Friday, 18 March 2011

The Nexus 6 transmitter


After this long helpful preface I think you must be able to understand now the issues around the Nexus 6 QRP transmitter, so let's get now on to the real stuff. Nexus 6 is currently under development so I will present you the transmitter step by step as I develop it. Let's start with the transmitter. The picture below shows the crystal oscillator and the oscillator PSU of the Nexus 6.

Crystal oscillator and oscillator PSU of the Nexus 6

The picture shows an ultra low phase noise low distortion crystal oscillator, along with it's power supply. This type oscillator has been discussed in detail with Charles Wenzel from Wenzel Associates, to define it's performance. You could expect a phase noise better than -150dBc from this circuit and this is far better than any PLL can do. That is why I use a crystal oscillator and not any other kind of PLL/DDS. The trick is not to overload the crystal and thus degrade it's high-Q. The crystal used in that place, acts as a filter too, which helps eliminating some of the unwanted signals at the output of the oscillator.

The 25pF variable capacitor is used to shift the frequency of the crystal a few tens or hundreds of Hz in order to achieve fine tuning. Do not shift the crystal frequency too much or the phase noise will be degraded. I have performed different power and frequency measurements on the oscillator to understand it's linearity, but since linearity may be depend on the specific crystal used, I would not like to present the measurement results here. In general, the oscillator is more linear at 7-18MHz and presents higher output levels, whereas the power level is a bit reduced at the low and high ends of the shortwave bands (160m and 10m).

As far as concern the mechanical construction, use a two-pole four-position panel switch to switch between different bands. Use a panel mount crystal holder in order to change crystals for different bands. Try to keep the leads lengths as short as possible. The crystal and the variable capacitor will be switched between the oscillator and the receiver filter using relays, but I will show this later on. For the time being, leave some empty space for a relay there. Use a panel mount air variable capacitor, preferably silver plated, in order not to degrade the Q of the crystal too much. If you cannot find silver plated capacitors use aluminum or nickel plated, but always use air dielectric ones. Warning, both poles of the variable capacitor must be insulated from the chassis. Additionally, connect the capacitor such as the pole that you touch with your finger is connected at the 22 Ohm resistor side and not at the oscillator side! If you do it the other way, the oscillator frequency will change a bit every time you touch the capacitor with your finger. Even if you use an insulating knob for the capacitor, it is a good idea to connect it as I mentioned.

The power supply of the oscillator is composed of a BC547 transistor and a 2N4401. The BC547 section behaves like a capacitor multiplier, multiplying the 100uF at the base of the transistor with the 100uF shunt capacitor, to give a total of 10000uF. This should suppress any potential hum, but to achieve a lower phase noise oscillator I have added the 2N4401 section taken out from Wenzel Associates.

System designers often find themselves battling power supply hum, noise, transients, and various perturbations wreaking havoc with low noise amplifiers, oscillators, and other sensitive devices. Many voltage regulators have excessive levels of output noise including voltage spikes from switching circuits and high flicker noise levels from unfiltered references. The traditional approach to reducing such noise products to acceptable levels could be called the "brute force" approach - a large-value inductor combined with a capacitor or a clean-up regulator inserted between the noisy regulator and load. In either case, the clean-up circuit is handling the entire load current in order to "get at" the noise. The approach of the 2N4401 circuit described here uses a bit of finesse to remove the undesired noise without directly handling the supply's high current.

The key to understanding the "finesse" approach is to realize that the noise voltage is many orders of magnitude below the regulated voltage, even when integrated over a fairly wide bandwidth. For example, a 10 volt regulator might exhibit 10 uV of noise in a 10 kHz bandwidth - six orders of magnitude below 10 volts. Naturally, the noise current that flows in a resistive load due to this noise voltage is also six orders of magnitude below the DC. By adding a tiny resistor, R, in series with the output of the regulator and assuming that a circuit somehow manages to reduce the noise voltage at the load to zero, the noise current from the regulator may be calculated as Vn/R. If the resistor is 1 ohm then, in this example, the noise current will be 10uV/1ohm = 10uA - a very tiny current! If a current-sink can be designed to sink this amount of AC noise current to ground at the load, no noise current will flow in the load. By amplifying the noise with an inverting transconductance amplifier with the right amount of gain, the required current sink may be realized. The required transconductance is simply -1/R where R is the tiny series resistor.

The 2N4401 circuit is suitable for cleaning up the supply to a low current device. A 15 ohm resistor is inserted in series with the regulator's output giving a 150 millivolt drop when the load draws 10 mA - typical for a low-noise preamplifier or oscillator circuit. The single transistor amplifier has an emitter resistor which combines with the emitter diode's resistance to give a value near 15 ohms. The regulator's noise voltage appears across this resistor so the noise current is shunted to ground through the transistor's collector. The noise reduction can be over 20dB without trimming the resistor values and the intrinsic noise of the 2N4401 is only about 1 nanovolt per root-hertz. Trimming the emitter resistor can achieve noise reduction greater than 40 dB.

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