: the SAA1057.
The current state of things is that I have a working VCO (voltage controlled oscillator):
Left: voltage; right: frequency in MHz
Notice that the frequency of the oscillator has to be 10.7MHz higher than the frequency of the incoming signal, in order to produce a 10.7MHz IF frequency. For more information regarding heterodyning, look here and here.
The entire circuit
Not quite finished...
That's the IC socket for the SAA1057. The loop filter is on the top right, and the varicap is just obscured by the probe.
Click for larger version
Click for full version
Download the Eagle schematic + PCB here (for the ECC85 tube).
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